A. E. Bell

Simultaneous selection for two correlated traits in Tribolium

Simultaneous selection by independent culling levels of two correlated traits in all four combinations of directions was investigated with Tribolium in a replicated experiment extending over nine generations. In addition to the two primary traits, 13-day larval weight and pupal weight, four secondary traits (pupation time, adult emergence time, adult weight and larval number) were observed. The observed responses for both selected and unselected traits agreed with theoretical expectations after the latter were adjusted for changes which occurred in genetic and phenotypic parameters. Phenotypio variances for the selected traits were correlated positively with population means, yet genetic variances and heritabilities declined in all selected populations. No change was detected in the genetic correlation between selected traits even though the divergent two-trait selection was designed especially to ‘break’ the positive correlation of + 0·55 ± 0·12 present in the base population. Striking changes in growth and developmental patterns resulting from the divergent selection were discussed in terms of metamorphic limits and ‘stabilizing’ genetic correlations.

Two-way selection for body weight in Tribolium on two levels of nutrition.

Parameters necessary for predicting direct and correlated responses for large and small 13-day larval weight in T. castaneum on two levels of nutrition were estimated in the base population. Larval weight in the GOOD environment was approximately twice that observed in POOR. Heritabilities (estimated from the ratio of sire component to total phenotype variance) of larval weight on the two rations were similar, 0·21 ± 0·06 and 0·19 ± 0·05 for GOOD and POOR, respectively. Heritabilities based on dam-offspring covariances were similar to these, but those obtained from full-sib covariances were much larger (0·97 ± 0·07 for GOOD and 0·69 ± 0·07 for POOR). This suggested that considerable dominance rather than maternal effects were present. The genetic correlation between growth on GOOD and growth on POOR was estimated as + 0·60 ± 0·21. The selection experiment was replicated four times with each replication extending over eight generations. Good agreement between predicted and observed values for direct selection was observed for large selection in both environments and small selection in POOR. However, response to small selection in GOOD was significantly greater than predicted in all four replications and was associated with increased selection differentials. Realized heritabilities were approximately the same for both directions in GOOD yet asymmetrical responses occurred for all replications due to unequal selection differentials. On the other hand, realized heritabilities were asymmetrical in POOR. Those observed for small selection were more than twice the size of those calculated for large lines. However, the responses in POOR were symmetrical since the selection differentials varied inversely with the realized heritabilities. Because of the asymmetry observed for heritabilities and selection differentials, correlated responses were poorly predicted. The average effective genetic correlation between growth in GOOD and growth in the POOR environment agreed remarkably well with the base estimate, yet asymmetry of the genetic correlation was a consistent phenomenon with values for the large lines being less than the base parameter while small lines were uniformly larger. Asymmetries of the various genetic parameters were not anticipated from base estimates. They were not caused by sampling or chance fluctuations since all four replications were remarkably consistent. Asymmetry for any one genetic parameter (e.g. heritability) was associated with a particular environment or direction of selection while other genetic parameters reacted asymmetrically in populations exposed to a different set of environmental treatments. For maximum performance in a single environment, these results show that selection should be practiced in that environment. With regard to mean performance in GOOD and POOR environments, selection for large size in POOR gave some 25% more gain than selection in GOOD. Selection for small size in either environment was equally effective in obtaining minimum size in both environments.

Genetic analysis of a population of Tribolium. I. Corn oil sensitivity and selection response.

IN most quantitative genetic studies the hypothesis of polygenic inheritance and random environmental effects is plausible. However, in one of several populations of Tribolium castaneum developed by Yamada and Bell (1963) to study the effectiveness of selection for growth in different environments this hypothesis was not satisfactory. The selection response in this particular population suggested a peculiar genotype by environment interaction. Although the situation was confirmed by biometrical analyses, these techniques did not suffice to answer questions concerning the underlying genetic mechanism on which selection had acted. It was our objective to identify the genetic basis of this peculiar response to selection and consequently more precisely define genotypes and genotypic differences. A preliminary report of this work was given by Costantino et al. (1966).

Genetic analysis of a population of Tribolium. II. Metabolic pattern of corn oil sensitive anomaly.

IN earlier papers (Costantino et al., 1966, 1967), two properties of a unique population of Tribolium castaneum were established. Firstly, corn (maize) oil inhibits larval growth and the degree of inhibition is a function of the concentration of corn oil in the diet; and secondly, the genetic basis of this inhibition is an autosomal recessive gene. Furthermore, these two properties accounted for certain unexpected phenotypic changes observed in the growth characteristics of this population during a long-term selection study. Our attention is now focused on (1) describing the mutant phenotype, and (2) elucidating the biological mechanism(s) responsible for the populations response to corn oil.

