Van Goor, J. | Search bioRxiv

bioRxiv2021

Extraordinarily precise nematode sex ratios: Adaptive responses to vanishingly rare mating options

Sex ratio theory predicts both mean sex ratio and variance under a range of population structures. Here, we compare two genera of phoretic nematodes (Parasitodiplogaster and Ficophagus spp.) associated with twelve fig-pollinating wasp species in Panama. The host wasps exhibit classic Local Mate Competition: only inseminated females disperse from natal figs, and their offspring form mating pools that consist of scores of the adult offspring contributed by one or a few foundress mothers. In contrast, in both nematode genera, only sexually undifferentiated juveniles disperse, and their mating pools routinely consist of eight or fewer adults. Across all mating pool sizes, the sex ratios observed in both nematode genera are consistently female-biased (~0.34 males), which is markedly less female-biased than is often observed in the host wasps (~0.10 males). In further contrast with their hosts, variances in nematode sex ratios are also consistently precise (significantly less than binomial). The constraints associated with predictably small mating pools within highly subdivided populations appear to select for precise sex ratios that contribute both to the reproductive success of individual nematodes, and to the evolutionary persistence of nematode species. We suggest that some form of environmental sex determination underlies these precise sex ratios.

evolutionary biology↗

bioRxiv2020

The enemy of my enemy is my friend: Nematode infection of pollinating and non-pollinating fig wasps has net benefits for the fig-fig wasp pollination mutualism

Mutualistic associations between species pairs are ubiquitous in nature but are also components of broader organismal community networks. These community-level associations have shaped the evolution of individual mutualisms through interspecific interactions ranging from secondarily mutualistic to intensely antagonistic. Our understanding of this complex context remains limited because identifying species interacting with focal mutualists and assessing their associated fitness benefits and costs is difficult, especially over space and through time. Here, we focus on a community comprised of a fig and fig wasp mutualist, eight non-pollinating fig wasp (NPFW) commensals/antagonists, and a nematode previously believed to be associated only with the pollinator wasp mutualist. Through repeated sampling and field experiments, we identified that all NPFWs are targets for infection by this nematode. Further, this infection can impact NPFWs more severely than either mutualistic partner, suggesting a novel role of density-dependent facultative mutualism between fig and wasp mutualists and the nematode.

ecology↗