Introgression, Natural Selection, And Thermal Tolerance In Blue Mussels (Genus Mytilus)
My dissertation addressed questions of the origin and maintenance of biodiversity, and the genomic response to environmental change in blue mussels (genus Mytilus). In chapters One and Two I explored the nature of species barriers and the consequence of hybridization. The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. I investigated the relative importance of natural selection and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native M. trossulus in a hybrid zone in central California. I used double-digest restriction-site associated DNA sequencing (ddRADseq) to genotype 1,751 randomly-selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in Mytilus. I found that ecologically based selection plays only a small direct role in maintaining reproductive isolation in the California hybrid zone, and that colonization history is an important control on the movement of genetic elements (i.e. introgression) during hybridization. Despite only low rates of hybridization between invasive Mediterranean blue mussel (M. galloprovincialis) and native blue mussel M. trossulus, introgression is occurring, and the geographic spread of M. galloprovincialis appears to drive the majority of introgression into the invasive species. My work reinforces the idea that demographic processes mediate the role played by natural selection in maintaining species barriers.
Chapter Three focused on the genetic consequences of large-scale environmental change. I developed new techniques using mRNA sequencing and ancestral state reconstruction to estimate rates of structurally stabilizing substitutions in blue mussels. I found that warm-adapted Mytilus galloprovincialis have higher rates of stabilizing substitutions than cold-adapted M. trossulus, which suggests that natural selection can efficiently modify structural properties of proteins to fine-tune thermal tolerance based on small changes in temperature of just several ˚C. As a whole, my dissertation reiterates the importance of demographic processes in controlling the movement of genetic material during hybridization, indicates introgression may contribute to invasive success, and documents subtle natural selection for changes in protein properties of warm adapted M. galloprovincialis.