Gene flow and allozymic population structure in the clam Venus antiqua (King of Broderip), (Bivalvia, Veneriidae) from Southern Chile

Using horizontal electrophoresis, 13 allozyme loci were screened in 144 adult clams (Venus antiqua) (King of Broderip) from 5 geographic samples in southern Chile. Polymorphism was detected at 11 loci across the sampling localities. Gene frequency differences rather than the alternative fixation of alleles characterize these collecting sites. Direct-count heterozygosity fluctuated between 6.8% and 10.3%, with a generalized and significant heterozygote deficiency detected in every locality. Similar levels of genetic diversity were detected in both exploited banks and one presumed unmanaged population. Substantial estimates of demic structuration were found at the total population (F_ST=0.107) and regional levels (F_ST=0.143), with coinciding high inbreeding estimates (F_IS=0.57). Although no statistically significant evidence of genetic substructuring was found at the within-region comparison (F_ST=0.007), a generalized heterozygote deficiency (F_IS=0.57) also characterized that hierarchical level. Spatial correlation of gene frequencies indicated a nearly random distribution of genotypes along the latitudinal distribution of samples. The high rates of global, and the within-region rates of gene flow suggest substantial larval interchange via long-distance dispersal that prevent differentiation by drift alone. Accordingly, no discrete genetic units can be recognized for the species at any spatial scale. Apparently, high recruitment rates help in replenishing genetic variants lost by continuous overfishing, thus explaining why human extraction has not resulted in major losses of genetic variation for individual populations. The slow-growing rate and delayed sexual maturation of this clam species are apparently stronger limiting factors than human-induced genetic erosion for the conservation of natural beds and viable captive management of V. antiqua.