Genetic parameters in two populations of chickens under reciprocal recurrent selection1

Synopsis Genetic information on two traits, age at sexual maturity and survivors’ per cent production, was obtained from two strains of the White Leghorn breed under a reciprocal recurrent selection scheme where the reciprocal crosses were raised at three locations representing diversified environments. Genotype by environment interactions were detected by a number of methods; (1) from analyses of variance of sire means within years and over locations where sires, locations, and sire by location interactions were important sources of variation; (2) from genetic correlations of the same trait at different locations; (3) from comparisons of the components that constitute the genetic correlations between the two traits at the same versus different locations. The sire by location interaction variance components when taken as a percentage of the total variation, although not large, 10 per cent for sexual maturity and 8 per cent for egg production, were found to be significant sources of variation. The genetic ...

The nature of the purebred-crossbred genetic covariance

Epistatic models are examined for conditions which result in a negative covariance between a sires purebred and test-cross progenies. It is found that heterozygote superiority is not a necessary condition for its occurrence. The frequencies of the same allele in the selected population and in the tester should be divergent. The implications of using the covariance in prediction are discussed. The important result to note is that selection in the crossbred may not be optimum even with a negative covariance.

Estimation of maternal, sex-linked and additive x additive epistatic gene effects for body size of Tribolium

SummaryThe genetic structure of two quantitative traits, 13-day larval weight and pupal weight, in two unselected populations of Tribolium castaneum was investigated by the genetic model of Carbonell et al. (1983). The variability among two and three-way crosses was analyzed into components due to: general and specific combining abilities, maternal, sex-linkage, specific reciprocal and additive-by-additive epistasis. Also, indirect evidence of higher order epistasis was studied. It is concluded that the heterotic trait larval weight is highly affected by sex-linked genes and by non-additive gene action with additive-by-additive as well as higher order epistasis playing major roles. Pupal weight, on the other hand, is determined mostly by additive gene action although epistasis is also a significant source for genetic variability. Both traits are significantly influenced by maternal effects.

Estimation of genetic gains in populations with overlapping generations.

Synopsis A technique for estimating genetic improvement from an “internal genetic control” was developed for populations which fulfil the following requirements: (1) The population can be divided into distinct subpopulations (e.g. sire families or inbred lines) with several sets of offspring such as hatches or litters produced during the life‐span of each subpopulation. (2) The replacement of subpopulations or breeders in the selection programme takes place more or less continuously to provide overlapping generations. (3) The effect of ageing of the parents on progeny performance can be measured. The genetic change in such populations independent of environmental time trends can be estimated directly from the breeding data without any need for a separate control population. The technique was applied to a practical meat‐type poultry breeding operation to yield an estimated annual genetic improvement of 100 g body weight for each bird at 9 weeks of age.

Crossbred response from purebred selection, an experimental check on selection theory withTribolium

SummaryTwo unrelated populations ofTribolium castaneum were subjected to full-sib family selection for purebred 13-day larval weight. Over 30 selection generations, replicate lines became differentiated with respect to the trend of phenotypic variance and the path of response. All lines were responding in the latter generations indicating no tendency toward plateau. Crossbreds between the two populations responded over the total selection period. In the first ten generations, observed crossbred responses were much greater than those predicted by selection theory; it is postulated that additive maternal effects were responsible. In the remaining generations, the agreement between observed and expected response was reasonably good.

Non-additive gene effects in populations under different methods of selection.

SummaryThe genetic parameters of two quantitative traits, 13-day larval weight and pupal weight, in Tribolium populations developed by reciprocal recurrent selection (RRS) and by within-line purebred selection (WLS) were compared each with the other and also with the parameters of the unselected base populations using the genetic model of Carbonell, Nyquist and Bell. The variability for two and three-way crosses of inbred lines derived from “companion” populations (two strains, breeds, or varieties used for a terminal cross or hybrid) was analyzed into genetic effects: autosomal additivity (*g), autosomal heterosis (*s), sex-linked additivity (L), sex-linked heterosis (LL), general maternal (m), specific maternal or reciprocal (r), additive by additive epistasis (aa), and deviations from the model due, among other causes, to higher order epistasis (dev). One series of crosses involved companion populations with diverse origins. For contrast, a second series of crosses involved companion populations originating from a common heterogenous base population. For the heterotic trait larval weight, *g and *s effects were equally important and accounted for over 50% of the total variation. The aa epistasis contributed another 20% and was followed in importance by higher order epistasis and general maternal effects. For the more highly heritable trait, pupal weight, *g effects were most important with *s, aa, and m effects having smaller but significant influences. Sex-linked and reciprocal effects were statistically significant for many crosses, but they were relatively unimportant overall. In general, the unselected base populations showed higher *g variation than either RRS or WLS populations with the reverse true for *s effects. In agreement with theoretical expectations, RRS was more effective than WLS in exploiting *s effects. The aa epistatic effects for larval weight were of major importance in the unselected populations, but RRS and WLS did not differ significantly for exploiting superior aa gene combinations. Companion populations with diverse origins revealed significantly larger variation due to *g and *s effects in crosses than did populations initiated from a common heterogeneous base.