GENETIC POPULATION-STRUCTURE AND GENE FLOW IN THE ATLANTIC COD GADUS-MORHUA - A COMPARISON OF ALLOZYME AND NUCLEAR RFLP LOCI

High levels of gene flow have been implicated in producing uniform pat terns of allozyme variation among populations of man) marine fish spec ies. We have examined whether gene flow is responsible for the limited population structure in the Atlantic cod, Gadus morhua L., by compari ng the previously published patterns of variation at 10 allozyme loci to 17 nuclear restriction fragment length polymorphism (RFLP) loci sco red by 11 anonymous cDNA clones. Unlike the allozyme loci, highly sign ificant differences were observed among all populations at the DNA mar kers in a pattern consistent with an isolation-by-distance model of po pulation structure. The magnitude of allele frequency variation at the nuclear RFLP loci significantly exceeded that observed at the protein loci (chi(2) = 24.6, d.f. = 5, P < 0.001). Estimates of gene flow fro m the private alleles method were similar for the allozymes and nuclea r RFLPs. From the infinite island model, however, estimates of gene fl ow from the DNA markers were fivefold lower than indicated by the prot eins. The discrepancy between gene flow estimates, combined with the o bservation of a large excess of rare RFLP alleles, suggests that the A tlantic cod has undergone a recent expansion in population size and th at populations are significantly displaced from equilibrium. Because g ene flow is a process that affects all loci equally, the heterogeneity observed among populations at the DNA level eliminates gene flow as t he explanation for the homogeneous allozyme patterns. Our results sugg est that a recent origin of cod populations has acted to constrain the extent of population differentiation observed at weakly polymorphic l oci and implicate a role for selection in affecting the distribution o f protein variation among natural populations in this species.