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Fishery Bulletin 98(2) 



for black sea bass but detected significant divergence 

 between Gulf of Mexico and Atlantic populations. 



The results of our study are consistent with the 

 null hypothesis that the tautog form a single genetic 

 stock within the species range. Although tagging 

 studies in Rhode Island have suggested that adult 

 movement into and out of the area is negligible 

 (Cooper, 1966), tautog have a relatively long pelagic 

 larval stage of approximately three weeks which 

 could result in enough gene flow among geographic 

 regions to prevent the genetic differentiation of 

 subpopulations. 



The current system off of the mid-Atlantic coast 

 of the United States consists of three major fea- 

 tures: the northeastward flow of the Gulf Stream, the 

 southwestward flow of the along-shelf current, and 

 the across-shelf-flowing warm-core ring streamers 

 that split off from the Gulf Stream (Brooks, 1996). 

 The direction of flow is also influenced by local river 

 runoff, seasonal wind patterns and meteorological 

 events, and by the onshore movement of Ekman cur- 

 rents. The flow patterns in the mid-Atlantic region 

 suggest that a high degree of transport of larvae 

 among regions is possible (Hare and Cowen, 1993), 

 especially for species such as the tautog that spawn 

 in nearshore waters and have pelagic eggs. Larval 

 transport has been suspected as the cause of genetic 

 homogeneity over wide geographic ranges for many 

 marine species (Avise, 1994; Hedgecock, 1994). 



Other mtDNA analyses of stock structuring of 

 coastal fish species in the mid-Atlantic region have 

 likewise detected no population subdivision, e.g. 

 weakfish (Graves et al., 1992b), bluefish (Graves et 

 al., 1992a), summer flounder (Jones and Quattro, 

 1999), and Atlantic croaker (Lankford et al., 1999). 

 None of these studies detected significant genetic 

 heterogeneity among samples collected at different 

 sites within the geographical region spanning from 

 Rhode Island to Chesapeake Bay. For many fish spe- 

 cies, the extensive mobility of juveniles and adults 

 coupled with larval dispersal is likely to result in 

 substantial gene flow. 



We were unable to reject the null hypothesis of 

 genetic homogeneity among three sites in the north- 

 em, middle, and southern portions of the tautog's 

 range but we did not not conclusively prove that 

 tautog in the mid-Atlantic region constitute a single 

 genetic stock. Population structure not resolved by our 

 study might be detected by an examination of addi- 

 tional polymorphisms, with rapidly evolving markers 

 better suited for detecting recent subpopulation diver- 

 gence. Improved resolution would also be gained by 

 increasing sample sizes; the current analysis with a 

 sample size of 24 individuals per population had lim- 

 ited power (40%) to detect significant heterogeneity 



among populations with the observed haplotype-fre- 

 quency distributions. However, it is unlikely that by 

 simply increasing sample sizes with the same set of 

 molecular markers the picture of minimal geographic 

 heterogeneity would alter substantially. If we had 

 found the same haplotype proportions in a study with 

 quadrupled sample sizes (n=96 individuals per site), 

 estimates of gene flow (Nm) would still be substantial 

 (Nm=8.6 between RI and DE; 28 for VA-RI; 73 for 

 DE-VA). We conclude that the sampled populations 

 are probably genetically homogeneous, as the result 

 of contemporary or recent gene flow. 



In many genetic studies of population structure, 

 PCR amplification of DNA is coupled with a restric- 

 tion fragment length polymorphism (RFLP) analy- 

 sis or direct sequencing. Although PCR is relatively 

 simple, the subsequent analyses can become expen- 

 sive and time-consuming when large numbers of 

 individuals must be evaluated. These drawbacks can 

 be alleviated through the use of mutation detection 

 techniques such as DGGE and DNA heteroduplex 

 mobility assays (Lessa 1992; Grompe, 1993; Lessa 

 and Applebaum, 1993 ). These methods can be used to 

 compare DNA fragments to determine rapidly which 

 individuals have the same haplotype. Only one rep- 

 resentative of each haplotype needs to be further 

 characterized, and large numbers of individuals can 

 be rapidly and efficiently screened. High-resolution 

 DNA screening techniques such as DGGE and het- 

 eroduplex analysis provide the sensitivity of DNA 

 sequencing and make it possible to screen greater 

 numbers of individuals for less cost and effort than 

 standard sequencing techniques. 



Our study employed a DGGE-heteroduplex assay 

 to screen regions of the mitochondrial and nuclear 

 genomes of the tautog for polymorphisms useful for 

 the analysis of population structure. Both the cyto- 

 chrome b and cytochrome c oxidase products, as well 

 as the LDH intron, appeared to be invariant in the 

 tautog samples, raising the question of whether the 

 DGGE-heteroduplex technique was providing the 

 high degree of resolution anticipated. 



In order to provide an independent assessment of 

 the DGGE-heteroduplex technique, we examined the 

 cytochrome b fragment from eight spot (Leiostomus 

 xanthurus), a species that has been shown to have 

 a high genetic diversity in a RFLP analysis of the 

 ATPase 6 and control regions (Lankford et al., 1999), 

 with the same protocol used to evaluate tautog sam- 

 ples. Four to six haplotypes were observed in the 

 eight individuals. It is clear that DGGE-heteroduplex 

 analysis is capable of revealing DNA sequence diver- 

 sity in amplified mtDNA, as has been reported by 

 other investigators (e.g. Campbell et al. 1995; Michi- 

 kawa et al. 1997; Tek Kay et al. 1997). 



