Sexual selection on population-level mating opportunities drives morph ratios in a fig wasp with extreme male dimorphism

James M Cook

doi:10.1186/s12862-021-01898-3

Abstract

BackgroundAlternative mating tactics are widespread in animals and associated with extreme morphological polymorphism in some insects. Some fig wasps have both highly modified wingless males and dispersing winged males. Wingless males mate inside figs before females disperse, while winged males mate elsewhere after dispersal. Hamilton proposed a model for this system with morphs determined by alternative alleles. This has an equilibrium where the proportion of winged males equals the proportion of females dispersing unmated; i.e. the proportion of matings that they obtain. Previously, we have shown qualitative support for this prediction across nine wing-dimorphic fig wasp species. Here I test the quantitative prediction in the fig wasp Pseudidarnes minerva. In addition, some fig wasp species that lack winged males, but have two wingless morphs, show a conditional strategy with morph determination influenced by the number of wasps developing in a patch. I also test for this alternative pattern in the wing-dimorphic P. minerva.ResultsI sampled 114 figs that contained a mean of 2.1 P. minerva wasps from 44 trees across four sites in Sydney, Australia. At the whole population level, the proportion of winged males (0.84 or 0.79 corrected for sampling bias) did not differ significantly from the proportion of unmated females (0.84), providing strong quantitative support for the prediction of Hamilton’s model. In addition, there was no evidence for other factors, such as local mate competition or fighting between wingless males, that could violate simplifying assumptions of the model. Meanwhile, the proportion of winged males was not correlated with the number of wasps per fig, providing no evidence for a conditional strategy.ConclusionThe morph ratio in P. minerva is consistent with Hamilton’s simple Mendelian strategy model, where morph ratios are set by average mating opportunities at the population level. This contrasts with some fig wasps from another subfamily that show conditional morph determination, allowing finer scale adaptation to fig-level mating opportunities. However, these conditional cases do not involve wing polymorphism. Male polymorphism is common and variable in fig wasps and has evolved independently in multiple lineages with apparently different underlying mechanisms.

Highlights

Alternative mating tactics are widespread in animals and associated with extreme morphological polymorphism in some insects
This is consistent with simple Mendelian segregation of alleles, but not with morph determination that is conditional on mating opportunities in the local patch, as seen in some other fig wasps, where the proportion of wingless males increases with the number of wasps in a Fig. [23, 27]
Wingless males are not known for P. cooki, but it seems likely that they may exist. This detailed study of a fig wasp species with male wing-dimorphism supports the hypothesis that the frequencies of winged and wingless males are maintained by sexual selection on their mating opportunities

Summary

Introduction

Alternative mating tactics are widespread in animals and associated with extreme morphological polymorphism in some insects. Hamilton proposed a model for this system with morphs determined by alternative alleles This has an equilibrium where the proportion of winged males equals the proportion of females dispersing unmated; i.e. the proportion of matings that they obtain. The first involves a simple genetic mechanism, involving alternative alleles at one or a few loci, where different phenotypes arise from different genotypes [7, 8] This model requires morphs to have equal fitness to coexist, since otherwise the fitter strategy would become fixed, and predicts that the equilibrium proportion of each strategy equals the proportion of offspring that it produces [7,8,9]. Most known examples involve male polymorphism, but female genetic morphs occur in Ischnura damselflies and appear to be widespread in damselflies and dragonflies [17]

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