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.