By P.I. Tzenov
Sericulture Experiment Station,
24 Mito Orozov Str., Vratza 3000, Bulgaria
Paper contributed to Expert Consultation on Promotion of Global Exchange of Sericulture Germplasm Satellite Session of XIX th ISC Congress, September 21st - 25th Bangkok, Thailand
Food and Agriculture Organization of the United Nations
Rome
2002
In Bulgaria until 1896 the silkworm eggs were produced only by the farmers themselves. Every sericulture farmer's family separated some fresh cocoons after the harvesting and the moths laid their eggs directly on the cocoons. The first commercial silkworm eggs in Bulgaria were produced in 1896, following the Pasteur's cellular method. At this time silkworm eggs have been imported also from France, Italy and Turkey. The French and Italian races had yellow cocoons and the Turkish - white.
After 1896 the egg production in Bulgaria was developed very fast, reaching about 270, 000 boxes (12g) in 1926. In the North and South-West part of the country were grown yellow cocoon races and in South-East part - white cocoon races. In the 30th and 40th years two main races were reared in Bulgaria, namely Yellow local, with yellow cocoons and cream color of raw silk and White Baghdad, having white cocoons.
The White Baghdad race is considered as the oldest at the Balkans. In Bulgaria the White Baghdad had 3 lines, namely Edirne, improved Bulgarian and Bulgarian. The White Baghdad had been selected by the ordinary farmers for many years and the characteristics of this race manifested comparatively high level of productivity and good national tradition in sericulture. White Baghdad race was characterized by long larval duration(40 days), high cocoon yield (40kg from 10g of eggs), big and heavy cocoons (2.8-3g), big size eggs (1200 eggs in 1g), very well adapted for the South Bulgarian region. Simultaneously the race was sensitive to flachery and NPV and it had too low raw silk percentage (29%) and reelability.
The Yellow local race had shorter larval duration, higher pupation rate, higher raw silk percentage (32-34%), but lower cocoon yield.
One of the main targets of Sericulture Experiment Station (SES) in Vratza since its establishment in 1896 and during more than one century old activity has been the improvement of silkworm races and hybrids in order to increase the cocoon and raw silk yield and quality at national level.
Up to the beginning of 30's of 20th century the station had not maintained its own silkworm genetic resources, but tested the races imported from different countries (mainly France and Italy) before their introduction at the private egg producing companies.
In the 40's the station introduced several yellow cocoon races like Askoli, Var, Abrucio, Almeria from France and Italy and using this material some local yellow cocoon lines like N1, N2, N13 and N24 were evolved.
During the period 1944-1952 hybrids between the Yellow local and Golden Chinese race were tested at the station and introduced to the field. In order to improve the lines of the Yellow local race, European yellow cocoon races Askoli, Var and Alpen were imported during the period 1949-1950. Using these races were selected the new Bulgarian lines N25, N26 and N27.
By crossing the race Askoli with the races Var and Alpen were selected the yellow cocoon lines 371 and 373, which, together with N25 and N 27 shifted gradually the other silkworm races from the field level. During the period 1952-1964 new hybrids between lines 371, 373 and Golden Chinese race were created and adopted in the field by SES-Vratza. In the end of this period white cocoon silkworm races and hybrids became more popular in the world due mainly to better silk dyeing.
From 1965 to 1970 at the station were selected many new silkworm lines by segregation from Italian and Japanese white cocoon F1 hybrids, like 11-J, 48-I, 157 K, 157 J etc. In 1969 the government approved the first white cocoon Bulgarian F1 commercial hybrids, selected at SES-Vratza, namely 11J × 48I; 48I × 157K and (48I × 11J) × (48I × 157K).
After 1970 the silkworm breeding work at SES-Vratza was expanded and many new races and hybrids had been introduced and created until nowadays. Taking into account the very big importance of the genetic resources for a successful breeding work the number of races, maintained at SES-Vratza was increased from 20-25 at the beginning of 70's to 183 in 2002.
In the early 70's were imported many silkworm races from Japan, like Thaihei, J-124, C-122, Chungetsu, Hosho, Kenko, Chinpaku, Taihei, Choan, Shunrei, Shogetsu, Honen, Kenpaku, Doei, Kohaku, Kinshu and Showa. These pure races were used as breeding source for selection of new highly productive races and hybrids at SES-Vratza.
In the middle 70's and 80, s from North Korea were received 8 races against giving to them Bulgarian accessions. A joint research program was started and developed with North Korea until the beginning of 90's, when the political system in Bulgaria was changed.
In 1979 was signed an agreement for scientific and technical cooperation between SES-Vratza and Sericulture Experiment Station, Merefa, Ukraine as well as Middle Asian Sericulture Research Institute in Tashkent, Uzbekistan. Through this cooperation, SES-Vratza received more than 20 Ukrainian and 7 Uzbekian silkworm races and gave to them in equivalent basis the same number of Bulgarian races. As results of the cooperation with Ukraine in Bulgaria were selected 16 silkworm races and 3 commercial hybrids and in Ukraine were created 2 commercial hybrids. In the late 80's were introduced 2 races from Georgia, 2 races from Syria and 2 races from Rumania.
SES-Vratza gave to Vietnam several highly productive races and received 2 Vietnamese lines. In the middle 80's in Bulgaria were imported several new Japanese lines, like KS (analogue of Kinshu), ShV (analogue of Showa), AS (analogue of Asahi) and TV (analogue of Tokai). These races were used for breeding of new commercial hybrids.
The last import of silkworm races from Japan was in 1997/98 through an agreement for cooperation between SES-Vratza and National Institute of Genetics, Mishima, Japan. SES-Vratza received 8 races, including 4 sex-limited for larval markings and 1 trimolter.
During the period 1998-2000 following a contract with the Egyptian company AgroMier, Cairo, SES-Vratza received 37 Egyptian silkworm races. Most of these races are originally from South Korea, China and India and are characterized with high productivity or some special, very precious for the breeding work. characteristics. The Egyptian races are still under evaluation and are considered as very promising breeding material for the future.
The research work in the field of silkworm genetics and breeding was directed to studies on the interactions between the genotype and environment, heterosis expression in F1, positive transgressions expression in F2, combining ability, heritability, correlations, regressions, inheritance of the main qualitative and quantitative characters, implementation of the intensive inbred-lines breeding, methods for creation of initial populations for silkworm breeding, breeding of sex-limited races, use of partheno and androgenesis in the silkworm breeding, breeding of silkworm races and hybrids for summer-autumn rearing, and having higher tolerance to adverse rearing conditions.
The recent hybrids selected at the station, like Vratza35 × Merefa2 and Super1 × Vratza35 × TV × Merefa2 manifested comparatively high productivity, namely cocoon weight 2.3-2.4g, shell weight 0.550-0.620g, filament length 1200-1500m. However these hybrids require to provide excellent conditions for silkworm rearing, otherwise due to their sensitivity they suffer from diseases (NPV) and the cocoon yield at farmer's level is poor. This is the reason to select new hybrids, having lower productivity, but higher tolerance to adverse environment in the recent years.
As it was mentioned above, now 183 silkworm accessions are maintained at Sericulture Experiment Station in Vratza. In Bulgaria some silkworm lines are also maintained in Agricultural university of Plovdiv (20-25), Trakian university in Stara Zagora (10-15) and Svila J.S.C, Haskovo (10-15).
Most of these races are analogues of some races, maintained in SES-Vratza. Now the main problem for silkworm germplasm maintenance in Bulgaria is the bad economical situation leading to poor financing of the budget sector. For example the share of subsidy in the annual budget of SES-Vratza is only 40% and the rest is provided by self incomes.
