Genetics of the butterfly Danaus chrysippus (L.) in a broad hybrid zone, with special reference to sex ratio, polymorphism and intragenomic conflict

The aposematic savanna butterfly Danaus chrysippus seems to be an exception to the rule that Batesian models and Mullerian mimics are not polymorphic. Throughout Asia and in much of Africa the species is in fact monomorphic and polytypic, as expected. It is, however, polymorphic for 3–4 colour genes over a large area of central and eastern Africa, where deviant sex ratios and non-Mendelian segregations also occur. All-female broods are widespread and frequent, often outnumbering bisexual progenies and giving rise to heavily female-biased populations. Full and partial sex linkage, which is not controlled from the X or Y chromosomes, and male-biased broods also occur. Genetical analysis for the BC-autosome carrying colour genes suggests there are two, probably mitochondrial, cytotypes (microbe-induced early male death syndrome is considered unlikely) and an autosomal, incompatibility (I) gene, two alleles of which are male-specific killers. F2 and backcross matings by females heterozygous at the I-locus give progenies which are either thelygenic, all males dying at or soon after hatching, or bisexual but showing full or partial sex linkage. Male death is attributed to nuclear-cytoplasmic incompatibility (NCI). Females achieve reversion from a thelygenic to a bisexual line by mating with males of compatible (maternal) cytotype. A second NCI system causes meiotic drive in female oogenesis for a cytoplasmically compatible A-autosome, with no effect on SR. The array of non-Mendelian segregations for sex and two autosomes, with switching between bisexual and unisexual lineages, indicates autosomal intragenomic conflict with the outcome that one autosome is preserved from elimination in females and the other condemned in dead males. The geographical distribution of the colour marker genes suggests that the polymorphisms have a hybrid origin involving several once allopatric subspecies, some of which have divergent cytotypes. Secondary contact probably followed range expansion induced by the periodic spread of savanna during the Pleistocene and massive deforestation by man in recent times. The unusual extent of the hybrid zone is attributed to the large number of subspecies involved in its origin, and to introgression promoted by the butterfly's extensive migratory activity.