Polygenes and polymorphism

Estimating single gene effects on quantitative traits 1. A diallel method applied to Est 6 in D. melanogaster

A modified diallel cross is used to estimate effects of alleles at the esterase 6 locus, relative to strain and environmental variance, in Drosophila melanogaster. Three strains homozygous for Est 6 (s) and three homozygous for Est 6 (F) were crossed in all 36 combinations. Male progeny were scored for mating speed, copula duration and esterase 6 enzyme activity, and all progeny for developmental time. These alleles show a significant additive effect on mating speed, but not on the other traits. Copula duration, developmental time and enzyme activity show additive strain genetic variance. Enzyme activity and developmental time also have maternal or X-chromosome strain variance, and these two traits are significantly correlated. This modified diallel method is generally useful because it permits the partition of trait variance into additive and dominant locus, background genetic and environmental components.

Relative mating activity of the sexes in homokaryotypes of Drosophila persimilis from a Redwoods population

Previous tests for mating activity of Drosophila persimilis homokaryotype KL (Klamath) and MD (Mendocino) chromosomal arrangements (northern California population: Redwoods) had shown KL to mate faster on the average than MD in homogamic tests. A strain (double-cross hybrid of four KL lines from the same population) with reliable high mating activity was developed for testing the sexes separately. Five pairs of KL-MD homokaryotype strains were chosen to be tested by the criterion that each pair had been derived from a separate wild KL/MD progenitor. Strains were crossed within arrangements in a diallelic design (20 inter- and five intrastrain crosses tested in 16 replicates per cross) to provide mating activity indices of four sets: KL females, KL males, MD females, MD males. Mating tests employed ten virgin experimental flies with ten tester (double-cross hybrid) flies of the opposite sex in 30-min observation periods. All flies were matured for 5 days at 25 degrees C before testing. Among parental strains, females were consistently higher in mating activity than males for both KL and MD arrangements. Most interstrain hybrids were heterotic, with KL and MD females not significantly different. However, hybrid MD males displayed greatest variation and had lowest mating activity, while KL males were the least variable and highest in mating activity. With heterosis in the hybrids, there was no predictability (additivity) from performance of parental strains to hybrid offspring. Mating activities of the two sexes were uncorrelated, indicating either that the sexes have independent genetic systems controlling mating activity or that the expression of the same genetic system is influenced by sex. Since the hybrid males of the two karyotypes displayed different courtship activity while the females were at about an equal level of receptivity, intrasexual selection among males is likely to be important in nature.

The genetics of resistance to long-term exposure to CO2 in Drosophila melanogaster; an environmental stress leading to anoxia

Genetic heterogeneity in populations of D. melanogaster has been described for resistance to long-term exposure to CO2 (4 to 5 hours). Crosses between inbred strains, and between strains set up from single inseminated females collected in the wild show the importance of additive genes. Genetic activity for resistance and sensitivity was found on the X, 2 and 3 chromosomes.The mechanism of resistance was shown to be an anoxia effect since the effect of an N2 atmosphere was the same as that of CO2. A study of 18 strains collected in the wild revealed a positive correlation between metabolic rate as measured by O2 uptake and mortality under CO2, and negative correlations were found between body weight, and both mortality under CO2 and metabolic rate. These results are consistent with an anoxia effect. A further variable correlated with body weight is resistance to desiccation. Thus the anoxia effect is correlated with factors determining the distribution of the species in the wild.

Genetic heterogeneity in natural populations of Drosophila melanogaster for ability to withstand dessication

Strains set up from single inseminated females of D. melanogaster derived from two wild populations have been shown to differ in their ability to withstand dessication, as measured by mortalities after 16 hours in a dry environment, thus there are genes segregating in wild populations for ability to withstand dessication. A more detailed study on strains from one of the wild populations, showed that strains with high wet and dry weights lose water by dessication relatively less rapidly and have lower mortalities, than strains with lower wet and dry weights.Variability within and between five inbred strains was studied with results as above. Heritabilities for wet weight, dry weight, and mortality were 0.40, 0.41 and 0.60 respectively, showing the likelihood that the traits would be amenable to further genetic analysis.The relevance of the results are discussed in relation to stress to high temperatures, and the ecology of the species in general.