However, the government provides every year some subsidy for maintenance of silkworm germplasm at SES-Vratza. In the last two years FAO through the project TCP/BUL/0065 "Rehabilitation of sericulture in Bulgaria" provided to SES-Vratza comparatively modern laboratory equipment and technical advises, allowing to maintain the silkworm genetic resources at higher scientific and technical level.
The main purpose in the silkworm accessions maintenance is to conserve their characters closed to the characteristics in the passport, therefore the selection is made only for the qualitative characters, but the main quantitative traits values are also checked. Other important purpose of the maintenance methodology is to avoid inbreeding.
Now all silkworm accessions from the germpalsm, maintained at SES-Vratza are reared only once per year, namely during the most favorable spring season-May/June.
There were some investigations regarding the methods for maintenance of the polyvoltine races (Tzenov, 1998; Tzenov et al., 2001). It was proved that in Bulgaria is possible to maintain the polyvoltine race Bonde 517 by only one rearing per year within 4 years.
During the papionage 21 layings are produced per each accession and preserved under the standard regime up to the next spring. SES-Vratza has excellent facilities for cold storage. Before putting into incubation 8 layings, having biggest number of normal eggs are selected and treated by 2% formaline solution for disinfection.
The layings are hatched mixed and the larvae are grown also mixed. After incubation the hatchability is only checked but no any selection is made for higher hatchability. Only the larvae, hatched on the day of "mass" hatching are brushed for rearing. After the 2nd molt 200 larvae are counted per each accession and grown together up to the cocooning.
After cocoon harvesting the following quantitative characters are checked: average fresh cocoon weight, shell weight, shell percentage, pupation rate, fresh cocoon yield by 1 box of silkworm eggs. Out of 150-190 cocoons, produced per race 80-100 cocoons are selected for the qualitative characters, namely cocoon color, shape and nature of grains. The cocoons selected are put mixed for the papionage and the eggs are produced on cartoons.
Once per 3 years a random cocoon sample is taken from each accession in order to investigate the cocoon filament characters by reeling test.
The main purposes in the maintenance of the pure lines, which are parents of the commercial hybrids is to keep the values of the main breeding characters at least at the level of their selection by the breeders and to improve the grainage characters like, moth emergence percentage and fecundity. Also the inbreeding must be avoided obligatory.
Now due to smaller egg production the pure lines of the commercial F1 hybrids are maintained in 2 categories, namely P3 and P1.The P2 category is not produced and P1 is obtained directly from P3. The pure lines that are parents of single hybrids are maintained in 2 sublines each by the following methodology:
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Super 1 |
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Hesa 2 |
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P3 |
Line 1 |
Line 2 |
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Line 1 |
Line 2 |
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P1 |
Line 1 × Line 2 |
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Line 1 × Line 2 |
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F1 |
Super 1 × Hesa 2 |
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The pure lines that are parents of four-way hybrids are maintained as population, without any sublines, following the methodology:
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P3 |
KK |
Hesa 1 |
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Vesletz 2 |
Gergana 2 |
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P1 |
KK × Hesa 1 |
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Vesletz 2 × Gergana 2 |
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F1 |
(KK × Hesa 1) × (Vesletz 2 × Gergana 2) |
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The P3 category of pure lines is grown once per year, during the most favorable season in May/June. During the papionage 250 layings are produced per each pure line/subline. Before incubation 120 layings, having the biggest number of normal eggs are selected in each line/subline.
The layings are disinfected by 2% formaline solution. After the hatching, 55 layings, having hatchability more than 98% are selected for silkworm rearing. Only the larvae, hatched on the day of "mass" hatching are brushed for rearing.
Each laying is reared separately. After the second molt 200 larvae are counted from each laying for growing up to cocooning. During the larval period the layings (if any) having not typical for the race larval marking are rejected (separated for production of P1 or F1). Also the layings having not uniform larval growth or some diseases (NPV) are rejected. After cocoon harvesting, pupation rate, the cocoon yield, average cocoon weight, shell weight and shell percentage are checked for each laying.
Usually from the 55 layings reared, 20-25 layings, having the highest pupation rate are selected for reproduction of P3.After that only 8-12 layings out of them are selected based on the highest cocoon weight, shell weight and shell percentage. The cocoons of the selected 8 layings are assorted for alive pupa, cocoon color, shape, texture and shell hardness manually by experienced researcher in silkworm breeding and after that cut and the pupae are separated by sex. The female pupae of the first four layings are mixed for papionage with the male pupae from the rest 4 layings and vice versa in order to avoid the inbreeding. Once per 3 years random samples for making silk reeling test are taken from each pure line.
For control of pebrine disease after hatching, the egg shells of each laying are microscoped, samples of silkworm skins after each molt, early spun cocoons etc. are taken for microscope examination. The cocoons of the rest layings, produced are assorted and used for production of P1 or directly for F1.
2.2.3.1. Qualitative characters
Egg serosa color. It is determined visually on all silkworm layings, produced from each accession. The color is green, gray, brown, mixed.
Egg chorion color. It is determined visually on 3 replicates having 200 eggs or 2 layings each. The color is white, yellow and mixed.
Larval markings. It is determined visually on the 5th-7th day of the 5th instar on the all larvae reared per each accession. The larval markings are plain, normal marking and zebra.
Cocoon shape. It is determined visually after harvesting and floss removal, on the whole amount of good quality cocoons produced per each accession. Cocoon shape is oval, elongated oval, oval with constriction, elongated, elongated with constriction, spindle.
Cocoon color. It is determined visually after harvesting and floss removal on the whole amount of good quality cocoons produced per each accession. Cocoon color is white and colored.
Cocoon size. It is determined on random sample of 100 good quality cocoons. Cocoon size is big, medium, small.
Cocoon nature of grains. It is determined on random sample of 100 good quality cocoons. It is fine, medium, coarse and flossy. As main qualitative characters for silkworm genetic resources at SES-Vratza characterization were accepted only serosa color, chorion color, larval markings, cocoon shape and color.
2.2.3.2 Quantitative characters
Hatchability in %: It is determined on 6 layings per accession and on each laying, selected for incubation in P3 category pure lines. It is calculated on the 3rd day after hatching by the following formula: Number of normal eggs in the layings - number of unhatched eggs in the layings/number of normal eggs in the layings × 100
Larval duration in h: The beginning is day and hour of larval brushing, the end is day and hour when the feeding is stopped and larvae mounted.
5th instar duration in h: The beginning is day and hour of the first feeding after the 4th molt, the end is day and hour when the feeding is stopped and larvae mounted.
Pupation rate in %: It is calculated by the formula: Number of cocoons with alive pupa/number of larvae counted × 100.
Number of normal eggs in the laying: It is determined on random sample of 15 layings, after 20th October of the current year.
Weight of the normal eggs in the laying in g: It is determined on random sample of 15 layings, after 20th October of the current year.
Abnormal eggs in the laying in %: It is calculated by the formula: number of abnormal eggs in the laying/number of normal eggs in the laying × 100.
Number of normal eggs per 1 g: It is determined on 3 replicates, having 0.5g loosing eggs each.
Fresh cocoon grades in %: It is determined immediately after cocoon harvesting and floss removal. The cocoons are assorted in good quality (having alive pupae), double cocoons and unreelable cocoons. After the assorting all the three categories are weighed.
