THE BUTTERFLY DANAUS-CHRYSIPPUS (L) IN EAST-AFRICA - POLYMORPHISM ANDMORPH-RATIO CLINES WITHIN A COMPLEX, EXTENSIVE AND DYNAMIC HYBRID ZONE

Samples of the polymorphic butterfly Danaus chrysippus are analysed fr om six well separated sites in East Africa. Morph-ratio dines are desc ribed for four diallelic genes A, B, C and L, each of which influences the visual phenotype. Each of the four dines has a different orientat ion, consistent with an hypothesis thai the polymorphism originated fr om hybridization between a number of polytypic demes which have at Var ious times undergone range expansion. Allopatric subspeciation in isol ated Pleistocene refugia is postulated. The phenotype of each geograph ical race is shared with one of the morphs within the hybrid zone; oth er sympatrically maintained polymorphic forms are normally confined to the hybrid zone. Wright's isolation-by-distance model best explains t he present distribution of gene frequencies. Morph-ratios differ signi ficantly between the sexes and are sometimes associated with heterozyg ote excess; gametic and genotypic disequilibria are general throughout the region and suggest the dines are maintained by strong natural sel ection. Seasonal cycling of phenotype frequency is believed to result from extensive migratory movements rather than natural selection. Fema le-biased sex-ratio, which is also seasonal, and Haldane rule effects, result from hybrid breakdown when genetically distinct demes meet and interbreed. Oscillating sex-ratios and frequency of colour genes are functionally linked by negative feedback. The polymorphism owes its or igin to allopatric evolution but is now maintained sympatrically. (C) 1997 The Linnean Society of London.