Male‐killer dynamics in the tropical butterfly,Acraea encedana(Lepidoptera: Nymphalidae)

Abstract

Sex ratio distortion in the tropical butterfly Acraea encedana is caused by infection with a male-killing bacterium of the genus Wolbachia. Previous research on this species has reported extreme female bias, high bacterial prevalences, and full sex role reversal. In this paper, we provide an assessment for the dynamics of the male-killer, based on a survey for sex ratios and Wolbachia prevalences among wild populations of A. encedana in Uganda. The study reveals that Wolbachia infection showed considerable variation over both spatial and temporal scales.

Biased sex ratios and aposematic polymorphism in African butterflies: A hypothesis

“…These expectations can serve as a base for testing this hypothesis against field data. In a thorough investigation for population sex ratios, male-killer prevalences and morph ratios in D. chrysippus, A. encedon and A. encedana conducted in Uganda (Hassan et al 2012a(Hassan et al , 2012b, two temporal patterns were found, which are highly consistent with theoretical predictions. Firstly, comparing the bacterial prevalences recorded during (2005)(2006)(2007) with those recorded a few years later (1998)(1999) (Jiggins et al 2000a(Jiggins et al , 2000b(Jiggins et al , 2000c for the three species has shown that the male killer undergoes extensive population dynamics.…”

Male-killing in African butterflies

“…[45][46][47] To begin with, Wolbachia prevalences were found to vary between different regions in Africa as well as between different populations of the same region. In a thorough investigation for the dynamics of Wolbachia infection in Uganda, Hassan et al 48,49 found an overall prevalence of 44.7% in A. encedon and 79.1 % in Acraea encedana; considerably lower estimates than these of Jiggins et al 42,43 More importantly, re-investigating the same habitat patches studied by Jiggins et al revealed consistently lower prevalences than that previously reported. This pattern can be explained by considering the male-killer's impact on host metapopulations; populations with extremely high prevalence that were observed by Jiggins were extinct due to the lack of males, and the habitat patches were then recolonized by new immigrants from nearby populations, with initially low bacterial prevalences.…”

“…77 We have suggested a selective scenario, in which colour polymorphism is maintained in the mimicry complex through the repeated extinction episodes experienced by the sympatric populations of the three species as a consequence of male-killers' spread. 80 In aposematic, mimetic species there are two selective forces acting to sustain monomorphism, and both are mediated by predation pressure; first, selection toward within-species convergence, favoring the most abundant form in the species, and second, selection toward between-species convergence, favoring the most abundant form in the mimicry complex. It is easy to see that the output of such selective system is colour monomorphism, as long as within-species and between-species selections are acting harmoniously for long time.…”

“…To conclude, in a system of polymorphic mimetic species subjected to repeated population extinctions and recolonizations, selection will continue to favor the state of monomorphism; however, no single colour pattern will be favored for a period long enough to actually promote monomorphism. 80 Hybrid mating and male-killing Invasion by a male-killer, if occurs in a lineage which recently undergone, or is currently undergoing speciation, might have fundamental consequences on the reproductive isolation which defines the species boundaries. If a considerable proportion of females in the infected species are virgins, and if the sex ratio in the sibling species (or the partially isolated subspecies) is normal, then it may pay females to mate with males that belong to the other species and produce less fit progeny, because the only available alternative is to stay virgin and not to have progeny at all; the sex ratio bias would thus select against the assortative mating induced by reinforcement, 81 leading to an interruption or a reversal in the ongoing speciation process.…”

African Queens find mates when males are rare

“…In some parts of this contact zone, sex ratios are highly female‐biased (Smith et al, 2019 ), raising the possibility that the scarcity of males may result in a failure of females to mate. In other areas, both infection rates and sex ratios vary between the wet and dry seasons (Hassan et al, 2013 ; Herren et al, 2007 ), possibly associated with seasonal migration of the different color morphs (Smith et al, 1997 ).…”