Fresh cocoon weight, and shell weight in g: There are two methods: 1. All good quality cocoons/shells in the laying/replicate are weighed and after that divided by their number. 2. A random sample consisted of 30 female and 30 male good quality cocoons/shells is taken and after weighting their weight is divided by the number. In the silkworm breeding the measuring is individual for each cocoon/shell.
Shell percentage: It is calculated by the formula: shell weight/fresh cocoon weight × 100.
Fresh cocoon yield by one box of eggs (20000 eggs) in kg. It is calculated by the following formula: fresh cocoon yield by laying/replicate in kg × 20000 × (hatchability × 0.01)/number of larvae counted.
Filament length in m: It is determined on a random sample of 30 good quality cocoons after single cocoon reeling test.
Filament weight in g: After the cocoon reeling the filament is dried to constant weight and weighed.
Filament size in denier: It is calculated by the formula: filament weight in mg/filament length in m × 9.
Reelability in %: It is calculated by the formula: filament weight/the weight of filament + frizen + pelet × 100
Raw silk percentage: The formula used is: filament weight/dry cocoon weight × 100.
Raw silk yield by one box of silkworm eggs: The following formula is used: fresh cocoon yield by 1 box of eggs × (good quality cocoons percentage × 0.01) × (percentage of dried cocoons obtained from the fresh cocoons after drying to constant weight × 0.01) × (raw silk percentage × 0.01).
Consumption index (CI) = dry weight of the mulberry leaf ingested in mg during the feeding period/feeding period duration in days × average dry weight of one larva during the feeding period in mg
Growth rate (GR) = body gain during the feeding period/feeding period duration in days × average dry weight of one larva during the feeding period in mg.
Leaf ingestibility (AI) = dry weight of the mulberry leaf ingested in mg/mulberry leaf supplied in mg dry weight × 100.
Food digestibility (AD) = dry weight of the mulberry leaf ingested in mg - dry weight of the excrements in mg/dry weight of the mulberry leaf ingested in mg.
Efficiency for conversion of the food supplied in products (ECS) = dry weight of the products obtained, namely body gain during the feeding period or cocoon shell or pupa or eggs/mulberry leaf supplied in mg dry weight × 100.
Efficiency of conversion of the food ingested in products (ECI) = dry weight of the products obtained, namely body gain during the feeding period or cocoon shell or pupa or eggs/dry weight of the mulberry leaf ingested in mg during the feeding period.
Efficiency of conversion of the food digested in products (ECD) = dry weight of the products obtained, namely body gain during the feeding period or cocoon shell or pupa or eggs/dry weight of the mulberry leaf ingested in mg - dry weight of the excrements in mg.
The first step in the silkworm breeding process is the primary evaluation of the initial breeding material (races, lines, hybrids etc.). Usually the following evaluation is made:
· Phenotypic evaluation of the main qualitative and quantitative breeding characters
· Phenotypic variation of the values of the quantitative characters
· Heritability of the main quantitative characters
· Correlation between the main quantitative breeding characters
· Regressions between the main quantitative breeding characters
For breeding new silkworm races the following different methods were used at SES-Vratza:
By this method were selected most of the older silkworm races, like Vratza 1, Vratza 2, Vratza 3, Vratza 4, Vratza 5, Vratza 6 etc. Generally the method is as follows:
1. Initial population: J-124 × C-122 (Japanese hybrid)
2. Selection of larvae, having markings and elongate shaped cocoons in F2
3. Mating the selected moths
4. Batch rearing, selection for larval markings, cocoon shape, texture, main quantitative characters for 11-16 generations.
5. New race: Vratza 1
By this method the pure line KK was selected. It is common that in the F1 hybrids can be found some individuals from the mother pure line due to mistakes in the sex discrimination, leading to mating within the pure line. The method is the following:
1. Initial population: South Korean hybrid
2. Selection of cocoons, having elongated with constriction shape and mating between them
3. Inbreeding for 4 generations, batch rearing, selection for cocoon shape and main quantitative characters.
4. Autbreeding for 9 generations, selection for cocoon shape and main quantitative characters.
5. New race: KK
By this method were selected many races like 157 K, Vratza 7, Super 1, Super 2, Hesa 1, Hesa 2, Gergana 1, Gergana 2, Vesletz 1, Vesletz 2, Kom 1, Kom 2 etc. As an example we will give the breeding of the line Super 1/11by Petkov (1976):
1. Initial population; J 124
2. Selection of 6 layings as initial parents for future sublines.
4. Inbreeding for 7-8 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters.
5. Checking the combining ability of each line by crossing with the race of Chinese type 157 K (general combining ability) and rejection 2 of the lines due to poor combining ability.
6. Inbreeding for another 7-8 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters, with simultaneous checking the combining ability of each line with 3 other promising races of Chinese type (specific combining ability).Some of the sublines are rejected due to inbreeding depression.
7. It was detected that the best specific combining ability had the line Super 1/11.
8. New race: Super 1/11, ready for direct hybridization.
Other example is the breeding of pure lines Hebar 1/18 and Hebar 2/1 by the following method:
1. Initial population: Kinshu
2. Selection of 3 sublines
3. Inbreeding for 3 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters.
4. Rejection 2 of the sublines
5. Inbreeding in another 3 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters.
6. New race: Hebar 1/18
By this method were selected the pure lines Vratza 35, Merefa 2, the races Vratza 51, Vratza 53, Vratza 52, Vratza 54 etc. Here we are giving an example with the breeding of line Vratza 35 (Nacheva et al., 1998):
1. Initial population: Hebar 1 × Tashkent 11
2. Inbreeding in 2 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters.
3. Autbreeding for 16 generations, batch rearing, selection for larval marking, cocoon shape, texture, main quantitative characters.
4. New race; Vratza 35
By this method as initial material were used sex limited races, crossed with other races, having plain larvae or yellow eggs. After that the hybrid population was maintained by batch rearing for 4 generations and with inside batch mating (inbreeding). On the 5th generation an inter batch crossing was made, followed again by inbreeding in the 6th and 7th generations. At the 8th generation the two inbred lines were crossed and the new race was created.
Using this method Petkov (1996) selected 2 sex limited for egg color races (T15/4 and HT-215/38) and 5 sex limited for larval marking races (B2/6, BTV-2/64, TBV-2/24, HB-2/22 and TV-3/2).
It is well known that the reason to use sex limited races is in order to make easier, and more effective silkworm egg production, especially the sex discrimination. To create a commercial hybrid is a tedious job, therefore if we could use the same pure lines, but having sex limitation it would be a good achievement.
The method is based completely on the back crosses and by using it Tzenov (2001) selected 5 new lines, namely SN1/H1, SN1/AS, Iva 1, Nova 2 and Magi 2, analogues of the pure lines Hesa 1, AS, KK, Vesletz 2 and Gergana 2, but sex limited for larval marking. Here we give an example with the breeding of the line Iva 1:
1. Initial population: cross between females of Nig 1 (sex limited for larval marking) and males of KK(having plain larvae).
2. Backcrosses for 9 generations, mass rearing
3. Backcross for 2 generations, batch rearing
4. New race: Iva 1
Practically the selection is made only with the female individuals of Iva 1, using selected for P3 males of KK. The males of Iva1 are used only for hybridization. In this case the basic pure line (KK) is maintained in big volume, allowing to make the necessary selection and the sex-limited analogue is maintained in lower volume (8-10 layings during rearing).
Our studies proved that the sex limited analogues of the pure lines demonstrated nearly the same values of the main quantitative characters like the original pure lines.
