Graves et a\ Genetic analysis of Cynoscion regalis stock structure 



473 



the nucleon diversity, which is sensitive to the number 

 of enzymes employed, increased for only one of the two 

 samples. The within-sample percent mean nucleotide 

 sequence diversity, which is not as sensitive to the 

 number of restriction sites surveyed, was slightly lower 

 for both samples (Table 4), and the corrected values 

 of percent mean nucleotide sequence divergence be- 

 tween the DE91 and S091 samples was essentially the 

 same whether based on 6 or 13 informative restriction 

 enzymes. 



Discussion 



The presence of alleles unique to samples from par- 

 ticular geographic locations has been used to infer in- 

 traspecific genetic structuring and to determine levels 

 of gene flow among collection sites (Slatkin 1989). This 

 model assumes that increased frequencies of "private" 

 alleles are a direct result of limited gene flow. An in- 

 spection of the above data reveals several genotypes 

 that are present at very low frequencies (Table 2); 

 however, it is interesting to note that most genotypes 

 represented by more than a single individual occur in 

 two or more geographic samples. For example, the 

 genotype AAAAFA was encountered three times in 

 the analysis of 370 weakfish, occurring in the NY88, 

 NC88, and DE89 samples. The lack of spatial partition- 

 ing of rare alleles is strongly suggestive of a high rate 

 of gene flow among collection locations. 



The level of mtDNA variation found within the weak- 

 fish is among the lowest reported for any species of 

 fish. While it is difficult to compare nucleon diversities 

 from different studies because the value is dependent 

 upon the number of restriction sites surveyed, relative 

 levels of variability can be determined from com- 

 parisons of studies involving about the same numbers 

 and types of restriction endonucleases. The nucleon 

 diversity of the 1991 weakfish samples surveyed with 

 13 enzymes was 0.157, a value that falls well below the 

 range of 0.473-0.998 reported by Avise et al. (1989) 

 for other fishes analyzed with about the same number 

 of enzymes. The nucleon diversity of the weakfish was 

 also substantially below the mean value of 0.943 re- 

 ported for the red drum Sciaenops ocellatus (Gold and 

 Richardson 1991). Relatively low values of nucleon 

 diversity have been found for the black drum Pogonias 

 cromis (0.584) and the spotted seatrout Cynoscion 

 nehulosus (0.531), a congener of the weakfish (C. Fur- 

 man and J.R. Gold, Texas A&M Univ., College Station, 

 pers. commun., Aug. 1991). However, these values are 

 still substantially larger than those we found for the 

 weakfish. Comparisons of nucleotide sequence diver- 

 sities among these species also indicate that the 

 weakfish is relatively depauperate in terms of mtDNA 



variation. The mean percent nucleotide sequence diver- 

 sities within 7 black drum samples (0.142) and 5 spotted 

 seatrout samples (0.222) are substantially higher than 

 that within the 7 weakfish samples (0.10) surveyed in 

 this study. 



The finding of relatively low levels of mtDNA varia- 

 tion within the weakfish is consistent with the lack of 

 allozyme variation reported by Crawford et al. (1989). 

 Low levels of mtDNA variation have generally been 

 attributed to small effective population sizes of females, 

 resulting in relatively rapid sorting of gene trees (Nei 

 1987, Avise et el. 1988, Chapman 1990, Bowen and 

 Avise 1990). Variations in weakfish abundance over the 

 last 110 years have been reflected in commercial 

 catches, which have fluctuated from a high of 44.5 

 million pounds in 1908 to a low of 3.1 million pounds 

 in 1967 (Vaughan et al. 1991), but it is unlikely that 

 such variations over recent history have drastically 

 reduced the effective population size of female weak- 

 fish. Population bottlenecks on a larger time-scale (e.g., 

 glaciation events) or cyclical fluctuations in population 

 size may have resulted in the reduced genetic diver- 

 sity within the weakfish, but such explanations are 

 merely speculative and do not necessarily agree with 

 the observation that other sciaenids with similar 

 distributions and life histories do not exhibit such low 

 levels of mtDNA diversity. 



Reductions in effective population size can also occur 

 due to differential reproductive contribution, resulting 

 from skewed sex ratios, limited mating opportunities, 

 or varying of survival among progeny. While little is 

 known of weakfish spawning behavior or differential 

 recruitment success, the sex ratio tends to be very close 

 to 1.0 (Wilk 1979). Thus, the cause or causes con- 

 tributing to the low genetic variation observed among 

 weakfish relative to other fishes is problematic. 



In addition to low levels of within-sample variation, 

 we detected little temporal or spatial genetic differen- 

 tiation among weakfish samples. Because there were 

 few variant mtDNA genotypes, and almost all of the 

 rare variant genotypes occurred in more than one 

 population, the uncorrected mean nucleotide sequence 

 divergences among weakfish samples were of the same 

 magnitude as mean nucleotide diversities found within 

 samples. Thus, the mean difference among mtDNA 

 genotypes randomly drawn from within a single sample 

 was equivalent to the mean difference among mtDNA 

 genotypes drawn from different samples. 



Low levels of within-group mtDNA variation do not 

 preclude the occurrence of significant between-group 

 differentiation. Bowen and Avise (1990) recently re- 

 ported low values of mtDNA diversity within samples 

 of Atlantic and Gulf of Mexico black sea bass Cen- 

 tropristis striata (within-sample percent nucleotide 

 sequence diversity of 0.03), yet their study revealed 



