HART, M.W.; Dalhousie Univ.: Reproductive variation and population genetics in marine invertebrates

Many larval biologists have become population geneticists in order to study the population genetic consequences of reproductive variation among marine invertebrates. Most studies focus on the effects of dispersal differences on among-population differentiation (as F_ST or its analogs). We have explored instead the effects of reproductive variation (including dispersal differences) on within-population differentiation or inbreeding (as F_IS). In our empirical work on microsatellite allele frequency variation in a sea urchin with broadcast spawning of gametes and a lengthy planktonic larval stage, we found no significant differentiation among populations on a scale of about 1000 km, but highly significant differentiation within populations: large heterozygote deficits and a relatively high F_IS. We reviewed F_IS estimates from recent studies of marine invertebrates and found two biologically significant patterns: no significant association between F_IS and the presence or absence of a dispersing planktonic larva; and a significantly higher mean F_IS for species with broadcast spawning of sperm in comparison to species with direct sperm transfer or copulation. We suggest that these high inbreeding coefficients are caused by high variation in paternity rates among free-spawning males. A simulation model using field measurements of individual fertilization success shows that high among-male variance in fertilization rate is correlated with large among-cohort genetic differentiation and heterozygote deficits in mixed-cohort populations.