In Bulgaria, only 3 races (SB1, VB1, HB 2) were selected by this method in the recent years (Tzenov, 1999; Vasileva, 1999-2001) by using crosses between the uni-bivoltine pure lines Super 1 and Hesa 2 and the polyvoltine race Bonde 517. The method is following:
1) Super 1 × Bonde 517 F1
2) Super1 × Bonde517 F2.There was segregation in colored and white cocoons with different shape and texture. Seven male cocoons out of 32 kg cocoons, having white color, elongated shape, similar to those of the uni-bivoltine line texture were selected and mated with selected females of the line Vratza 35.
3) Inbreeding for 6 generations, batch rearing, selection for cocoon color, shape, texture and the main quantitative characters.
4) Outbreeding for 5 generations, batch rearing, selection for cocoon color, shape, texture and the main quantitative characters.
5) New race VB 1, the breeding work will be continued.
The newly selected races manifested a higher tolerance to adverse rearing conditions, but their productivity was comparatively low. The idea is to use them as components of four-way hybrids with highly productive but, sensitive to bad environment pure lines. For example a possible four-way hybrid which will be tested in the next year is VB 1 × Vratza 35 × HB2 × Merefa 2.
The breeding targets are generally the following:
Japanese type races: marked or plain larvae, elongated with or without constriction shaped cocoons, white in color.
Chinese type races: plain larvae, oval and white in color cocoons.
Hatchability: as high as possible. Checked individually.
Larval duration: 27-29 days. Checked for the laying.
Fifth instar duration: 7-9 days. Checked for the laying.
Pupation rate: as high as possible. Checked for the laying.
Number of normal eggs in the laying: more than 550. Checked individually
Weight of the normal eggs in the laying: more than 0.330 g. Checked individually.
Good quality cocoons: more than 95%. Checked for the laying.
Fresh cocoon weight: the individuals having the highest or medium cocoon weight for the race are selected. Checked individually.
Cocoon shell weight: the individuals, having the highest for the race shell weight, the highest or average cocoon weight and the highest or average shell percentage are selected. Checked individually.
Shell percentage: the individuals, having the highest or average for the race shell percentage are selected. Checked individually.
Fresh cocoon yield by 1 box of eggs: The layings, having the highest values are selected. Checked for the laying.
Filament length: This character is checked during the primary evaluation of the initial population for breeding and every year/generation during the breeding process. During the primary evaluation are selected only populations/races/lines having total cocoon filament length more than 950 m and unbreakable filament length more than 800 m. During the breeding process if any line showed values of the trait less than the limit for more than 2 generations is rejected.
Raw silk percentage: This character is checked during the primary evaluation of the initial population for breeding and every year/generation during the breeding process. During the primary evaluation are selected only populations/races/lines having raw silk percentage more than 39%. During the breeding process if any line showed values of the trait less than the limit for more than 2 generations is rejected.
Some of the selected silkworm races are used as pure lines for commercial hybridization.
The first precondition one race to be selected as pure line is to cover all the breeding characters criteria mentioned above, to have low variability of the quantitative characters values and the inheritance of the characters to be stable in the generations.
The chosen (or especially selected) races are tested for the combining ability and those one showed the highest combining ability between them are used as pure lines of the commercial hybrid. After that the new hybrid is tested at least for 3 years at the station/institute where it has been selected, and after that the hybrid is tested for at least 2 years in the system of the State Executive Agency for Varieties Testing.
During the testing as control is used the most widely adopted in the field silkworm hybrid for the last 5 years. To be recognized as new and original the hybrid must display difference from other hybrids/races at least in 1-2 qualitative characters and higher than the standard at least fresh cocoon and raw silk yield by one box of silkworm eggs. Now in Bulgaria are produced single and four-way F 1 commercial hybrids.
2.3.8.2 Methods of creation tolerant to adverse rearing conditions F 1 hybrids
2.3.8.2.1 By using selected for tolerance to adverse rearing condition races.
By this method were created the hybrids SB1 × HB2, VB1 × HB2 and the opposite. Really, under highly adverse rearing conditions these hybrids manifested a high tolerance, but under optimal rearing their productivity was lower than the control.
However our target was to create a hybrid, tolerant to adverse environment, but under good rearing condition to give a productivity near to the control. The reason is that if the hybrid is reared by a good farmer to ensure him/her a yield like the ordinary highly productive hybrid.
Because of this reason the other method, described below was implemented further.
2.3.8.2.2. By testing accessions of the silkworm germpalsm for higher tolerance to adverse rearing conditions and further hybridization.
For this purpose from the silkworm germplasm, maintained at SES-Vratza were chosen 15 accessions of the Japanese type and 13 accessions of the Chinese type, which had comparatively high values of the main quantitative characters.
After that the races were tested for three times regarding their tolerance to adverse rearing conditions during the 4th and 5th instars (t 28-310C, RH 75-80%, feeding amount and rearing space-reduced by 50%, without ventilation). It was proved that the races KK, Hesa1, AS, Vesletz2, Gregana2, ShV and Hebar2 had higher pupation rate under the adverse rearing conditions.
After that these races were crossed between them to check their combining ability. KK × Hesa1 and KK × AS appeared to be the best crosses among the Japanese type of races and Vesletz2 × Gergana2 among the Chinese races respectively. The combination (KK × Hesa1) × (Vesletz2 × Gergana2) gave the best results both under adverse and optimal rearing conditions.
Simultaneously, the races were mass reared for 6 generations under adverse conditions and for reproduction were used only the individuals survived and having the highest cocoon weight, shell weight and shell percentage. After that the races have been maintained following the standard methodology for pure lines.
As a result was created the new hybrid (KK × Hesa1) × (Vesletz2 × Gergana 2) and the opposite, having higher tolerance to adverse rearing conditions and simultaneously satisfactory high productivity under optimal rearing.
Silkworm germplasm database, maintained at SES-Vratza contains data for the main qualitative and quantitative characters values for each accession from the year of receiving/introduction up to now.
The main qualitative characters, like egg serosa and chorion color, larval markings, cocoon shape and color etc. are checked and recorded every year. Also, every year are checked the values of hatchability, pupation rate, fresh cocoon weight, shell weight and shell percentage. However the characteristics of the cocoon filament are studied by reeling test once per 3 years.
Now the average data for the silkworm germplasm are translated in English and converted to electronic files.
It is necessary and very essential to create a software for integrated database and descriptor preparation as well as query based program for silkworm genetic resources in order to upload them in Internet.
There are 183 accessions from 15 different countries maintained at silkworm germplasm in SES-Vratza (Table 16 & 17, chart 16). Most of the accessions are Bulgarian-81 (44%), followed by Egyptian - 37(19%), Ukrainian-20 (11%), Japanese-12 (7%), North Korean-8(4%), Uzbekistan (4%), Chinese-5(3%), Georgia, Rumania, Syria and Viatnam-having 2 accessions each, Madagaskar, Austria, Italy, having 1 accession each. For most of the races bred in Bulgaria as initial material were used Japanese pure breeds or hybrids. The races from Egypt originate from South Korea, India and China.
Nearly all the accessions are uni and bivoltine, excluding only two races which performed as polyvoltine. The main qualitative characters, characterizing the silkworm genetic resources in Bulgaria are shown in Tables 16 and 17. It is evident from the tables that most of the accessions have gray or/and green egg serosa color, white and/or yellow egg chorion color, marked or/and plain larvae, elongated with or without constriction or oval cocoon shape. The races with white cocoons are prevailing.
