4. Genetics and selection
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Genetics of rabbit breeds and populations
Coat colour and hair structure
Groups of breeds by adult size and origin
Local populations and strains
Genes and the environment
Genetic improvement: selection and crossing
Domestication of the rabbit is relatively recent and most of today's breeds and populations have been bred by man in the last 200 to 300 years.
The rabbit has been used as an experimental animal in genetics and reproduction physiology since the beginning of the century, but it was not until 1950 that the first findings on quantitative genetics were published, in Venge's study of maternal influence on rabbit birth weight (Verge, 1950). This work opened the way for research on the genetic improvement of the rabbit for meat production. INRA scientists in France began research and development work in their area in 1961, followed by work in other research laboratories in many countries. The excellent bibliography by Robinson (1958), Genetic studies of the rabbit, giving reliable genetic and physiological bases, has already been dated by this new work.
A report on French research on rabbit selection in the 10 years from 1970 to 1980 was recently drawn up by Rouvier ( 1981). However, experience gained in European conditions cannot be transferred directly to developing countries. To upgrade their rabbits breeders should use the local animals, either native or from imported populations that have been acclimatized, and make use of the genetic variability that is available.
It does seem that priority should be given to research on rural and backyard rabbit production. These would be small, thrifty, autonomous units requiring little investment and using local resources. They would be reasonably productive.
Table 26 gives the taxonomic classification of the rabbit.
Genetics of rabbit breeds and populations
Perhaps the best of the various definitions of breed is Quittet's: "A breed is a collection of individuals within a species which share a certain number of morphological and physiological characters which are passed on to their progeny as long as they breed among themselves."
One way of assessing the genetic uniqueness of different breeds is to study their origins. A breed is the outcome of the combined impact of artificial and natural selection (environmental adaptation). It is difficult to define, and its background can be difficult to trace. Artificial selection may be based on a number of different criteria, not necessarily all to do with productivity. Animals may be selected in artificial or natural conditions, the environment may gradually change, and so on.
TABLE 26.-TAXONOMIC CLASSIFICATION OF THE RABBIT (Oryctolagus cuniculus)
CLASS: MAMMALIA: - Superorder: Glires
- Order: Lagomorpha
- Order: Rodentia
Family: Leporidae (hares, rabbits)
+ Sub-family: Palaeolaginae
- Genus: Pentalagus (East Asia)
- Genus: Pronolagus (southeast Africa)
- Genus: Romerolagus (Mexico, one sole species, R. nelsoni)
+ Sub-family: Leporinae
- Genus: Lepus (hares: numerous species distributed throughout the Old and the New World)
- Genus: Macrotolagus (sometimes considered a sub-genus of Lepus, living in North and Central America)
- Genus: Oryctolagus (true rabbit, living in Europe and North Africa. One species, Oryctolagus cuniculus, with a few sub-species)
- Genus: Sylvilagus (Americad rabbits; numerous species)
- Genus: Coprolagus (Asiatic rabbits)
- Genus: Nesolagus (Sumatra, one sole species, N. netscheri)
- Genus: Brachylagus (pygmy rabbit, living in North America)
- Genus: Ochotona (one sole genus for the different Ochotonidae; living in the northern regions of Europe, Asia and America)
Source: Grassé and Dekeyser, 1995.
Rabbit breeds or populations can also be defined in terms of gene frequencies. This is possible with genes identifiable through their visible or major effects on progeny. Coloration and hair structure are classified as visible effects. Thanks to advanced observation techniques the genes governing blood groups, biochemical and protein polymorphism and hereditary anomalies are now also known.
Rabbit populations can also be defined in terms of the strength of breeding stock, natural selection trends, origin and geographical area, and characteristics of breeds, strains, inbred lines and local populations -quantitative characters affected by large numbers of genes that cannot be pinpointed individually.
Rabbit chromosomes carry 2n = 44 genes. Genes situated at 70 loci are known and 6 linkage groups have been established. Among the known loci roughly one-third concern the coat (colour, hair structure), one-third govern blood groups and the production of antibodies, and most of the rest govern hereditary diseases.
The rabbit can support a slow and gradual increase in inbreeding, hut research suggests that mating programmes for small populations should minimize its extent and rate of increase among the stock.
