TEMPORAL STABILITY OF LACTATE DEHYDROGENASE-A CLINES OF THE HIGH COCKSCOMB, ANOPLARCHUS PURPURESCENS.

Abstract

In marine species with planktonic larval stages, strong geographic differentiation among resident adult populations is noteworthy in that identification of the forces and mechanisms which create such local differentiation allows an assessment of the relative importance of natural selection in promoting genetic divergence. Williams et al. (1973), and later Koehn and Williams (1978), focused on this problem in their analysis of genetic differentiation in the American eel, Anguilla rostrata. Their study centered on the interaction between age-dependent differentiation (elvers vs. resident adults) and spatial differentiation of groups along a latitudinal transect. They found different patterns of differentiation which depended upon the specific genetic marker under analysis. At the Mdh locus, allele frequencies were homogeneous among age-groups and among localities, suggesting recruitment of elvers from a common zygotic pool with no subsequent divergence between groups of colonists in situ. In contrast, at the Sdh locus, they observed a significant spatial component of variation across latitude, but one which was equally pronounced in both age groups. This result suggested (in light of the life history of the American eel) that divergence occurred prior to larval recruitment and was not further accentuated in situ. Finally, at the Phi-II locus, they observed that allele frequencies of newly recruited larvae were uniform across the transect (once again suggesting a common zygotic pool), but that geographic differentiation of adults was significant. This latter pattern would be consistent with a predominant role of local selective factors in promoting genetic divergence and it suggested selection intensities of about 10% per generation. Although the power of their analysis lies largely in the peculiar migrational history of the American eel, there is no reason to believe that the differences in genetic composition between life stages observed by Williams et al. (1973) and Koehn and Williams (1978) are in any way unique to Anguilla. As they point out, it is reasonable to expect that age-specific selection would operate in other species with planktonic larvae, but similar analyses have been rarely conducted. Actually, the degree of geographic differentiation in the American eel is rather slight; for example, systematic variation in Phi-II allele frequencies is on the order of 5% per degree of latitude (Koehn and Williams, 1978). In other marine fishes with planktonic larvae, more pronounced examples of geographic variation are known. In Anoplarchus purpurescens, an intertidal blennioid fish of the northwestern Pacific Ocean, relatively sharp differences in allele frequency over short distances have been reported (Johnson, 1971, 1977; Sassaman and Yoshiyama, 1979; Yoshiyama and Sassaman, unpubl.). Here we explore temporal (age-group) aspects of allele-frequency variation along two pronounced gradients of gene-frequency change in Anoplarchus. Our analysis follows the same logical format of Williams et al. (1973) and Koehn and Williams (1978). In addition we examine yearto-year stability of these gene-frequency clines by reference to previously published information (Johnson, 1971; Sassaman and Yoshiyama, 1979) for the same sampling sites. Since the presumption of extensive