From the data presenting the qualitative characters of the accessions having particular traits/mutations is evident that the following mutant alleles are possibly presented in the silkworm germplasm:
Ze, zebra larval marking; p, plain larval marking; p+, normal pattern larval marking; lethal; Pk, pink cocoon; Y, yellow blood; M3, moltinism, trimolting; G, green cocoon color; W2, white egg 2; pnd, pigmented, non-diapause egg; npnd, non-pigmented, non-diapause egg; Ng, no glue egg;
For the breeding purposes the following accessions, having particular characters/mutations could be of interest:
· As sources of genes for higher sturdiness-polyvoltine races Bonde 517 and Daizo, bivoltine KK and Vesletz 2.
· As sources of males having yellow colored eggs for mating with sex-limited for egg color females - 1A1, 1A-2-I.
· As source of zebra marked sex limited females-Ze Chinese 108, Ze Aojuku, SN1/AS, SN1/H1, Magi 2.
· As source of normal marked sex limited females - NIG 1, NIG2, B2/6, BTV-2/64, TVB-2/24, HB-2/22; TV-3/2, Iva 1, Nova 2.
· As source of sex limited females for egg color-T15/4, HT 215/38, E-22, E-23.
· As source of high parthenogenesis ability-E-11, E-12, Parthen colon 1, Parthen 2, P11/22, P15/18, Par+.
· As source of lethal genes (only male progeny)-E-18, E-20 and E-21.
The data regarding the hatchability, larval duration, pupation rate and the fresh cocoon yield by one box of silkworm eggs are presented in Tables 19, 20, 21 and charts 17, 18. It is evident from the tables that most of the silkworm accessions manifest comparatively high hatchability.
The races, having shorter larval duration (less than 650 h) are Vratza 24, Vesletz1, Ogosta1, Belopol2, Merefa 1, Iskar 1, Iskar 2, Ogosta 1/20, Belopol, Merefa 7, UN, Ukrainian 9, Ukrainian 10, Ukrainian 11, Ukrainian 13, Ukrainian 16, Sh-3, Sh-26-32, Tashkent 10, Tashkent 11, Tashkent 16, Line 22, N6, N7, MNB, Chinese 108, Mir 4, E-14, China, Shandon, Daizo, Bonde 517, E-26, E-27, E-28, E-29.
The data regarding the 5th instar duration in the pure lines, presented in Table 21 show that it vary from 180 h in Gergana 2 to 218 h in AS. The lines Gergana 2 and KK have shorter 5th instar than the other parents of F1 hybrids.
The silkworm accessions, manifested high pupation rate (more than 95%) are Vratza 37, Valve 111, Ukrainian 15, Ukrainian 17, Sh-4, Tashkent 11, Tashkent 12, Mziuri 1, N 6, N7, 110-C, 115-J, Changu 112, E-4b, 1A-2-I, 1A-2-II, Jena, China, E-26 and B2/6. It is evident from Chart 17 that 80% of the silkworm accessions have pupation rate higher than 85%, namely 26% of the races have pupation rate from 85 to 90%, 39% have pupation rate from 90 to 95%, and 15% of the races manifest pupation rate higher than 95%.
Since the data for pupation rate were obtained by rearing of silkworms under optimal conditions they do not give correct information about the survivability of the races under adverse environment.
Tzenov et.al. (2000) studied the pupation rate and fresh cocoon yield in different Bulgarian silkworm races and hybrids under optimal and adverse rearing conditions during the last two larval instars.(Tables 22 and 23). It was estimated that there existed clearly expressed genetically determined differences between the races and hybrids for pupation rate under adverse silkworm rearing conditions. However no any correlation was detected between the pupation rate in the optimal and adverse rearing conditions in one the same strain. The highest pupation rate under adverse rearing conditions manifested the races KK, AS and Hesa1 of the Japanese type, Vesletz 2, Gergana 2 and ShV of Chinese type and the hybrid KK × Vesletz 2.
The races KK and Vesletz 2 could be recommended as sturdy white cocoon bivoltine genotypes, having simultaneously comparatively high productivity.
Silkworm accessions manifested high fresh cocoon yield by 1 box of eggs (more than 35kg) are as follows: Super 1, Hesa 2, Vratza 35, Merefa 2, Hebar 1/18, Hebar 2/1, Vratza 33, Vratza 40, Vratza 50, BC-1, Kom 1, Belopol 1/18, Belopol, Valve 222, Merefa 7, Ukrainian 14, Ukrainian 15, Ukrainian 18, Ukrainian 20, Sh-4, Mziuri 1, Baniasa alba, Changu 112, A-14, TV, Mir 4, Mir 5, JH-3, JH-4, E-4a, E-5a, E-4b, E-5b, E-1, E-2, E-3, E-6, E-8, E-9, E-10, E-13, E-15, E-16, Vratza 54, 1A-1, 1A-2-II, PS-4BE-23, E-30, P11/22B2/6, BTV-2/64. The data in Chart 18 display that 93% of silkworm accessions have cocoon yield higher than 25 kg. Most of the races (37%) manifested cocoon yield from 30 to 35 kg, 29% - cocoon yield from 35 to 40 kg and 5% - from 40 to 45 kg.
The data for fresh cocoon weight, shell weight and shell ratio are presented in Tables 21, 24, 25 and charts 19, 20, 21. The following races manifested comparatively high (more than 2.150 g) fresh cocoon weight - Vratza 35, Merefa 2, Super 1, Hesa 2, Vratza 33, Vratza 38, BV-3B, Ukrainian 14, Ukrainian 19, E-4a, E-5a, E-4b, E-5b, E-1, E-6, E-9, E-13, E-15, E-16, B2/6, BTV-2/64, TBV-2/24, TV-3/2. The average fresh cocoon weight is the highest in E-5a-2.558 g, E-1-2.535g, E-6-2.445 g, Vratza 33-2.438 g, E-4b-2.363g, Vratza 35, 2.248 g and Merefa 2 - 2.295 g.88% of the races have cocoon weight higher than 1.7 g. Most of the accessions manifested cocoon weight from 1.8 to 2 g - 43%, and 35% of them - cocoon weight higher than 2 g.
Shell weight is the highest (more than 0.450g) in Vratza 35, Merefa 2, Super 1, Hesa 2, Hebar 2/1, Vratza 33, Kom 1, BV-3B, Ukrainian 18, Ukrainian 19, A-14, E-5a, E-4b, E-1, E-6, E-15, Vratza 51, Vratza 53, Vratza 52, Vratza 54, PS-12Sh, B2/6, BTV-2/64, TBV-2/24, HB-2/22, TV-3/2. The following races manifested the highest shell weight: Vratza 35-0.535 g; Merefa 2-0.525 g; E-5a-0.503 g; Vratza 53-0.517 g; Vratza 51-0.509 g; Vratza 52-0.512 g; B2/6-0.535 g; BTV-2/64-0.523 g; TBV-2/24-0.525g; HB-2/22-0.518 g; TV-3/2-0.531 g.
75% of the races have shell weight higher than 0.350 g, namely from 0.350 g to 0.400 g - 35%, from 0.400g to 0.450 g - 26%, from 0.450 to 0.500 g - 8% and higher than 0.500 g - 6%.
The following accessions showed high (more than 22%) shell ratio Vratza 35, Merefa 2, Super 1, Hesa 2, Hebar 1/18, Hebar 2/1, Vratza 33, Super 4, Kom 1, Kom 2, Ukrainian 18, Vratza 51, Vratza 53, Vratza 52, Vratza 54, KS, ShV, Tashkent 15, Nig 2, T15/4, HT 215/38, B2/6, BTV-2/64, TBV-2/24, HB-2/22, TV-3/2.70% of the accessions displayed shell ratio higher than 19% and 29%- shell ratio higher than 21%.