Breeds created by selectors, amateurs and fanciers in the United States and, especially, western Europe now conform to official standards. The book of the Société centrale d'aviculture de France (SCAF) on standards for rabbits describes 44 breeds. They have been bred from animals of local and regional populations, or by crossing breeds, or using mutants for changes in coat colour or structure. Mass selection for size and body morphology has separated these breeds into giant, medium, small and very small.
The characters by which an animal conforms to a breeding standard, such as body size, whether or not it is compact, coat colour and density, and ear size, may be related to its resistance to variations in climate. In fact, such factors as coat, skin, body area and weight affect the animals' body temperature.
The currently known genetic determinants of variations in colour and structure are listed below. Coat colour has always been of great interest to breeders.
Coat colour and hair structure
In The genetics of domestic rabbits, published in 1930, Castle described 6 mutations in coat colour and 2 mutations in pattern; 3 mutations in hair structure; 1 mutation in the yellow colouring of the abdominal fat, and 2 linkage groups. A convenient way to detect the effects of various mutations is to describe the rabbit's "wild" colouring. The coat consists of 3 types of hair: the longer, rectrix guard hairs, stiff at the base; the more numerous tectrix barbed hairs forming the major part of the coat, which share a hair follicle with the third type-the shorter hairs making up the undercoat.
The wild coat colour, the so-called agouti, consists of grey dorsal fur with a much lighter or white ventral area. The long guard hairs are black but appear deeper black at the tips and bluer at the base. The barbed hairs have zones of colour: black at the tips, with a yellow band in the middle and bluish at the base. The fibres of the underfur are bluish at the base and fringed with yellow at the tips. Colouring is thus basically due to the distribution of black and yellow pigments (eumelanin and phaeomelanin) in the hair, especially in the barbed hairs, and over the whole coat (sides and back in relation to belly fur). Mutations in different loci modify this colouring.
Locus A, agouti: the nonagouti mutation a produces animals without a yellow band in the hair and a lighter belly. Their colouring is uniform. A is dominant over a. A third allele has been described at this locus, a' (tan pattern), which is recessive to A and dominant over a.
Locus B, black pigment: a recessive b allele produces a chocolate brown pigment instead of black in agouti hair. This mutation is one of the latest to be discovered (1900).
Locus C: the C gene is required for the development of pigments in the fur, skin and eyes and hence for the expression of colouring. The recessive c gene inhibits the expression of colouring, causing albinism in recessive cc homozygotes. There are several alleles at this locus, quoted below in dominant-to-recessive order:
C full expression of colouring.
cch chinchilla, suppression of colour in the intermediate band of the coat.
ch Himalayan. Only the hairs at the body extremities are black. The expression of this gene depends on the ambient temperature.
The albinism locus is epistatic over the colour loci. The cc genotype covers the expression of colour genes situated at other loci.
Dilution, D, d: the recessive mutant d allele affects the intensity of the pigmentation, causing a dilution of the pigment granules. The dominant D allele produces normal pigmentation density. The recessive dd homozygote is found in the genotypes of blue (black diluted to blue) or beige (yellow diluted to beige) rabbits.
Normal extension of black (E) or yellow (e): the e gene mutation causes increased yellow pigment in the hair, tending to replace the black (or brown) pigment. Grey, black or brown breeds have the E gene. Yellow and red breeds are recessive ee homozygotes.
Vienna White locus: Vienna White rabbits have completely unpigmented fur but coloured eyes (blue). The original gene is called V and its mutated form v. Rabbits of the Vienna White breed are therefore recessive vv homozygotes. Crosses of this breed with albino rabbits produce coloured progeny.
Mutations producing a mottled coat: these mutations involve the loci for English (En, en) and Dutch (Du, au). The Papillon rabbit is of the En en heterozygous genotype. The En gene is incompletely dominant. The En En homozygotes are whiter than the heterozygotes, while recessive homozygotes are blacker. The colour genotype of the Papillon rabbit (Giant Checker in English, Mariposa in Spanish) cannot be pinpointed. At the other locus the du du genotype produces the white belt characteristic of the Dutch rabbit.
Angora: this recessive mutation produces unusually long hair. The normal length L dominant gene has mutated into a recessive l gene to produce the Angora. The mating of two Angora rabbits always produces Angora offspring. Two rabbits with normal hair can sometimes produce a fraction of Angora progeny if they are Ll heterozygotes.
