Low variation at allozyme loci and differences between age classes at microsatellites in grass goby (Zosterisessor ophiocephalus) populations

Allozymes and microsatellites were used to assess the level and distribution of genetic variation in grass goby population samples collected from the Venice Lagoon between October 2001 and May 2002. Eighteen enzymatic loci were examined in 434 individuals, 14 of which resulted to be monomorphic, and 4 (GPI-B*, LDH-B*, PGM-A*, PGM-B*) showed 2 alleles scored in 6 individuals only. Comparison with previous data suggests that genetic variation has been eliminated in the Venice Lagoon population during the last few years at three loci. In contrast, analysis of 11 microsatellites in a subset of 192 individuals revealed substantial molecular variation. Analysis of molecular variance showed a lack of genetic differentiation inside the lagoon with respect to site and date of collection, sex, and level of pollution. Significant variation in allelic frequencies was found at microsatellite loci when small (one year old) males were compared to large males (two and three years old), suggesting that a complex population dynamics occurs in this species. The very low level of polymorphism of allozymes could be due to the evolutionary history of the species, or, considering the difference between small and large males, could be the result of recent effects of drift. The second hypothesis is supported by the comparison with previous allozyme studies of the species in the same area, that suggests that loss of heterozygosity at three loci occurred in the last 10 years.