Most progress in understanding the genetics of speciation has come through work on model systems like Drosophila, with particular emphasis on the special role of their sex chromosomes. A more comprehensive understanding will require studies of alternative systems, including the vast diversity of species lacking sex chromosomes (most plants, mollusks, marine invertebrates and reptiles). This study uses a combination of breeding and molecular assays in a tidepool invertebrate, the copepod Tigriopus californicus, to examine how speciation occurs when there are no sex chromosomes. Predictions will be tested that address the role of genetic incompatibilities, conflict between nuclear and cytoplasmic genes, and the degree to which incompatibilities are masked by other genes. The genetic underpinnings of sterility vs. viability vs. morphological differences will be examined, as well as the possibility that mild sex-specific second-generation patterns in this species are driven by mechanisms similar to those underlying the strong sex-specific first-generation patterns commonly found in species bearing sex chromosomes.
This work will develop genetic resources (single nucleotide polymorphisms, a linkage map) that should prove valuable for a range of future studies by the growing community of researchers focusing on Tigriopus. The research also has conservation implications: it addresses how biodiversity is generated in organisms lacking sex chromosomes. Lastly, this research will enhance outreach to diverse communities and will provide professional training for one postdoctoral associate, one graduate student, and numerous undergraduates, including individuals of underrepresented groups.
Most progress in understanding the genetic basis of speciation has come through work on model organisms like the fruitfly Drosophila, with particular emphasis on the role of sex chromosomes. A more comprehensive understanding will require studies of alternative systems, including the vast diversity of species lacking sex chromosomes (many plants, mollusks, marine invertebrates and reptiles). This study used a combination of breeding and molecular assays in the copepod Tigriopus californicus to test explicit predictions about the mechanics of speciation in the absence of sex chromosomes. In addition to its importance to our fundamental understanding of the speciation process, such work is relevant to practical conservation questions concerning the consequences of intentional and unintentional hybridization between populations. As a first step in this work, we have developed a linkage map for T. californicus, with 191 molecular markers distributed across the species 12 chromosomes. This is the first linkage map for copepods, one of the most abundant groups of animals on the planet. DNA sequence and mapping information for these markers have been archived on appropriate public databases. In addition to providing the backbone for our own lab’s research, the linkage map is important for collaborative work now underway to assemble the T. californicus genome, and will be useful for future genetic and evolutionary studies of related taxa. As populations diverge from each other they accumulate genetic differences that may cause conflicts when the populations are hybridized. We raised >1500 second generation interpopulation hybrid copepods and screened individual animals for our mapped molecular markers and for a series of fitness and morphometric traits. Associations between molecular markers and phenotypic traits were used to infer gene regions involved in hybrid conflicts. Results showed patterns that were very different from those typically seen in species with sex chromosomes, including relatively slow accumulation of hybrid conflicts, with most hybrid conflicts impacting survival, body size or development rate rather than fertility. Conflicts between nuclear gene regions (on chromosomes inherited from both parents) and mitochondrial DNA (on an organelle inherited only from the mother) were found to be more important than has been found in more commonly-studied organisms. The genetic basis for hybrid survival differed substantially between sexes. These differences cannot be attributed to sex chromosomes, and were instead attributed to greater vulnerability in more metabolically active males. Beyond the specific objectives of the proposal, grant funds supported a series of related studies on the beneficial and detrimental consequences of interpopulation hybridization over varying time periods, as well as studies of evolutionary relationships among populations and species in the genus Tigriopus. Broader impacts of the research include student training and public outreach. Grant funds supported research by 3 postdoctoral associates, 4 graduate students (all women), 1 technician and 15 undergraduates (12 women, including 2 under-represented minorities). Ten undergraduates are coauthors on resultant papers. Edmands lab members presented their work at a number of local and national conferences, including annual meetings of the Society for the Study of Evolution in 2010, 2011 and 2012. Funds from the grant helped support outreach to diverse groups in the greater Los Angeles area, including a new collaboration with the Aquarium of the Pacific in Long Beach aimed at enhancing public understanding of evolutionary processes.