Rex: this recessive mutation produces the reverse effect, unusually short hair. The symbol for the Rex gene is r and for the dominant allele R.
Hairlessness: this is caused by a recessive mutation and is usually lethal.
The genotype of the coat colour and structure in rabbit breeds can be predicted when these loci are known. So far not much gene interaction visibly affecting body colour and breeding characters has been found, but there has been very little research in this area. The function of the Angora and Rex genes, of course, is to produce Angora wool and Rex fur.
Groups of breeds by adult size and origin
There are different kinds of breeds:
Breeds are conveniently grouped by adult size, which is also related to production characters such as precocity, prolificacy, growth rate and age at maturity. A major determinant of adult size is the origin of the breed.
Adult weight exceeds 5 kg. The growth potential of the heavy breeds can be exploited, especially in crossbreeding. The Bouscat Giant White, French Lop, Flemish Giant and French Giant Papillon are examples. The fur of the Lop varies greatly in colour and can be white, agouti, iron grey, or black. Its body build would make it a good meat rabbit. However, it is bred for show and therefore found only in small units, at least in France. The breed is more important in other European countries such as the Federal Republic of Germany and Denmark.
The Bouscat Giant White is a synthetic albino breed. It is a large rabbit known for its prolificacy and fast growth rate in traditional French rabbitries. The Flemish Giant from Belgium comes in several colours. It is one of the largest rabbits (potential adult weight 7 kg) and is still farm-raised. This breed could furnish a gene pool for improving growth in other breeds; Flemish Giants could be purebred for this purpose.
Adult weight varies from 3.5 to 4.5 kg. These are the basic stock of breeds used for intensive rabbit production for meat in western Europe and are the most numerous. Only a few examples are described here.
Silver rabbits are found in several countries (English Silver, German Silver). These varieties differ from the Champagne d'Argent in adult size (English Silver is lighter) and colour. Like Burgundy Fawn, Champagne d'Argent is an example of a breed that has developed with selection over many years from a regional population (Champagne). The breed is known for both its fur, once much sought after? and its productivity: high fertility, quick growth, good muscle development and good meat quality. Its adult weight is 4-4.5 kg. It is farm-bred in France, usually on straw litter. Research has begun on intensive breeding of Champagne d'Argent.
The Burgundy Fawn is also of regional origin . It has spread throughout France and elsewhere in Europe (Italy, Belgium, Switzerland). The Burgundy Fawn Rabbit Breeders' Association has established a stud book for this breed, ensuring purebred selection.
The New Zealand Red was first developed in California, with a system of selection very similar to that used in France on the Burgundy Fawn, with the difference that the New Zealand breed was raised on wire-mesh floors which were introduced much earlier in the United States than they were in France.
The Californian is a synthetic American breed. It was presented for the first time in 1928 in California by its breeder, whose objective was a meat animal with very good fur. The adult weight of the Californian is 3.6-4 kg.
The New Zealand White originated as a breed in the United States. It is the albino offspring of coloured rabbits. From the outset it was bred selectively in large meat-production units, especially in southern California (San Diego area), for its breeding qualities: prolificacy, maternal performance, fast growth rate and precocious body development which makes it ready for slaughter at 56 days, the objective being a light carcass. The New Zealand White adult weight (4 kg) slightly exceeds that of the Californian. The New Zealand White was used in the the first studies on the rabbit at the Fontana Station in California (Rollins and Casady, 1967). Since 1960 this breed has spread through western Europe and other regions with the growing use of mesh floors for rabbit cages.
The Grand Chinchilla rabbit raised in Europe is of German origin. Its adult weight averages 4.5 kg. It can be bred for meat and fur.
These breeds have an adult weight of 2.5-3 kg. They include the Himalayan, the Small Chinchilla, the Dutch and the French Havana.
The Russian or Himalayan rabbit is white with black extremities. It is thought to have originated in China and spread from there to the USSR and Poland. It carries the Himalayan C" gene mutation. A strain of these rabbits bred in a French rabbitry has shown a strong rate of ovulation. This seems to be a biological characteristic of the strain, which is now being raised at the INRA centre at Toulouse.
