Changing the dynamics of malefemale sexual conflict through alteration of the population sex ratio

The direction and intensity of sexual conflict is dictated by the relative levels of parental care exhibited by male and females, and the population sex ratio. Microorganisms that bias the sex ratio of the individual that harbours them, such as feminizers and male-killers, have an effect on the population sex ratio of the host, and thus alter this balance.

The degree to which these microorganisms can alter this balance of course depends on the prevalence of the microorganism. The prevalence of malekillers is sometimes low (1-5% of females infected), more often modest (20-50% of females infected), but sometimes very high (>95% of females infected) (Hurst et al., 2003). Effects on patterns of mate competition and sperm competition are expected with a 50% prevalence (gives rise to a 2 females per male population sex ratio), and gross mating system alteration may occur with higher prevalence levels (80-99%, corresponding to a 4:1 to 100:1 population sex ratio).

The example of Acraea encedon is a case in point of a host whose reproductive processes have been fundamentally altered by the presence of male-killing bacteria. In Ugandan populations of this species, the population sex ratio bias (14:1) associated with the high-prevalence male-killer infection has led to sex role reversal (Jiggins et al., 2000). Virgin females of this species congregate on hill tops, waiting for males with which to mate.

It is clear that as well as overt mating system differences, these parasites are likely to produce more covert changes in the reproductive biology of the species concerned. In particular, we would expect male ejaculate constitution to change with increasing population sex ratio bias, and corresponding changes in the ability of males to mate multiply. Preliminary evidence for this comes from studies of the nymphalid butterfly Hypolimnas bolina (Dyson and Hurst, 2004) (see Fig. 8.1). This species inhabits the island and mainland areas of the Pacific, and the prevalence of a male-killing Wolbachia varies between islands. In one island, Independent Samoa, prevalence is extreme (99% of females are infected), and the population sex ratio is 100 females per male. On this island, unmatedness of females is common, indicating that male access to females has become unlimited. The size of spermatophore delivered by males during copulation in Independent Samoa is less than 50% of that found on neighbouring islands with more moderate prevalence. This is consistent with consideration of adaptive male behaviour in the light of male-killer-induced sex ratio changes. It will be informative to ascertain whether the association holds when a wider array of islands with different male-killer prevalence is investigated.

Fig. 8.1. Female Hypolimnas bolina. This butterfly is host to a male-killing Wolbachia strain. In the population of Independent Samoa, 99% of females are infected, producing a population sex ratio of 100 females per male. This results in high levels of viriginity in the field. The consequences of varying male-killer prevalence on host reproductive ecology are currently being investigated.

Fig. 8.1. Female Hypolimnas bolina. This butterfly is host to a male-killing Wolbachia strain. In the population of Independent Samoa, 99% of females are infected, producing a population sex ratio of 100 females per male. This results in high levels of viriginity in the field. The consequences of varying male-killer prevalence on host reproductive ecology are currently being investigated.

A second study indicating the capacity of parasite-induced population sex ratio biases to select for change in reproductive biology comes from study of isopod Crustacea. Moreau and Rigaud (2003) measured the ability of males to inseminate females across seven species of woodlice, five of which were naturally infected with a feminizing Wolbachia, and two with CI Wolbachia. The five feminizer-infected species were homogeneous for mating rate, and this was nearly double that found in the two species that did not bear the feminizer. Whilst not a formal comparative analysis, this result is clearly consistent with the intuitive notion that sex ratio distorters, via the alteration in population sex ratio they produce, select for changes in reproductive tactics.

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