Tringali and Wilson mtDNA analysis of two stocks of Sardmella aurita 



367 



within the geographic regions sampled. Thus, the evi- 

 dent lack of significant heterogeneity in mtDNA 

 haplotype frequencies among the Gulf specimens of S. 

 aurita is probably not due to an inadequate level of 

 mtDNA genome sampling relative to the sequence vari- 

 ability present. 



Accepting that the mtDNA variability among sample 

 locations of S. aurita was sufficiently characterized, at 

 least two possibilities could account for the observed 

 genetic homogeneity between our sample locations in 

 eastern Gulf of Mexico. This lack of genetic structur- 

 ing is the result of recent or ongoing genetic exchange 

 between the sampled locations, or there may have been 

 insufficient time for discrete Gulf populations, if they 

 do exist, to diverge through mtDNA lineage sorting. 

 The first explanation seems the more likely one, con- 

 sidering the life history of the species. S. aurita is 

 known to be highly migratory (Hildebrand 1963, 

 Simpson & Gonzalez 1967) and to spawn repeatedly 

 throughout much of the year (Grail 1984). Although 

 the pattern of adult migration of S. aurita in the east- 

 ern Gulf is still poorly understood, spawning occurs 



continuously between the Panhandle and Tampa Bay 

 with no apparent geographic partitioning in either the 

 distribution or abundance of eggs or larvae (Houde 

 1976). There are no known physical barriers to dis- 

 persal or migration in this region. 



Significant geographic variation in mtDNA haplotype 

 frequencies does exist between Spanish sardines in 

 the eastern Gulf of Mexico and waters off southern 

 Brazil, based upon the results of goodness-of-fit test- 

 ing. This variation is due to two related factors. One is 

 that the haplotype most common among the pooled 

 specimens from the eastern Gulf of Mexico, 

 AAAAAAAAA, possessed by 46% of the specimens, has 

 a much lower occurrence (6%) among the specimens 

 from southern Brazil. The other is the difference in 

 the distribution of the Pst-l digestion pattern. Whereas 

 5 of 16 (31%) of the Brazilian specimens possessed the 

 G-type pattern (Table 2), this pattern was absent in 

 the Gulf specimens (re=57). Relative to the common A- 

 type pattern of Pst-l present among the Gulf speci- 

 mens, this G-type pattern represents a site gain at 

 Pst-I. 