The lightweight breeds usually develop very quickly and make excellent mothers. They eat less than the medium and large breeds and could be crossed or used pure in developing countries to produce a light, meaty carcass of 1-1 .2 kg.
These breeds weigh about 1 kg at maturity. They are represented chiefly by the Polish rabbit, with its many variations of coat colour. Selection for small size has led to very low fertility and a marked decrease in growth rate. These breeds cannot be used for meat production. They are bred for show, for the laboratory and as pets.
Local populations and strains
Purebred animals are usually raised in small groups, and their selection for breeding characters is in its infancy. These breeds could therefore constitute interesting potential gene pools for improving local populations.
Most rabbits raised for commercial meat production belong to populations which may resemble one breed or another (a question of appearance only, as they do not meet the criteria for that particular breed in terms of origin and standards) and sometimes resemble no breed at all. These are "Common" rabbits-grey, spotted or white-the outcome of various unplanned crosses. They may belong to local populations. Some examples of local populations in developing countries are the Baladi rabbit of Sudan (baladi means native or local in Arabic), the Maltese rabbit of Tunisia and the Creole rabbit of Guadeloupe. Countries planning to develop rabbit production should first identify existing local populations and research their biological and breeding traits and adaptability before designing selection programmes and suitable production systems.
Finally, there are rabbit strains. The strain is a genetically closed group, small in number, with no outbreeding for several generations. Characteristics of a strain are the number of breeding animals, the year and way the group was constituted, and possibly the mating programme (selection or no selection). These strains can be found in research laboratories which keep them to study their biological and breeding characteristics in order to make the best use of them in selection. The INRA centre in Toulouse conducts selection experiments on strains (Table 27).
Private breeders have fairly recently begun selecting rabbit strains, along the lines of the poultry selection that has been practiced since 1930. But some breeders or small groups of breeders, at village level for instance, may also have created strains without realizing it.
Some research laboratories, such as the Jackson Laboratory at Bar Harbor, Maine, keep inbred rabbit strains or lines for use solely as laboratory animals.
TABLE 27.-SELECTION EXPERIMENTS AT INRA, TOULOUSE
White A 1077,
|Litter size at weaning||Combined selection
index based on
performances of doe,
her sisters, her mother
Selection by truncation
on index of females,
White A 9077,
|No selection||-||22 males
A 1027, closed
growth rate, 28-77
|Individual selection in
both sexes. Percentage
qm = 10%;
qf = 40%;
generation interval = 9
|Control line of
Source: Rouvier, 1979.
The expression of breeding characters depends on environment and the breeder. A comparison of results from several different environments and geographical locations can reveal general characteristics of the breeds or species. Prolificacy, growth rate, and tissue development in young rabbits are three sets of basic breeding characters.
Prolificacy is measured by the number of live births or total births per litter. It varies significantly according to several factors which may be inherent in the animal. Litter size increases by 10-20 percent from the first to the second litter, increases again but less from the second to the third, with no change from the third to the fourth. After the fourth the size may decrease. Inbreeding may reduce prolificacy.
TABLE 28.-MEAN DOE PROLIFICACY BY ADULT SIZES OF BREEDS
|Litter size at birth|
|Very small breed|
|Polish (4 references)||4|
|Small Himalayan (4 references)||5.3|
|New Zealand White (10 references, 13 445 litters)||7.5|
|Californian (5 references)||7.6|
|Chinchilla (5 references)||6.8|
|Large Himalayan ( 1 reference)||3.7|
|Flemish Giant (3 references)||8.1|
Prolificacy also depends on the season and the reproductive rate imposed on the doe. In healthy does receiving normal feed and 12-14 hours light, prolificacy seems to be linked to adult size. Ovulation potential does increase, on the average, with size. The first factor affecting prolificacy is the ovulation rate (number of eggs) followed by the viability of blastocysts and embryos before birth.
