Population genetics of the green sea urchin, Strongylocentrotus droebachiensis

I used microsatellite genotypes and mitochondrial DNA sequences to study the population genetics of the green sea urchin (Strongylocentrotus droebachiensis) in the northwest Atlantic Ocean. I developed microsatellite markers for S. droebachiensis that are also suitable for genetic studies in S. purpuratus. I measured within and among population genetic differentiation at four microsatellite loci in eleven populations throughout the north Atlantic and northeast Pacific. I found small but significant genetic subdivision at the largest spatial scale (F ST = 0.0625, P = 0.0002). One northwest Atlantic population recently affected by disease was significantly differentiated from some others, but otherwise there was little differentiation among populations within Atlantic Canada. All of these populations were highly inbred (F IS = 0.1499).; I used COI haplotype data to investigate the phylogeographic history of populations in the northwest Atlantic, and to understand the relative influence of gene flow from Pacific and northeast Atlantic populations on the genetic variation in northwest Atlantic populations. Using analysis of molecular variance, statistical parsimony, mismatch, and nested clade analyses I found strong evidence of the survival of northwest Atlantic populations during recent Pleistocene glaciations, limited gene flow across the north Atlantic, and introgession of haplotypes from the congener S. pallidus.; I used an age class analysis of both microsatellite genotype and mitochondrial haplotype data to address the possibility that sea urchins are susceptible to a large variance in reproductive success. Significant genetic variation among age classes supported the hypothesis that reproduction in sea urchins can be viewed as a sweepstakes in which relatively few adults contribute to each spawning event.; I reviewed the effect of reproductive variation on the non-random component of inbreeding (FIS). FIS values were significantly associated with the mode of male spawning: by release of planktonic sperm versus some form of direct sperm transfer to females or benthic egg masses. Such free-spawning often produces low fertilization rates and highly variable fertilization success among individual males. This broad pattern among species is consistent with my observations of population genetic variation within S. droebachiensis, and suggests that the population genetic consequences of free-spawning could be widespread and significant.