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Fishery Bulletin 90(3). 1992 



the analysis, distributed among the remaining 25 in- 

 dividuals. In each case, variant genotypes were close- 

 ly related to the common genotype, differing by no 

 more than two restriction site changes. Restriction site 

 gains or losses were inferred from completely additive 

 changes in fragment patterns. No length polymor- 

 phisms or heteroplasmy were observed. A total of 43 

 restriction sites were detected in the 6-enzyme survey, 

 and the average individual was scored for 29 sites, 

 representing about 1.0% of the weakfish mtDNA 

 genome. 



The common mtDNA genotype, AAAAAA, occurred 

 in the great majority of fish in all samples, ranging in 

 frequency from 0.905 (DE88) to 0.960 (DE89, S091), 

 with a value of 0.932 for the pooled sample (Table 2). 

 The next most-common genotype, AAAAAC, occurred 

 in the pooled sample at a frequency of 0.017, and was 

 present in 4 of the 7 samples. Because of the predom- 

 inance of a single genotype in all samples, nucleon 

 diversities were relatively low (Table 3), ranging from 

 0.079 (DE89) to 0.180 (DE88), with a value of 0.130 

 for the pooled samples. As all variant mtDNA geno- 

 types were related to the common genotype by no more 

 than 2 restriction site changes, the percent mean 

 nucleotide diversity within each sample was also quite 

 low (Table 3), ranging from 0.06 (DE89) to 0.18 (NY89). 



Little genetic differentiation was detected among 

 weakfish samples collected at the same location over 

 2 or more years (Table 4). Among samples collected in 

 Delaware during 1988, 1989, and 1991, and in New 

 York during 1988 and 1989, the percent mean nucleo- 

 tide sequence divergences, corrected for within-sample 

 variation (Nei 1987), ranged from 0.00 (DE88/DE91, 

 NY88/NY89) to 0.01 (DE89/DE91). 



Little genetic differentiation was encountered among 

 samples of weakfish collected along the mid-Atlantic 

 coast. The nucleotide sequence divergences among 

 samples collected at geographically distant sites dur- 

 ing the same spawning season ranged from 0.00 

 (NY88/NC88) to 0.03 (NY89/DE89). These values are 

 of the same magnitude as those found among samples 

 of weakfish collected at the same site over 2 or more 

 years, indicating a lack of spatial genetic structuring. 



An analysis of the distribution of mtDNA genotypes 

 also revealed no significant heterogeneity among tem- 

 poral or spatial collections. To avoid a bias caused by 

 including expected values <1, we initially pooled all 

 alternate genotypes for an analysis of heterogeneity. 

 The results of a G-test (Sokal and Rohlf 1981) revealed 

 no significant spatial or temporal differences in the 

 distribution of the common and pooled rare genotypes 

 among the 7 samples (Gh 2.88, 0.75>p>0.50). Ex- 

 panding the analysis to include the common genotype 

 and all 10 rare genotypes separately (each with ex- 

 pected values < 1 in one or more collections) did not 



significantly change the outcome. Once again, the null 

 hypothesis of homogeneity could not be disproved 

 (Gh 51.62, 0.50>p>0.25). 



The low level of variation detected in our analysis 

 of weakfish mtDNA could be the result of many fac- 

 tors. After reviewing the 1988 and 1989 results, we 

 felt that we might have biased our estimates of mean 

 nucleotide sequence divergence by using 6 restriction 

 endonucleases that, by chance, were not variable within 

 weakfish. To test this hypothesis, we analyzed the 

 DE91 and S091 weakfish collections with an additional 

 7 restriction endonucleases. The average individual in 

 the 13-enzyme analysis was scored for 65 restriction 

 sites, or approximately 2.4% of the weakfish mtDNA 

 molecule. Of the 49 fish in the two samples surveyed, 

 only one variant mtDNA genotype was found (one fish 

 from the S091 sample with the common 6-enzyme 

 mtDNA genotype exhibited a site gain relative to the 

 common pattern for the enzyme Bell). As a result. 



