DISSERTATION RESEARCH: Reinforcement of isolation between parasite host races

进化生物学中一个长期存在的争论是，在没有地理障碍的情况下，物种之间的繁殖障碍是如何产生的。如果寄生在一个新的寄主物种上的寄生虫之间存在成功繁殖的障碍，那么新的寄主物种通常可以在感染一个新的寄主物种后形成。这个项目结合了对一种原生寄生植物——沙漠槲寄生的观察、实验和基因组研究来解决这个问题。寄生植物虽然表现出与许多其他寄生虫相同的流行病学模式，但它允许直接测试有关不同寄生虫谱系中交配障碍进化的假设，这在其他系统中是不可行的。这项研究将有助于阐明促进寄生虫物种形成和适应新宿主的生态机制。研究正在经历多样化的寄生虫的平行进化的潜力为研究物种相互作用的基因组学提供了一个独特的机会。此外，研究人员将通过与对本地植物感兴趣的当地外展团体合作，将研究结果传播给广泛的受众，并将为研究生和本科生的培训做出贡献。这项工作将测试关于合子前和合子后分离机制在同位和异位体中的相对作用的既定理论，研究对象是一种本地半寄生性植物——加利福尼亚Phoradendron californicum （visicaceae）——在它们重叠的范围内的两个寄主小种。将在亚利桑那州的多个地点研究感染爪金合欢（Senegalia greggii）和丝绒豆科植物（Prosopis velutina）的宿主种群，这些宿主种群在同域（局部散布）和异域（距离最近的其他物种感染宿主3公里）。研究人员将对这些种群的开花时间进行选择梯度分析，以测试在同种中，寄主种族之间对不同开花时间的选择（一种合子前隔离机制）是否比异种中更强。此外，研究人员将进行手工授粉实验，以量化宿主种族之间交配的成本和基因流动的后期障碍的强度。最后，利用RAD测序技术对来自这些寄主种族的槲寄生进行群体基因组分析，将揭示寄主种族是单一寄主切换事件的结果，还是在整个范围内重复、平行分化和强化的结果。研究人员将进一步利用这些序列数据来研究这种寄生植物中寄主种形成和对寄主物种适应的基因组特征。

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.