The aposematic savanna butterfly Danaus chtysippw seems to be an exception to the rule that Batesian models and Miillerian 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. AU-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 nuclearcytoplasmic 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
The colour polymorphism of the Danaus rhyippus population at Dar es Salaam, East Africa, is controlled at three major loci, each with two alleles. Two of the loci, one governing ground colour and the other forewing pattern, are closely linked. The third locus, determining hindwing pattern, assorts independently.Thirty-eight broods raised from wild mated pairs, F1 and F2 generations gave 857 offspring of 23 genotypes (out of 27 possible). The forewing length, taken as an index of size, was investigated in relation to the genotype. Heterosis is evident at all three loci. The two linked lori show epistatir interaction of an unexperted kind : double heterozygotes are smaller than heterozygotes at only one locus but larger than double homozygotes. The hrterotic effert at the third, unlinked locus is the most pronounced and is additive to that at the other two. Heterosis is more marked in males than females.The possibility that body size has importance in connexion with sexual selection, food recources and mimetic relationships is discussed.Analysis of gene and chromosome frequenries in the wild parents of 61 broods suggests that double heterozygotes for the two iinked loci may have heterozygous advantage. Seventy-eight per c-ent of chromosomes are repulsion phase: thus, there is pronounced linkage disequilibrium which must be maintained by selection as crossing over is almost 2%. In particular, the chromosome carrying both dominant alleles in coupling is rare.Consideration of the centres of distribution and present ranges of the alleles at all three loci suggests that three geographiral races, aegyptzus, dorippus and alczppus, were isolated by forest barriers, during wet periods in the Pleistocene, in south-west, north-east and north-west Africa respectively. They have probably expanded their ranges in the post-glacial period to overlap and interbreed in central and east Africa. Either heterozygous advantage o r seasonal (directional) selection or a combination of both is responsible for the persistence of the polymorphism.
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