DISSERTATION RESEARCH: Reinforcement of isolation between parasite host races

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A long-standing debate in evolutionary biology is how reproductive barriers among species can arise in the absence of geographic obstacles. New parasite species can often form following infection of a novel host species if barriers to successful reproduction between parasites on the two species exist. This project combines observational, experimental, and genomic studies of a native parasitic plant, desert mistletoe, to address this question. While showing the same epidemiological patterns as many other parasites, parasitic plants allow for direct tests of hypotheses concerning the evolution of barriers to mating in diverging parasite lineages that would be infeasible in other systems. This research will help elucidate the poorly understood ecological mechanisms that promote parasite speciation and adaptation to novel hosts. The potential for studying parallel evolution of parasites undergoing ongoing diversification provides a unique opportunity to study the genomics of species interactions. Additionally, the researchers will disseminate the results to a broad audience through collaborations with local outreach groups interested in native plants and will contribute to the training of graduate and undergraduate students.This work will test established theory about the relative roles of pre-zygotic and post-zygotic isolation mechanisms in sympatry and allopatry using two host races of a native aerial hemiparasite of plants, Phoradendron californicum (Viscaceae), across their overlapping ranges. Populations of the host races infecting catclaw acacia (Senegalia greggii ) and velvet mesquite (Prosopis velutina) in sympatry (locally interspersed) and allopatry ( 3 km from nearest infected host of other species) will be studied at multiple sites across Arizona. The researchers will conduct selection gradient analyses for flowering time across these populations to test if selection for divergent flowering times (a pre-zygotic isolation mechanism) between the host races is stronger in sympatry than in allopatry. Additionally, the researchers will perform hand-pollination experiments to quantify the cost of mating between host races and the strength of late-acting barriers to gene flow. Finally, population genomic analyses of mistletoes from these host races using RAD sequencing will reveal whether host races are the result of a single host-switching event or repeated, parallel divergence and reinforcement across the range. The researchers will further use the sequence data to investigate the genomic signature of host race formation and adaptation to host species in this parasitic plant.