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

Abstract

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

Incomplete sexual isolation in sympatry between subspecies of the butterfly Danaus chrysippus (L.) and the creation of a hybrid zone

“…(3) Within the hybrid zone at Kampala, Nairobi, Mombasa and Dar es Salaam, but not reported elsewhere, populations are periodically highly female biased (Owen and Chanter, 1968;Smith, 1975c;Gordon, 1984;Smith et al, 1998;Jiggins et al, 2000;D. Schneider, 1997, in litt.…”

“…The formal colour genetics of D. chrysippus ( Figure 2) is fairly well known (Owen and Chanter, 1968;Clarke et al, 1973;Smith 1975aSmith , 1980Smith , 1998Gordon, 1984;Smith et al, 1998). Hindwing colour and pattern are controlled by the autosomal A locus: 2a and 2c have the genotype AA, whereas 2b and 2d are aa.…”

“…In Africa, the species is divided into three largely vicariant subspecies (Talbot, 1943) (Figure 1), alcippus in the western Sahel, south of the Sahara Desert, dorippus in the north-east (northern Kenya and Somalia), and chrysippus (formerly known as aegyptius) in the remaining areas (Owen, 1971;Pierre, 1973;Smith et al, , 1998. We have examined over 15 000 specimens of D. chrysippus in European museum collections and find that the geographical relation of alcippus and chrysippus (Figure 1) differs profoundly between the two sides of Africa.…”

“…In Uganda, 40% of females (n ¼ 90) were found to be infected, compared to only 4% at Watamu on the eastern coast of Kenya. Estimates of infection frequencies elsewhere in East Africa are obtained from the progenies of wild-caught females; Smith et al (1998) reported that 18% (n ¼ 77) of females at Dar es Salaam, Tanzania, and 69% (n ¼ 62) from Nairobi, Kenya, produced female-biased progenies, predominantly all female broods. On the other hand, small samples of female D. chrysippus from Ghana (n ¼ 6), South Oman (Salalah) (n ¼ 15), South Africa (n ¼ 4) and Zambia (n ¼ 3) were screened for the presence of Spiroplasma; all tested negative.…”

Mitochondrial DNA Clocks and the Phylogeny of Danaus Butterflies

“…According to independent analyses, then, D. chrysippus is not a phylogenetic species as defined by Cracraft (1983). And yet, this finding is paradoxical because its subspecies dorippus, chrysippus and alcippus hybridise extensively in East Africa (Smith et al, 1998;Lushai et al, 2003a, b) to produce offspring fertile in both sexes: thus, D. chrysippus has generally been regarded as a biological species (Mayr, 1942) that is both polymorphic and polytypic (Smith et al, 1997). Because the dorippus result, in particular, challenges taxonomic orthodoxy, the COI genes of dorippus and other representative taxa were sequenced twice using different primers (Table 1); in every case the original sequences were confirmed.…”