Genetic structure of tautog (Tautoga onitis) populations assayed by RFLP and DGGE analysis of mitochondrial and nuclear genes

The tautog (Tautoga onitis) is one of two temperate labrid species commonly inhabiting the coastal marine and estuarine waters of the mid-Atlantic coa st of the United States. To delineate population structure throughout its p rimary range, we examined samples collected from three sites (Rhode Island, Delaware, Virginia). Five regions of the mitochondrial genome (COI, ATPase 6, cyt b, ND2 and control region) and one nuclear intron were amplified by PCR and screened for sequence variation with a battery of restriction enzy mes (RFLP analysis), or by denaturing gradient gel electrophoresis (DGGE). With RFLP analysis an average of 129 restriction sites per individual were revealed and 532 bases per individual were surveyed. Polymorphisms were obs erved in the ND2 and control region fragments, but not in the COI, ATPase 6 , or cyt b fragments. Mean within-sample haplotype diversity was 0.6905 (+/ -0.00184), within the range of values reported for other marine species. Ho wever, mean nucleotide diversity was 0.000782, one of the lowest values rep orted for a marine teleost. Corrected nucleotide divergence between samples was essentially zero, suggesting the absence of population structuring alo ng the mid-Atlantic Coast. DGGE analyses of GOT, cyt b, and a lactate dehyd rogenase (LDH) intron revealed little additional variation; each product po ssessed a single common haplotype and occasional rare variants. The low level of genetic diversity observed in the tautog may reflect a sma ll effective population size resulting from historical population bottlenec ks or large variance in reproductive success. The apparent absence of geogr aphic differentiation suggests that tautog from Rhode Island to Virginia fo rm a single genetic stock: data from additional genetic polymorphisms are n eeded to confirm or disprove this conclusion.