Gregory (1932) said litter size depends on the number of eggs produced after mating and this number depends on the body size of the breed: 3.97 for Polish does and 12.88 for the Flemish Giant. The corresponding litter sizes at birth are 3.24 and 10.17. Small light breeds are generally less prolific than medium and large breeds. The prolificacy of the Small Himalayan breed raised by INRA at Toulouse is 6.7 total births and 6.3 live births per litter. Elamin (1978) gives these average figures from the Sudan for the Baladi, Californian and New Zealand White breeds:
|Live births per litter||3.5||6.67||6.94|
Table 28 shows variations in doe prolificacy among breeds of different adult sizes. This table is mainly based on old data from Pickard (1930). While only a small number of breeds are represented there does appear to be a positive correlation between adult size and prolificacy.
TABLE 29.-AVERAGE LITTER SIZES IN 7 FRENCH RABBITRIES
|Unit||Number of litters||Young per litter|
|Total young||Live births||At 21 strays||At 56 strays|
|1 to 7||5742||8.1||7.5||5.8||5.4|
A comparison of fertility among medium-size breeds reveals few differences. The following figures are in communications from the Genetics and Reproduction Section of the Second International Rabbit Breeding Congress, held at Barcelona in 1980, for New Zealand and Californian breeds researched in Spain by Garcia et al. (1980).
|New Zealand White||Californian|
|Live births per litter||7.04||7.23|
|Weaned per litter||5.46||5.72|
Roustan et al. (1980) give the following averages from 19 923 litters of medium-size does of various genotypes recorded in 8 rabbitries in the Toulouse region for litters unmodified by the fostering or removal of young rabbits.
Live births per litter 8.01 (range of 1 to 14)
Weaned per litter 6.41
Comparable values are given by Rouvier et al. ( 1973) from checks of on-farm performances for Burgundy Fawn (unit 1) and New Zealand White (units 2-7) in Table 29.
Paez Campos et al. ( 1980) give the breeding parameters of New Zealand White, Californian, Chinchilla and Rex breeds raised at the National Rabbit Breeding Centre at Irapuato in Mexico, a tropical zone tempered by an altitude of 1 800 metres (Table 30).
TABLE 30.-AVERAGE BREEDING PARAMETERS OF 4 BREEDS RAISED AT THE IRAPUATO NATIONAL RABBIT BREEDING CENTRE, MEXICO
|Strains||Litter size||Live births per litter||Rabbits weaned per litter||Age at first mating (days)||Weight at first mating (kg)||No. of litters examined||Number of does|
|New Zealand||8.5||8.0||6.5||144||3.49||3 723||600|
Ponce de León (1977) obtained the following results from 4 breeds researched in Cuba, in a tropical wet climate (see page 94 for characteristics of this institute and the breeds used).
|Total births||Live births|
|New Zealand White||7.0||6.2|
The high rate of still-births ( 11.6 percent) is explained by rearing conditions in the rabbitry.
Among the medium-sized breeds, doe prolificacy averages 7-8 young in widely varying countries and climates. This character is thus relatively independent of climate.
Prolificacy does seem to be a breed-linked character, basically independent of climatic conditions. However, this conclusion is based on only a few breeds. Many local breeds and populations are still not very well known. Once the ovulation potential and embryonic viability of these breeds are discovered the best strategy for upgrading them (pure or crossbreeding) under local conditions can be planned.
The study methodology has been established by Hulot and Matheron ( 1981) at the INRA Animal Genetic Improvement Station in Toulouse. The authors examined the numbers of corpus luteum, implantation sites and 16-day embryos of does ovulating after mating. These does came from two strains, one of New Zealand White and the other of Californian and Large Himalayan, established in 1966. The purpose of the experiment was to study nulliparous does (first litter), primiparous does (second litter) and multiparous does (third-fourth litters). Each group was fairly well represented at all seasons of the year. Table 31 shows that does of the Californian strain ovulated more often than the New Zealand White (difference of two eggs), but they later lost far more eggs, so the difference in the number of embryos counted 12 days after mating (genital tract removed by laparotomy) was not significant.
The does are reared in a rational French management system at semi intensive reproductive rates: the nulliparous does are serviced at 150 + 10 days. Those which kindle are mated again during lactation 10 or 11 days after kindling. Prolificacy rates observed for the two strains are not very different, with a slight superiority so far (but not in this study) for the New Zealand White. Prolificacy could be used as a universal criterion to describe breeds or strains if it were calculated from female acceptance of the buck rather than the number of kindlings per doe.