It is evident from the data, concerning the main productive characters that most of the pure lines (parents of the commercial hybrids) manifest one of the highest values.
The data about the cocoon filament characters are shown in Tables 26, 27, 28 and chart 22. The data manifest that 48 silkworm accessions have comparatively long (more than 1050 m) filament. It is the longest in Merefa 2 - 1310 m, Hesa 2 - 1251 m, Ukrainian 19 - 1281 m, E-16-1256 m, TBV-2/24-1292 m. 57% of the accessions manifested cocoon filament length higher than 900 m, 17% - from 900 to 1000 m, 21% - from 1000 to 1100 m, 12% - from 1100 to 1200 m, 6% - from 1200 to 1300 m and 1% from 1300 to 1400 m.
The average raw silk output, reeled by one cocoon is higher (more than 0.330g) in the races Super 1, Hesa 1, Vratza 35, AS, KK, Hesa 2, Merefa 2, Vesletz 2, Gergana 2, Hebar 1/18, Hebar2/1, Vratza 33, Vratza 50, Ogosta 1, Merefa 1, Valve 222, Valve 111, Ukrainian 18, Ukrainian 19, Sh-4, Mziuri 1, Mir 4, E-4a, E-5a, E-4b, E-5b, E-1, E-6, E-7, E-8, E-10, E-13, E-15, E-16, E-31, PS-5Sh, E-23, P11/22.
From the data discussed could be concluded that the following silkworm races are able to be used for direct commercial hybridization: Super 1, Hesa 1, Vratza 35, AS, KK, Hesa 2, Merefa 2, Vesletz 2, Gergana 2, Hebar 1/18, Hebar2/1, Kom 1, TV, KS, ShV, Vratza 51, Vratza 53, Vratza 52, Vratza 54, E-5a, E6, B2/6, TBV-2/24, TV-3/2, HT-215/38, T/15/4.
As promising silkworm races, which can be used in the future breeding programs appear to be: Vratza 33, Vratza 40, 157K, BC-1, BC-2, Super 4, Kom 2, Belopol 2, BV1, BV-3B, Iskar 2, Valve 222, Valve 111, Ukrainian 14, Ukrainian 15, Ukrainian 18, Ukrainian 19, Ukrainian 20, Mziuri 1, A-14, E-4b, E-5b, E-1, E-8, E-9, E-10, E-13, E-15, E-16, Iva 1, SN1/H1, SN1/AS, Magi 2, Nova 2.
In Table 29 are shown the data for silkworm egg hatchability in three pure lines and their F1 hybrids, characterizing the diapause duration of eggs laid by the spring moths generation in the end of June and further preserved under standard conditions. The results manifested that at the beginning of March all the races and hybrids already have normal and high hatchability of the eggs. They keep the high hatchability up to the beginning of July, therefore these eggs can be used for silkworm rearing for 4 months, namely from the beginning of March to the beginning of July. Since most of the silkworm genetic stocks in Bulgaria are reared only once per year these data should be considered in the supply of some silkworm genetic resources to other countries.
Vassileva and Tzenov (2001) tested 31 accessions from silkworm genetic resources for their ability for parthenogenetic development (Table 30). The results showed that the highest parthenogenetic eggs hatchability (more than 20%) was detected in the races ShV, TV, Hesa 2, Siria 2, Mysore 1, 70-42, MNB and China.
Tzenov(1996) studied the fecundity in 16 highly productive silkworm races (Table31) and detected that in the Japanese type races the moth emergence percentage was the highest in Kom 1 (93.32%) and the lowest in AS (61.88%). The moth emergence was comparatively higher in Vratza 35, Hebar 1, KS and Super 1. In the Chinese type races the moth emergence was the highest in ShV (88.04%), Hebar 2 (83.18%) and TV (92.51%) and the lowest in Merefa 2 (55.24%). It is evident from the table that in average the Chinese type races have higher values of the traits number and weight of normal eggs in the laying than the Japanese type.
Other important traits are those characterizing the food ingestion, digestion and utilization. From his study with the same 16 silkworm races, the tropical polyvoltine Bonde 517 and their hybrids (Table 32) Tzenov (1996) detected that there were significant differences between the races regarding the amount of dry mulberry leaves supplied during the whole fifth larval instar..The highest amount of leaf was supplied to the race AS (5.227g dry matter per 1 larva during the 5th instar) and the lowest amount-in Bonde 517-only 3.170 g.
In the Japanese type races the amount of food ingested was the highest in Vratza 51 (3.688 g) and the lowest in Hebar 1 (3.137 g). In the Chinese type races the food ingested was the highest in Vratza 54 (3.784 g) and the lowest in Hebar 2 (2.858 g).
The food ingested and digested was the lowest in polivoltine race - 0.943 and 0.347g respectively. The data manifested that most of the hybrids studied expressed different in degree, positive heterosis for the amount of food ingested and digested. The leaf ingestibility was the highest in the Chinese type race Vratza 54 (73.53%) and the lowest in the polivoltine race Bonde 517 - only 29.75%. The highest food digestibility was detected in the tropical race (36.80%), Super 1 (31.74%), and Vratza 53 (30.04%). The food digestibility was the lowest in Hebar 2 (25.72%), KS (26.07%) and Merefa 2(26.65%).
Most of the hybrids studied showed different in degree positive heterosis for the leaf ingestibility, but the heterosis manifested for the food digestibility was low or negative.
In Tables 33 and 34 are presented the data for food utilization. The consumption index was the highest in Vratza 51 (0.829 mg), Vratza 53 (0.825 mg), Vratza 52 (0.827 mg), Vratza 54 (0.809 mg), Bonde 517 (0.823 mg) and the lowest in Super 1(0.718 mg), Vratza 35 (0.732 mg), AS (0.744 mg) and Merefa 2 (0.741 mg). Unlike the other races who have high leaf ingestion and consumption index, the polivoltine race combined high consumption index with low food intake. As regards the growth rate character values the inter-racial differences were negligible. The polivoltine race manifested very high growth rate (0.267 mg).
The efficiency of conversion of the food supplied (ECS) for body gain was the highest in Super 1 (17.83%), Merefa 2 (17.37%), Vratza 35 (17.16%) and the lowest in Hebar 2 (13.53%) and Bonde 517 (9.65%).
The utilization of the food supplied for silk shell was the highest in Super 1 (7.65%), Vratza 54 (8.18%), Vratza 35 (7.87%) and the lowest in Hebar 2(6.54%) and Bonde 517 (2.08%). The utilization of the food supplied for eggs was the highest in Super 1 (2.62%), Hesa 2 (2.66%) and the lowest in AS (2.18%) and Hebar 2 (1.84%).
The data for efficiencies of utilization of the food ingested and digested, shown in Table 34 manifested that in most of the cases the races having comparatively high leaf ingestion and digestion showed lower food utilization and vice versa. The heterosis expression in F1 was low or negative.
Bozchkova et al.(1996) investigated the haemolymph lysozime activity in 16 silkworm races and its inheritance in 10 F1 hybrids (Table 35). Lysozime is an important agent of the natural resistance of the insects. The results obtained clearly bring out that there are significant differences between the races and hybrids as regards the haemolymph lysozime activity. Generally the races, having plain larvae and oval-shaped cocoons (Chinese type) displayed a higher lysozime activity than that observed in the races with larvae having markings and elongated cocoons (Japanese type). However there are some races of the Japanese type, having a higher lysozime activity. The F1 hybrids showed a partial dominance or overdominance for the higher parent or lower parent value.
