340 



Fishery Bulletin 98(2) 



One of the composite haplotypes was found only 

 in a single individual from Virginia, whereas the 

 remaining three were found in all three geographi- 

 cal regions. Mean within-sample haplotype diversity 

 was 0.6905 (±0.00184), and mean nucleotide diver- 

 sity was 0.000782 (Table 4). 



Corrected nucleotide divergence between samples 

 varied between -0.0077 and -0.0081%, suggesting 

 the absence of population structuring along the mid- 

 Atlantic Coast. This inference is further supported 

 by exact log-likelihood analysis of the distribution of 

 the four composite haplotypes across the three sites 

 (likelihood ratio statistic==2.82, 6 df, exact P=1.00). 

 A similar analysis of the larger data set (n=12, only 

 polymorphic sites examined) indicated no significant 

 heterogeneity (likelihood ratio statistic=6.05, 6 df, 

 exact P=0.469). Lack of population differentiation 

 was further supported by a mean Fg-j- value of -0.005 



•ilillt 



Figure 1 



Denaturing gradient gel of tautog cyt b PCR products. Each lane represents the 

 pooling of PCR products from two individuals, treated to induce heteroduplex 

 formation when two different DNA sequences were present. The single homo- 

 duplex bands in lanes 1-8 (from left to right I indicate sequence identity for the 

 nine tautog examined in pairwise combinations. Lanes 9-10 (the two lanes far- 

 thest to the right) illustrate the combination of the common haplotype and one 

 rare haplotype. Tlie two lower (faster-migrating) bands represent alternative 

 homoduplex molecules; the upper two bands are the heteroduplex molecules. 



(i.e. zero), and the analysis of molecular variance 

 (AMOVA), which found 100% of total haplotype vari- 

 ation to occur within populations. 



DGGE-heteroduplex analysis 



For the entire sample of 72 individuals, analysis of 

 the cyt b fragment revealed a total of three hap- 

 lotypes, two of which were unique and found only 

 in single individuals from Delaware and Virginia 

 (Fig. 1). Sequence analysis of the three cyt b haplo- 

 types revealed that the two unique haplotypes dif- 

 fered from the dominant haplotype by synonymous 

 single base substitutions. The unique Virginia hap- 

 lotype differed from the common haplotype by an A 

 to G transition at base 276, whereas the unique Del- 

 aware haplotype resulted from the same substitu- 

 tion at base 291 (Fig. 21. 



All 72 tautog appeared to possess a single COI hap- 

 lotype, a result consistent with the extensive restric- 

 tion enzyme screening of this region performed on 

 the original subset of 24 individuals. The LDH intron 

 was also relatively invariant. Of the 72 fish screened, 

 DGGE-heteroduplex analysis revealed only a single 

 heterozygous individual, collected in the Delaware 

 region. 



Discussion 



RFLP analysis revealed a mean mtDNA haplotype 

 diversity value (0.6905 ±0.00184) for tautog that 

 is intermediate within the range of 

 values reported for other marine 

 fishes. However, the overall nucleo- 

 tide diversity value (0.078%) for the 

 tautog is one of the lowest reported 

 for a marine fish. It should be noted 

 that values for haplotype diversity are 

 affected by the number of restriction 

 enzymes used and should be compared 

 across studies with caution. In addi- 

 tion, values for haplotype and nucleo- 

 tide diversity may be biased upwards 

 in analyses that include only polymor- 

 phic enzymes. 



Low levels of intraspecific sequence 

 diversity in mitochondrial genes are 

 generally attributed to low effective 

 population size (N^Jor to reduced rates 

 of mutation (Ovenden, 1990). Effective 

 population sizes inferred from mtDNA 

 data are often orders of magnitude 

 smaller than present-day census esti- 

 mates; this disparity may reflect the 



