Combined selection for tree height improvement in Prosopis cineraria - S.K. Jindal

Central Arid Zone Research Institute, Jodhpur, India

Introduction

Since selection has affected evolution, and can be used to direct the future evolution of populations, the study of selection and its effects has absorbed more interest and effort than any other genetic force, mutation or migration (Namkoong, 1979). To understand the effectiveness of different selection methods, a knowledge of heritability, genetic gain, and genetic gain as a percentage of the mean is prerequisite. With this view, a trial including eleven one-parent progenies of Prosopis cineraria was established at the Central Research Farm of CAZRI, Jodhpur, to ascertain the characteristics of this species in terms of the heritability of traits and genetic gain possible, in order to improve tree breeding programmes.

Materials and methods

Germplasm of P. cineraria was collected in Rajasthan in May 1984, with pods from individual trees being collected and threshed. Seeds were treated with concentrated sulphuric acid to break dormancy and sown immediately into polythene bags containing a mixture of sand, clay and manure in ratio of 2:1:1. Eleven progenies were transplanted to the field in July 1984 in a randomised complete block design with three replications. Each plot consisted of two rows of 7 plants each, at a 3 x 3 m spacing. Tree height was recorded every December from 1986 to 1990. Data was analysed tree wise, and variance components were calculated as per expected mean squares given in Table 1. Since the number of living trees per plot was not uniform, C values were calculated. Family heritability, single tree heritability with and without blocks, and genetic gain from the half-sib progeny trial, were calculated from the formulae below. Genetic gain for family selection was calculated by using the mean height of single family with highest mean, and for the mass selection 37 best trees were used.

% Family heritability

= (s²_p)/(s²_e/CW + s²_p/B + s²_p) x 100

% Single tree heritability (without blocks)

= (4 s²_p)/(s²_e + s²_bp + s²_p) x 100

% Single tree heritability (with blocks)

= (4 s²_p)/(s²_e + s²_bp + s²_p + a²_b) x 100

Genetic gain from family selection

= (selected families - average families) x family heritability

Expected mass selection gain

= (selected trees - average trees) x single tree heritability

Results and discussion

Mean family height growth varied significantly (p<0.01) from 0.77 to 1.55 m in 1986, 0.81 to 1.91 m in 1987, 0.76 to 2.29 m in 1988, 0.81 to 2.62 m in 1989 and 1.01 to 3.37 m in 1990 (Tables 2 and 3). Population mean height was 1.08, 1.21, 1.40, 1.62 and 2.07 m consecutively for five years. Most of the families retained their performance from first year onwards, for example Acc. no. 249, a collection from Jaisalmer was the poorest performer throughout, whereas Acc. No. 264 from Jodhpur district maintained its superiority. In the fifth year, only four families, those from Jodhpur, Nagore, Barmer and Tonk had high mean values above that of the mean height growth recorded for the experiment for the year, with a mean height superiority of 30.9-62.8%. Thus the presence of significant genetic variation among families in all the five years of study is encouraging. This could be profitably exploited for the selection of the best families. Estimates of heritability and percentage of variation due to blocks, progenies, their interaction and error are presented in Table 4. Values of single tree heritability with and without blocks were the same because variance due to blocks was negative (not normally possible, but it comes about because it is an estimate) and its value was taken as zero. Single tree heritability estimates were more than those for family heritability except in the first year, where family heritability was greater. These findings are contrary to those of Jindal et al. (1992), where family heritability values were higher than single tree heritability in Tecomella undulata. Heritability estimates were very high, ranging from 73.1-84.6% for single tree heritability and 72.3-83.0% for family heritability. Low values of single tree heritability for tree height are reported in Tecomella undulata (Jindal et al., 1992), Eucalyptus tereticornis (Kedharnath and Vakshasya, 1977) and Eucalyptus regnans (Eldridge, 1972). These high heritability values show that phenotypic selection of parents (mass selection) can be beneficial and the need for progeny testing may not arise.

Table 1. Expectations of mean squares

C = 12.18 as the number of plants per plot was not constant.

Table 2. Analysis of variance for tree height in P. cineraria (* p<0.05; **p<0.01)

Table 3. Mean tree height of 11 half-sib progenies of P. cineraria transplanted in 1984

Table 4. Family and single tree heritability and percentage variation due to components, for tree height in P. cineraria

* single tree heritability (without blocks) was same for all years, to single tree heritability (with blocks), because s²_b was negative and its value was taken as zero for computation of other values.

** since the value of s²_b was negative, therefore it was taken as zero.

The percentage of variation due to different sources showed that error contributed more than progenies and block-progeny interaction. The contribution of error variation was at a maximum in the first year and a minimum in fifth year, at the cost of block-progeny interaction, where the trend was the reverse over time. Variation due to progenies was almost stable, ranging from 18.3% in year 4 to 21.1% in year 2.

Mean height, genetic gain, genetic gain as a percentage of the mean due to family selection and mass selection showed that the mean height of the 37 best trees from the population was 36.1, 42.9, 47.2, 51.1 and 41.5% better than the best family from years 1 to 5 (Table 5). The mean height of the 37 best trees from the experiment was 2.11, 2.73, 3.37, 3.96 and 4.77 m in the five consecutive years. Genetic gain as a percentage of the mean was more than 25 in the case of family selection and 37 in the case of mass selection.

High values of heritability due to families and single trees, coupled with high genetic gain as a percentage of the mean suggested that improvement in tree height can be achieved by simple mass selection, which is perhaps the easiest breeding method in forestry. Phenotypes are selected according to their individual performance and open pollinated with unselected pollen. With this method the effective gene frequency of the favourable allele is halved and the gain is half of that usually computed in mass selection. Thus for the breeder, for maximum gains there should be control of pollen parents, which can be achieved by establishing progeny trials and selecting the best families and best plants within the families, i.e. combined selection. This selection is also based on the individuals’ own performance. There should be large numbers of families for combined selection for the effective improvement of tree height in P. cineraria.

Table 5. Genetic gain for tree height by selecting the best performing family (comprising 37 trees) and by selecting an equivalent number of best trees in the progeny trial of P. cineraria (mass selection)

References

Eldridge, K.G., 1972. Genetic variation in growth of Eucalyptus regnans. Bulletin No. 46. Dept. of Primary Industry, Forestry and Timber Bureau. Canberra, Australia. 72p.

Jindal, S.K., Manjit Singh, K.R. Solanki and N.L. Kackar, 1992. Changes in genetic parameters and ranks of tree height over six growth years in Tecomella undulata. Silvae Genetica 41: 213-216.

Kedharnath S. and Vakshasya, 1977. Estimates of components of variance, heritability and correlations among some growth parameters in Eucalyptus tereticornis. Proceedings of Third World Consultation on Forest Tree Breeding. Canberra, Australia.

Namkoong, G., 1979. Introduction to quantitative genetics in forestry. Technical Bulletin No. 1588, USDA. 17p.