Untangling Gene Flow and Reproductive Success in Highly Fecund Marine Species with Type-III Survival

Estimation of gene flow from Sewall Wright’s standardized measure of spatial variance of allele-frequencies, F_ST, is fraught with pitfalls, especially for marine species with dispersing planktonic larvae.  Adding to Whitlock & McCauley’s (1999 Heredity 82:117) earlier cautions about estimating numbers of migrants, Nm, from F_ST is Wakeley & Eldon’s (2009 Genetics 181:615) more recent theoretical demonstration that F_ST for selectively neutral markers in highly fecund marine species is determined by the balance of random genetic drift, migration, and sweepstakes reproductive success (SRS).  Thus, estimating gene flow under the assumption of drift-migration equilibrium is suspect unless supported by carefully controlled comparative studies or by evidence for long-term temporal stability of spatial patterns.  On the other hand, non-equilibrium population genetic approaches – analyses, in larval or juvenile cohorts, of kinship, genetic diversity, or linkage disequilibrium, for example – can provide direct evidence of reproductive processes and could permit, if implemented at a sufficient scale of sampling, real-time tracking of larval cohorts naturally tagged by SRS.  However, selection may be a significant factor shaping the genetics of kin-structured larval cohorts, as suggested by recent estimates that early mortality in the Pacific oyster, a typical, highly fecund, marine species with type-III survivorship, is largely genotype dependent.