Genetic Differentiation of Disjunct Gulf of California and Pacific Outer Coast Populations of Hypsoblennius jenkinsi

Abstract

jenkinsi. Specimens were collected from a single site in the northern Gulf of California and from two study sites in southern California waters. The standard genetic distance value (D) of 0.003 for the two populations in southern California waters is substantially less than the D values for either of these populations and the Gulf population (0.035, 0.038). Slight genetic differentiation of the southern California populations nonetheless is indicated by a significant (P < .001) difference between the local populations in allele frequencies at the Est-1 locus. The geographic proximity of the southern California populations coupled with the extended planktonic larval stage of H. jenkinsi suggest that the genetic differentiation of these local populations may be due to differences in local selection pressures rather than restricted gene flow. In contrast, statistically significant (P < .005 in five cases, P < .01 in one case) differences in allele frequencies were found between northern Gulf of California and southern California populations at six loci (Ada-A, Adh-A, Ck-C, Gapdh-B, Pep-2, Pgdh-A). The nature and magnitude of these differences suggest that, although no fixed allelic differences were found, there is limited gene flow at present between these disjunct populations. The low D values of the disjunct populations are compatible with the hypothesis that this disjunct distribution originated when the range of these temperate fish, continuous around Cabo San Lucas during the Pleistocene glacial periods, was disrupted in the Cape region due to post-glacial warming of surface waters. The distributions of all marine fish species that have been regarded as 'disjuncts' are discussed.