Biology subjects

Nason, J. D.

Publications and source records attributed to Nason, J. D..

5 recordsLinked to original sources

bioRxiv2022

Pollinator and host sharing lead to hybridization and introgression in Panamanian free-standing figs, but not in their pollinator wasps

Obligate pollination mutualisms, in which plant and pollinator lineages depend on each other for reproduction, often exhibit high levels of species-specificity. However, cases in which two or more pollinator species share a single host species (host sharing), or two or more host species share a single pollinator species (pollinator sharing), are known to occur in current ecological time. Further, evidence for host switching in evolutionary time is increasingly being recognized in these systems. The degree to which departures from strict specificity differentially affect the potential for hybridization and introgression in the associated host or pollinator is unclear. We addressed this question using genome-wide sequence data from five sympatric Panamanian free-standing fig species (Ficus subgenus Pharmacosycea, section Pharmacosycea) and their six associated fig pollinator wasp species (Tetrapus). Two of the five fig species, F. glabrata and F. maxima, were found to regularly share pollinators. In these species, ongoing hybridization was demonstrated by the detection of several first-generation (F1) hybrid individuals and historical introgression was indicated by phylogenetic network analysis. In contrast, although two of the pollinator species regularly share hosts, all six species were genetically distinct and deeply divergent, with no evidence for either hybridization or introgression. This pattern is consistent with results from other obligate pollination mutualisms, suggesting that, in contrast to their host plants, pollinators appear to be reproductively isolated, even when different species of pollinators mate in shared hosts.

evolutionary biology↗

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

Genome-wide sequence data show no evidence of admixture and introgression among pollinator wasps associated with a community of Panamanian strangler figs

Interactions between plants and their animal pollinators can shape processes of divergence and gene flow within associated lineages. For example, in the obligate mutualism between figs (Ficus) and fig pollinator wasps (family Agaonidae), each wasp species typically pollinates a single fig species, potentially reinforcing reproductive isolation among different wasp species. Multiple pollinator species, however, can sometimes reproduce in the same host fig species, potentially enabling hybridization and introgression between wasp species. In a community of Panamanian strangler figs (section Americana), we use genome-wide ultraconserved element (UCE) loci to estimate phylogenetic relationships and test for hybridization and gene flow among 19 pollinator species associated with 16 host fig species. Previous studies showing ongoing pollinator sharing and a history of pollinator host switching are consistent with documented genetic admixture in their host figs. Here we investigate if host sharing and a dynamic evolutionary history including host switching has also resulted in hybridization and gene flow between pollinator species. Phylogenetic analyses recover strong support for well-delimited wasp species coupled with high interspecific divergence. There is no evidence for ongoing hybridization or introgression, even among pairs of pollinator species currently reproducing within the same host. In contrast to work suggesting admixture among Panamanian host figs, we conclude hybridization and interspecific gene flow have not been important processes shaping the evolutionary history of their pollinating wasps.

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↗

bioRxiv2018

Inferring Processes of Coevolutionary Diversification in a Community of Panamanian Strangler Figs and Associated Pollinating Wasps

1The fig and pollinator wasp obligate mutualism is diverse (~750 described species), ecologically important, and ancient (~80-90 Ma), providing model systems for generating and testing many questions in evolution and ecology. Once thought to be a prime example of strict one-to-one cospeciation, current thinking suggests that genera of pollinator wasps co-evolve with corresponding subsections of figs, but the degree to which cospeciation or other processes contributes to the association at finer scales is unclear. Here we use genome-wide sequence data from a community of Panamanian strangler figs (Ficus subgenus Urostigma, section Americana) and associated fig wasp pollinators (Pegoscapus spp.) to infer the process of coevolutionary diversification in this obligate mutualism. Using a model-based approach adapted from the study of gene family evolution, our results indicate pervasive and ongoing host switching of pollinator wasps at this fine phylogenetic and regional scale. Although the model estimates a modest amount of cospeciation, simulations reveal this signal to be consistent with levels of co-association expected under a model of free host switching. Our findings provide an outline for testing how ecological and evolutionary processes can be modeled to evaluate the history of association of interacting lineages in a phylogenetic framework.

evolutionary biology↗