Ward et al Genetic variability in Cynoscion nebulosus, determined with DNA markers 



203 



seatrout from Indian River. Florida (which is near St. 

 John's River), to be genetically more similar to spot- 

 ted seatrout from Choctawhatchee Bay in the Florida 

 Panhandle than to other Atlantic Coast fish of the 

 same species. The analyses of the two studies, taken 

 together, indicate at least two distribution breaks in 

 the eastern Gulf and the Atlantic, the first between 

 Georgia and the upper Atlantic Coast of Florida and a 

 second between Charlotte Harbor and Tampa Bay. The 

 clustering of populations observed by Wiley and Chap- 

 man (2003) between Indian River and Choctawhatchee 

 Bay may, in light of our finding of a genetic discontinuity 

 between the intervening Charlotte Harbor and Tampa 

 Bay, reflect relative differences in genetic affinity dis- 

 cerned by the two data sets. Resolution of this possible 

 incongruence will require examination of numerous 

 sampling sites collected from both the Atlantic and Gulf 

 coasts of Florida. 



Spotted seatrout inhabiting the northwestern Gulf of 

 Mexico from the Laguna Madre to Grand Isle, Louisi- 

 ana, were not found to be subdivided into discernible 

 stocks or populations and there was little indication of 

 temporal differentiation within bays. Exceptions to this 

 lack of temporal differentiation were seen in Galveston 

 Bay and in Louisiana. Temporal differences in these 

 two sites may be due to sampling error (although the 

 n for each year's sample in the two sites appear to 

 be adequate), or these two large regions may harbor 

 populations that are temporally or spatially genetically 

 structured. Some indications of geographically coherent 

 spatial patterns were observed among spotted seatrout 

 in the northwestern Gulf of Mexico. For example, there 

 was an indication that this species on the lower coast 

 is genetically differentiated, albeit weakly, from conspe- 

 cifics inhabiting bays on the middle and upper Texas 

 coast. This finding is similar to that found in allozyme 

 (King and Pate, 1992) and mtDNA data (Gold et al., 

 1999) where differences between Laguna Madre sam- 

 ples and more northerly bays were observed. Galveston 

 Bay spotted seatrout were also found to be genetically 

 divergent, being genetically distinct from spotted seat- 

 rout in all other bays in 1999 and most bays in 2000. 

 Galveston Bay was, in addition, one of two sites where 

 the genetic structure of spotted seatrout was found to 

 be temporally heterogeneous. Gold et al. (2003) found 

 the upper Texas Coast to be a region of genetic transi- 

 tion; a notable shift in allele frequencies of the Soc201 

 locus was evident between Matagorda Bay and Sabine 

 Pass — a span that includes Galveston Bay. 



The lack of genetic population subdivision in the 

 northwestern Gulf is consistent with the observed de- 

 crease in heterozygosity in relation to Hardy-Weinberg 

 expectations. Similar heterozygote deficiencies in white 

 seabream (Diplodus sargus) (Lenfant and Planes, 2002) 

 were hypothesized to represent mixing of genetically 

 disparate individuals during some stage of recruit- 

 ment (the Wahlund effect). This phenomenon may be 

 characteristic of many marine fishes, especially those 

 with local populations recruited from a larval stage 

 with highly dispersive capabilities. Spotted seatrout. 



SL99 



SL97 



0.01 



SLOG 

 SA99 

 CC99 

 SL98 

 M98 



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LAOO 

 GB98 



scTCm 



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 MB98 



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EMOO 

 LA99 



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 GBOO 



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74 



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Figure 2 



Cavalli-Sforza and Edwards (1967) chord distance 

 neighbor-joining phenogram calculated for 33 spatial 

 and temporal samples of spotted seatrout ^Cynoscion 

 nebulosus). Values along branches indicate bootstrap 

 values as percentages of replicates, based on locus (above 

 line) or individual (below linel. Letter codes are defined 

 in Table 1. 



due to its unique life history characteristics, is not 

 an obvious example of a marine species expected to 

 exhibit high gene flow. Spotted seatrout are confined 

 to nearshore waters, spend most of their life within an 

 estuarine habitat, and spawn and select their nursery 

 area within the estuary (McMichael and Peters, 1989). 

 Results of tagging studies support the hypothesis of a 

 natal bay affinity based on spotted seatrout life his- 

 tory (e.g.. Baker and Matlock. 1993). Stock structure 

 among spotted seatrout was detected by the studies of 

 morphology, physiology, (Iverson and Tabb, 1962), and 

 genetics (Gold et al., 1999, and references therein). 

 This population structure was not detected in the Gold 

 et al. (2003) or the present analyses of variation in 

 microsatellite DNA loci — markers expected to yield a 

 high-resolution analysis of population-level genetic vari- 

 ability. The differences observed between microsatellite 

 variability and that seen in allozymes (Weinstein and 

 Yerger, 1976; Ramsey and Wakeman, 1987; King and 

 Pate. 1992) and mtDNA (Gold et al., 1999) may reflect 

 different evolutionary processes (Gold et al., 2003). 