While prolificacy does seem to be a sure characteristic of breed or population, which offers the added advantage of being measurable in a large production unit, ovulation rate is much more characteristic of a breed. Ovulation rate also indicates the maximum biological potential for prolificacy. The potential biological equilibrium between ovulation rate and egg viability in a breed should therefore be examined.
The data in Table 31 refer only to the two strains studied and cannot be applied to other rabbit populations. They do suggest a possible study approach for developing countries into the prolificacy of local rabbits- the identification of populations with high ovulation potential and high embryo viability could suggest possible crosses to combine these two advantages for improved fertility.
TABLE 31.-AVERAGE FIGURES FOR CORPUS LUTEUM, IMPLEMENTATION SITES AND EMBRYOS OF RABBITS OF 2 GENOTYPES
|Corpus luteum||Sites||Embrios||Embryo losses|
|All does having ovulated||Californian||56||13.0||7.9||7.2||45%|
|New Zealand White||56||11.1||8.8||8.1||26%|
|Does having at least 1 implantation site||Californian||44||13.1||10.3||9.4||28%|
|New Zealand White||51||11.1||9.4||8.7||22%|
TABLE 32.-VARIABILITY IN WEIGHTS OF YOUNG RABBITS FROM 28 TO 78 DAYS, AND CARCASS WEIGHTS, FOR 2 BREEDS
|Age in days||Small Himalayan||New Zealand White|
|Liveweight in g|
|66||1 245||10||2 066||11|
|73||1 387||10||2 300||10|
|78||1 476||10||2 503||10|
|Carcass weight in g|
The growth rates of young rabbits are strongly correlated with adult size and weight where there has been no marked dietary deficiency. Table 32 gives average weights of young rabbits at successive ages, from 28 to 78 days, as well as carcass weights at 78 days, for the Small Himalayan and New Zealand White. The table clearly shows the growth rate of young Small Himalayan rabbits (adult weight of breed 2.5 kg) to be slower than that of the New Zealand White breed (adult weight 4 kg). Moreover, at 78 days the New Zealand White is more mature than the Small Himalayan, when its liveweight is 63 percent of adult weight against 59 percent for the Small Himalayan. The coefficients of variation (v%). the ratio of the standard phenotype deviation from the mean, are typical of the intrabreed variability of these characters for a given feeding system. Variability is greater in young New Zealand White rabbits than in Small Himalayan. Medium breeds slaughtered at the same age also vary in growth performance and carcass composition. Table 33 gives data for young Burgundy Fawn, Champagne d'Argent and Large Himalayan rabbits slaughtered at 84 days. Champagne d'Argent has excellent growth, muscle tissue and fat development for meat production. Burgundy Fawn is a close second.
TABLE 3.3.-AVERAGE LIVEWEIGHT AT DAYS, CARCASS WEIGHT MUSCLE WEIGHT/BONE WEIGHT RATIO, WEIGHT OF FATTY TISSUE IN CARCASS, FOR 3 BREEDS
|Burgundy Fawn||Champagne d'Argent||Large Himalayan|
|Liveweight at 84 days (g)||2143||2460||2055|
|Carcass weight (g)||1305||1588||1287|
|Muscle weigh/bone weight ratio (%)||4.3||4.5||4.0|
|Weight of fatty tissue in carcass (g)||86||107||73|
Weight gain and the growth rate of the main tissues depend on the breed's biological characteristics and on production factors such as feeding. So the criterion for describing a breed in a particular production environment should probably be maturity in terms of weight, defined as weight at a given age divided by adult weight. The most interesting breeds from the production point of view are those with the best ratio of weight gain to adult weight, and which arrive early at the proper liveweight for market. Lightweight breeds could be utilized as purebreds or, better, crossed with medium-weight breeds for a light carcass with good muscle development and quality meat (sufficient fat) where there is consumer demand.
Genetics of breeding characters
The genetic improvement of breeding characters relevant to the production environment depends on the specific genetic variability expressed in that environment. This variability is expressed in animals of the same breed or local population as well as in different breeds and populations and in interpopulation crosses. Variability is an expression of genetic differences which selection and crossing try to exploit.
The question here is how genetic variability can be exploited in small scale production, preferably using local resources. Upgrading the potential of a species depends on its biological characteristics, the mastery of its reproduction and calculating the genetic parameters for selection.
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