Historical Ecology of the California Sea Mussel and Purple Sea Urchin Inferred from Multiple Gene Genealogies

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

Pogson: Historical ecology of the California sea mussel and purple sea urchin inferred from multiple gene genealogies. Climatic fluctuations throughout the Pleistocene are known to have had profound effects on the origins, extinctions, and geographical distributions of marine taxa. However, the effects of these dramatic changes in sea level, sea surface temperature, current patterns, and upwelling regimes on the evolutionary genetics of marine species remains unclear. The fossil record lacks the resolution to reasonably estimate historical abundance and is incapable of providing estimates of important parameters such as a species evolutionary effective population size (Ne). Recent advances in coalescent theory provide exciting new opportunities to estimate evolutionary Ne, to infer population growth histories, and to explicitly test various demographic scenarios (including population bottlenecks) through the examination of the structures of gene genealogies obtained from multiple independent loci. This study will investigate the historical ecologies of two abundant and widespread NE Pacific marine taxa (the California sea mussel, Mytilus californianus, and the purple sea urchin, Strongylocentrotus purpuratus) through the reconstruction of allelic genealogies at multiple nuclear and mitochondrial gene regions in samples that span the geographic ranges of both species (Baja California to SE Alaska). The large genealogies (150 alleles per locus) obtained from the multiple independent loci will provide an unparalleled ability to infer both species evolutionary effective population sizes and allow explicit tests of alternative demographic hypotheses. The possibility of range fragmentation and bottleneck events on mussel and sea urchin populations over the recent Pleistocene will be assessed by (i) applying a coalescent-based maximum-likelihood method that tests for the concordance of the timing and severity of diversity reducing events across multiple independent genealogies, and (ii) performing coalescent simulations that will impose population fragmentation and bottleneck events coincident with known glacial maxima over the past 500,000 years. The genetic signature of northward range expansion will also be tested by inferring the directionality of gene flow and by comparing the ages of mutations and the growth histories of northern and southern populations (that have a fossil record persisting throughout the Pleistocene). The results are expected to provide fundamental new insights into how the Pleistocene period has influenced the evolutionary histories of marine taxa in the NE Pacific region. Broader impacts of the proposed research: Understanding how the environmental history of the Pleistocene has affected present-day marine communities is a fundamental yet largely unresolved question. This research project will significantly advance our understanding in this area by inferring the historical abundances and population genetic histories of two ecologically important marine species of the NE Pacific region. The genealogical approach offers an exciting new framework to link paleoceanographic and paleoclimatic research with ecological and evolutionary studies on extant marine species. When extended to other intertidal and subtidal species, these coalescent-based methods will provide new insights into the stability of marine communities and the susceptibility of individual taxa to historical climate change. This project will also continue the strong tradition at UCSC of involving undergraduate students in research, aid in the training of minority and graduate students, and fund one postdoctoral position.