The average data for performance of Bulgarian F1 commercial hybrids which are given to the farmers are presented in Table 36. The larvae were reared under conditions, closed to those at the field level.
There are no significant differences between the hybrids regarding the larval duration. The 5th instar duration is significantly shorter in the four-way hybrid KKxH1 × V2xG2 and the opposite. The same hybrid manifested also higher pupation rate. The fresh cocoon yield by one box of eggs was significantly lower in the hybrids Vratza 35xMerefa 2, S1xV35 × TVxM2 and the opposite, due mainly to the lower pupation rate. The hybrids Vratza 35 × Merefa 2 and S1xV35 × TVxM2 had significantly higher single cocoon fresh weight, shell weight and shell percentage.
The basic Bulgarian silkworm hybrids were also tested under adverse rearing conditions by the State Executive Agency for Variety Testing and the results obtained are shown in Tables 37 and 38. The data manifested that the two four-way hybrids, namely KKxH1 × V2xG2 and KKxAS × V2xG2 had much higher pupation rate than the hybrid Super 1 × Hesa 2 under the adverse larval rearing conditions, but the differences between the hybrids in cocoon weight, shell weight, shell percentage, filament length, filament weight were negligible. The lower pupation rate in the hybrid Super 1 × Hesa 2 reflected in lower fresh cocoon and raw silk yield by one box of silkworm eggs.
Taking these results into account, for the last 2 years SES-Vratza has produced silkworm eggs from the hybrids KKxHesa 1 × Vesletz2xGergana2 and Super 1 × Hesa 2 and the opposite in order to ensure a reliable cocoon yield to the farmers. The field results confirmed this decision.
The breeding of improved and new silkworm hybrids is a continuos process at SES-Vratza. During the period 1997-1999 Petkov et al (2000) tested series of new highly productive double, three-way and four-way silkworm hybrids under laboratory condition.(Tables 39 and 40). The results manifested that the new hybrids surpassed the standard (Super 1 × Hesa 2) in most of the quantitative characters and their values were comparatively very high. There are no significant differences between the double, three and four-way hybrids, except for the fecundity characters that are higher in three and four-way crosses, compared with the double hybrids.
From the above discussion the following conclusions can be made:
1. There are 183 accessions from 15 different countries maintained at silkworm germplasm in SES-Vratza. Most of the accessions are Bulgarian - 81 (44%), followed by Egyptian - 37 (19%), Ukrainian-20 (11%), Japanese-12 (7%), North Korean-8 (4%), Uzbekistan (4%), Chinese-5 (3%), Georgia, Rumania, Syria and Viatnam-having 2 accessions each, Madagaskar, Austria, Italy, having 1 accession each. Nearly all the accessions are uni and bivoltine, excluding only two races which performed as polyvoltine.
2. For the breeding purposes the following accessions, having particular characters/mutations could be of interest:
- As sources of genes for higher sturdiness-polyvoltine races Bonde 517, Daizo, and bivoltine KK and Vesletz 2
- As sources of males having yellow colored eggs for mating with sex-limited for egg color females - 1A1, 1A-2-I.
- As source of zebra marked sex limited females-Ze Chinese 108, Ze Aojuku, SN1/AS, SN1/H1, Magi 2.
- As source of normal marked sex limited females - NIG 1, NIG2, B2/6, BTV-2/64, TVB-2/24, HB-2/22; TV-3/2, Iva 1, Nova 2.
- As source of sex limited females for egg color-T15/4, HT 215/38, E-22, E-23.
- As source of high parthenogenesis ability-E-11, E-12, Parthen colon 1, Parthen 2, P11/22, P15/18, Par +.
- As source of lethal genes (only male progeny)-E-18, E-20 and E-21.
3. The silkworm accessions, manifested high pupation rate (more than 95%) are Vratza 37, Valve 111, Ukrainian 15, Ukrainian 17, Sh-4, Tashkent 11, Tashkent 12, Mziuri 1, N6, N7, 110-C, 115-J, Changu 112, E-4b, 1A-2-I, 1A-2-II, Jena, China, E-26 and B2/6. 80% of the silkworm accessions have pupation rate higher than 85%, namely 26% of the races have pupation rate from 85 to 90%, 39% have pupation rate from 90 to 95%, and 15% of the races manifest pupation rate higher than 95%.
The races KK and Vesletz 2 could be recommended as sturdy white cocoon genotypes, having simultaneously comparatively high productivity. 93% of silkworm accessions have cocoon yield higher than 25 kg. Most of the races (37%) manifested cocoon yield from 30 to 35 kg, 29% - cocoon yield from 35 to 40 kg and 5% - from 40 to 45 kg.
4. The following races manifested comparatively high (more than 2.150g) fresh cocoon weight-Vratza 35, Merefa 2, Super 1, hesa 2, Vratza 33, Vratza 38, BV-3B, Ukrainian 14, Ukrainian 19, E-4a, E-5a, E-4b, E-5b, E-1, E-6, E-9, E-13, E-15, E-16, B2/6, BTV-2/64, TBV-2/24, TV-3/2. The average fresh cocoon weight is the highest in E-5a-2.558g, E-1-2.535 g, E-6 - 2.445 g, Vratza 33 - 2.438 g, E-4b-2.363 g, Vratza 35, 2.248 g and Merefa 2 - 2.295g. 88% of the races have cocoon weight higher than 1.7 g. Most of the accessions manifested cocoon weight from 1.8 to 2g - 43%, and 35% of them - cocoon weight higher than 2g.
The following races manifested the highest shell weight: Vratza 35-0.535g; Merefa 2-0.525g; E-5a - 0.503 g; Vratza 53 - 0.517 g; Vratza 51 - 0.509 g; Vratza 52 - 0.512 g; B2/6-0.535 g; BTV-2/64-0.523 g; TBV-2/24-0.525g; HB-2/22 - 0.518 g; TV-3/2 - 0.531 g. 75% of the races have shell weight higher than 0.350g, namely from 0.350g to 0.400g - 35%, from 0.400g to 0.450 g - 26%, from 0.450 to 0.500 g-8% and higher than 0.500g - 6%. 70% of the accessions displayed shell ratio higher than 19% and 29%- shell ratio higher than 21%.
5. 48 silkworm accessions have comparatively long(more than 1050 m) filament. 57% of the accessions manifested cocoon filament length higher than 900 m, 17% - from 900 to 1000 m, 21% - from 1000 to 1100 m, 12% - from 1100 to 1200 m, 6% - from 1200 to 1300 m and 1% from 1300 to 1400 m.
It is the longest in Merefa 2 - 1310 m, Hesa 2 - 1251 m, Ukrainian 19 - 1281 m, E-16-1256 m, TBV-2/24-1292 m. The average raw silk output, reeled by one cocoon is higher (more than 0.330 g) in the races Super 1, Hesa 1, Vratza 35, AS, KK, Hesa 2, Merefa 2, Vesletz 2, Gergana 2, Hebar 1/18, Hebar2/1, Vratza 33, Vratza 50Ogosta 1, Merefa 1, Valve 222, Valve 111, Ukrainian 18, Ukrainian 19, Sh-4, Mziuri 1, Mir 4, E-4a, E-5a, E-4b, E-5b, E-1, E-6, E-7, E-8, E-10, E-13, E-15, E-16, E-31, PS-5Sh, E-23, P11/22.
6. The highest parthenogenetic eggs hatchability (more than 20%) was detected in the races ShV, TV, Hesa 2, Siria 2, Mysore 1, 70-42, MNB and China.
7. In average the Chinese type races manifest higher values of the traits number and weight of normal eggs in the laying than the Japanese type.
