Phenotypic plasticity of abdominal pigmentation in Drosophila kikkawai: multiple interactions between a major gene, sex, abdomen segment and growth temperature

Drosophila kikkawai is known to be polymorphic for a single autosomal locus controlling abdomen pigmentation in females. Two strains homozygous at thi s locus (Abdomen pigmentation, Abp) were established from a polymorphic Ind ian population: one was homozygous (DD) for the dark allele, the other (LL) for the light allele. A Mendelian analysis of crosses at 25 degrees C conf irmed the occurrence of a major locus, with dominance of the D allele. Phen otypic variation of pigmentation according to growth temperature was then a nalyzed in DD and LL male and female flies, and in reciprocal F1. A slight difference was found between reciprocal F1 - females from a dark mother wer e darker but not at all temperatures. In females, the D allele exhibited an antero-posterior gradient of increasing expression from segment 2-7, with dominance over L and an increased expression at low temperatures. In males, abdomen pigmentation was uniformly light in segments 2-5, the D allele bei ng repressed by the sex genotype. In segment 6, the D allele was expressed but only at low temperatures, and was either recessive to L or codominant. Phenotypic plasticity that is, amount of change induced by different growth temperatures, was variable according to genotype and segment. It always co rresponded to a darkening of the fly at lower temperatures, but was general ly much less than in melanogaster. In D. kikkawai, climatic adaptation migh t occur more by changing the frequency of the D allele than by phenotypic p lasticity.