Evolution of nematode symbionts of solitary and social halictid bees

Symbiotic relationships are crucial to the diversity and complexity of life, as evidenced by the endosymbiotic origin of eukaryotic cells. Though we have gained a great deal of understanding regarding the evolution of symbioses, one area that has not been addressed is if host social structure can affect symbiont evolution. This dissertation took advantage of the labile social structure found in halictid bees by examining the relationship between social and solitary halictids and their nematode associates. First, a five year survey of bees in Virginia showed that six species of halictid bees associate with nematodes in the genus Acrostichus. A three gene molecular phylogeny suggested that each bee species associates with a host specific nematode. Furthermore, the nematode and host phylogenies indicate a history of cospeciation. To test where this relationship lies on the continuum between parasitism and mutualism, and whether this differs based on host sociality, laboratory colonies of a solitary and a social halictid species were reared. There was no significant relationship between the infection status and reproductive fitness of each nest. This result indicates that Acrostichus spp. are commensal with both social and solitary hosts. To quantify the degree that the bee's and nematode's reproductive interests are coupled, and to determine if this coupling differs based on host sociality, linkage disequilibrium between host and symbiont haplotypes was calculated. There was no evidence for linkage in either the social or the solitary host. Host social structure does not tighten the association between bees and nematodes. Furthermore, the lack of linkage indicates that the nematodes undergo some horizontal transmission within species, in addition to previously documented vertical transmission. Finally, an epidemiological model parameterized to the Acrostichus /halictid association was used to explore the levels of horizontal transmission and fitness effects on the host that correspond with the prevalence of infection found in nature. The modeling results, together with the natural history of the association, suggested that low levels of sexual transmission likely occur. Taken together, these studies provide insight into how symbiotic relationships evolve. Host social structure does not appear to affect symbiont evolution, at least not for this commensal relationship.