Population genomics of plant parasites

How are parasite populations genetically structured within and across host individuals? space? host species?

The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal, and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector-borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host-associated genetic structure in sympatry, little genetic variation due to geographic site, and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.

This work was done in collaboration with Noah Whiteman (UC-Berkeley), Jen Koop (UMass-Dartmouth), Nicolas Alexandre (UC-Berkeley), and Lauren Johnston. The results of this work are published in Molecular Ecology.

How are parasite species structured across their geographic range? How do ecological interactions (host species, biotic dispersal), environmental characteristics, and geography interact to determine that structure?

When natural selection operates repeatedly to produce reproductive isolation, parallel divergences can result across space. Determining the frequency with which parallel host-associated divergence can occur across space will provide insight into the propensity of parasites to undergo sympatric diversification. The targets of selection for adaptation to the host environment can be identified through studying the genomic patterns of divergence. In collaboration with Dr. Noah Whiteman and Tim O’Connor at the University of California – Berkeley, I am producing a reduced representation genomic library using RAD capture sequencing of hundreds of desert mistletoe individuals from across the species’ geographic range to answer the above questions and determine the genomic signature of adaptation. Studies of the population genomic structure will benefit from powerful new methods developed by the Bradburd Lab at Michigan State University. More information soon!