8. The highest amount of leaf was supplied to the race AS (5.227g dry matter per 1 larva during the 5th instar) and the lowest amount - in Bonde 517-only 3.170 g. In the Japanese type races the amount of food ingested was the highest in Vratza 51 (3.688g) and the lowest in Hebar 1 (3.137g). In the Chinese type races the food ingested was the highest in Vratza 54 (3.784 g) and the lowest in Hebar 2 (2.858g).
The food ingested and digested was the lowest in polivoltine race-0.943 and 0.347g respectively. The leaf ingestibility was the highest in the Chinese type race Vratza 54 (73.53%) and the lowest in the polivoltine race Bonde 517 - only 29.75%. The highest food digestibility was detected in the tropical race (36.80%), Super 1 (31.74%), and Vratza 53 (30.04%). The food digestibility was the lowest in Hebar 2 (25.72%), KS (26.07%) and Merefa 2 (26.65%). The utilization of the food supplied for silk shell was the highest in Super 1 (7.65%), Vratza 54 (8.18%), Vratza 35(7.87%) and the lowest in Hebar 2 (6.54%) and Bonde 517 (2.08%). The races having comparatively high leaf ingestion and digestion showed lower food utilization and vice versa.
9. In average the races, having plain larvae and oval-shaped cocoons (Chinese type) displayed a higher haemolymph lysozime activity than that observed in the races with larvae having markings and elongated cocoons (Japanese type).
10. The following silkworm races can be used for direct commercial hybridization: Super 1, Hesa 1, Vratza 35, AS, KK, Hesa 2, Merefa 2, Vesletz 2, Gergana 2, Hebar 1/18, Hebar2/1, Kom 1, TV, KS, ShV, Vratza 51, Vratza 53, Vratza 52, Vratza 54, E-5a, E6, B2/6, TBV-2/24, TV-3/2, HT-215/38, T/15/4.
As promising silkworm races, which can be used in the future breeding programs appear to be: Vratza 33, Vratza 40, 157K, BC-1, BC-2, Super 4, Kom 2, Belopol 2, BV1, BV-3B, Iskar 2, Valve 222, Valve 111, Ukrainian 14, Ukrainian 15, Ukrainian 18, Ukrainian 19, Ukrainian 20, Mziuri 1, A-14, E-4b, E-5b, E-1, E-8, E-9, E-10, E-13, E-15, E-16, Iva 1, SN1/H1, SN1/AS, Magi 2, Nova 2.
11. The best hybrids for the field level in Bulgaria are KKxHesa 1 × Vesletz2xGergana 2 and Super 1 × Hesa 2.
12. There are no significant differences between the double, three and four-way hybrids, made by one the same races, except for the fecundity characters which are higher in three and four-way crosses, compared with the double hybrids.
For the countries having comparatively high level of silkworm breeding work the sharing of germplasm information is very important.
The main problems in the sharing this information are:
* language-some countries, having comparatively rich silkworm genetic resources like those from the former Soviet Union (Ukraine, Uzbekistan, Azarbaidjan, Georgia) still have problems with communication in English.
* lack of enough techniques for fast and easy communication - some countries still do not have internet, fax communication is difficult.
* in some countries the silkworm genetic resources are not very well studied
* lack of internationally accepted methodology for silkworm qualitative and quantitative characters characterization and evaluation.
* in some countries this information is considered as "secret".
* in some countries the genetic resources belong to private companies and the government organizations don't have enough information about them.
* some countries are in competition between each other in selling silkworm eggs to third markets, hence they are not interested to share this information.
* there are political problems between some countries and they do not want to share the information.
Some of these problems can be solved but unfortunately others is impossible to solve.
In nowadays and in the near future the easiest way to share information about the silkworm germplasm for each country is by internet. For establishment of a successful system for internet uploading and sharing the silkworm germplasm information we suggest the following measures:
1. To establish under FAO and ISC a working group(committee), consisted of leading silkworm breeders, fluent in English from the sericulture science and technology advanced countries.
2. The working group to develop a standard methodology for characterization and evaluation of silkworm germplasm.
3. Each country/organization/company which wants to share information for their silkworm germplasm and to receive regular information for the germplasm in other participating countries to be invited by FAO/ISC to prepare, following the standard methodology in electronic files in English a complete information about their silkworm genetic resources.
4. FAO to provide funding for creation of internet site with information about silkworm genetic resources of each participating country/organization/company and a common site for global silkworm germplasm.
The sharing of silkworm races between different countries is much bigger problem than the sharing only information about them. Usually the sericulturally developed countries are not interested to share their genetic resources with other countries due to competition at the international silk market, competition at silkworm egg market or some restrictions in their legislation.
From the other hand most of the developing countries do not have experience and skillful staff to maintain the silkworm races at the level of their passport characteristics. Sooner or later they deteriorate the races and even loose them.
In our opinion sharing silkworm genetic resources need only countries having breeding work above a certain level. Now the African and some Latin American countries who newly introduced sericulture still do not need to obtain silkworm races, because they don't have enough trained and experienced staff to maintain and use them for breeding pure races and hybrids.
However the developing sericulture countries should get gradually experience in silkworm breeding and egg production in order finally to create their own silkworm germplasm and more important - to produce their own silkworm eggs.
Therefore in our opinion the exchange of silkworm races between different countries is possible based on a common interest. Usually the exchange of some races is made by equivalent (certain number of races from one country is exchanged with certain number of races from other country). By this way SES-Vratza has received many silkworm races from Japan, Ukraine, North Korea, Egypt, Uzbekistan etc.
In other cases some sericulturally developed countries who have rich genetic resources, but due to economical reasons the cocoon production went down are ready to give pure races to other countries in order to support their sericulture as a source of cocoons/raw silk for the silk industry in the developed country.
In our opinion one successful way to support the developing countries is to sell them P1 eggs. This trade is beneficial for both the sides because the price of P1 eggs is double than the F1, but the costs are not so much higher, by one box of P1 can be produced at least 80 boxes F1 eggs, the costs of the recipient for production of F1 eggs are much lower than those of the hybrid eggs at the international market, the developing country avoids the expensive and highly sophisticated work for maintenance of the parent pure lines. By this method the developing country can gradually get good experience in egg production, the parents of the hybrid are reared under indigenous conditions of the country, no any problems with egg transportation etc.
Following this way in 2001/2002 SES-Vratza gave to 3 African countries (Uganda, Ivory Coast and Ghana) P1 eggs of the Bulgarian four-way hybrid KKxHesa 1 × Vesletz 2xGergana 2. In Uganda they have already produced their own F1 eggs for rearing in the country.
Such trade with P1 eggs, based on a mutual interest is possible even between sericulturally comparatively developed countries. For example in June/July 2002 in Thailand, under the DOAE system were tested P1 Bulgarian eggs and the results obtained were promising.
We do not believe that most of the developing countries will succeed to obtain, preserve and maintain good pure races from sericulturally developed countries, therefore one way for them is to buy regularly P1 eggs and to produce the F1 eggs by themselves.
Chart 16. Geographical origin of the silkworm germplasm
|
Country |
No. strains |
Country |
No. strains |
Country |
No. strains |
|
Bulgaria |
81 |
Uzbekistan |
7 |
Vietnam |
2 |
|
Ukraine |
20 |
Georgia |
2 |
Madagaskar |
1 |
|
China |
5 |
Rumania |
2 |
Austria |
1 |
|
Japan |
12 |
Syria |
2 |
Italy |
1 |
|
Egypt |
37 |
North Korea |
8 |
Unknown |
1 |
