Population genetic structure of the California sea mussel, Mytilus californianus: Influence of the Pleistocene, biogeography, and micro-evolutionary processes

In order to fully understand the factors shaping within and among population genetic variation one must simultaneously think along ecological and evolutionary time scales. This involves a research program that considers the influence of geologic history, biogeographic patterns, and life history, as well as micro-evolutionary factors (natural selection, gene flow, and genetic drift) on estimated measures of population genetic structure. I selected the California sea mussel, Mytilus californianus (Conrad 1837), as a study system for investigating factors important to shaping the population genetic structure of sea mussels and more generally benthic marine organisms. Sea mussels are one of the most conspicuous and abundant invertebrates on rocky benches from southeast Alaska in the north to Baja California, Mexico in the south. They have a common bipartite marine invertebrate life history with planktonic veliger larvae and sessile juveniles and adults. Contemporary sea mussel populations have a continuous range that exceeds 4000 kilometers, extends well beyond the last glacial maximum to the north, and bounds two well recognized biogeographic boundaries. With their extensive range and common life history strategy, sea mussels are a good system for examining the potential influence of Pleistocene ice ages, biogeography, and life history on population genetic structure. To look at these factors I scored six allozyme loci in 1100 individuals from 11 populations spanning the species range including species boundary populations, isolated (island and range extremes), and edge (potential for physiological stress) populations. In addition to assessing the influence of the Pleistocene, biogeography, and life history on sea mussel population genetic structure, I was interested in the role of micro-evolutionary processes. In order to get at the relative importance of natural selection, gene flow, and genetic drift in structuring sea mussel population genetic structure I took an inter-locus approach using six allozyme loci and three single copy nuclear DNA RFLPs. The principle behind the inter-locus approach is that gene flow, genetic drift, and breeding structure should affect all loci in a similar way; therefore the only explanation for one, or more, outlying loci is some type of selection. Lastly, I was interested in testing the efficacy of Hedgecock's sweepstakes hypothesis for describing sea mussel population genetic structure. Hedgecock's sweepstakes hypothesis predicts that in marine organisms with high fecundity and high early mortality, at every generation only a fraction of the larval pool successfully recruit every generation due to a sweepstakes (chance) matching of reproductive activity and genetics to oceanographic (biological and physical) conditions conducive to spawning, fertilization, larval development and recruitment. In Chapter 1 I look at the influence of the Pleistocene, biogeography, and life history in shaping sea mussel genetic population structure. In Chapter 2 I do a comparative analysis of allozyme and single copy nuclear DNA RFLP markers to look at the relative importance of natural selection in driving sea mussel population genetic structure and to evaluate a suite of hypotheses that have been put forth to explain the pattern of heterozygote deficiency in bivalves. And finally, in Chapter 3, I test Hedgecock's sweepstakes hypothesis with four size classes of sea mussels.