De la Rosa-Velez et al : Genetic structure of Penaeus cahforniensis and P styliiostris 



681 



ago, when the Hoover Dam 

 was constructed up river and 

 flow was severely restricted. 

 Presently, hypcrsaline condi- 

 tions prevail in the upper 

 Gulf region. This dynamic and 

 extreme environment is topo- 

 graphically bounded to the 

 south by a submarine range. 

 Tiburon and Angel de la 

 Guarda Islands are the heights 

 of this submarine range (Fig. 

 1); south of these islands the 

 Gulf increases in depth to 

 around 3000 m in the Guay- 

 mas Basin. Thus, the islands 

 may be considered a geograph- 

 ical or ecological boundary 

 that reduces the free flow of 

 penaeids. Correa-Sandoval and 

 Cai"vacho-Bravo (1992) came 

 to a similar conclusion when 

 they described the distribution 

 pattern of brachyuran crabs in 

 the Gulf of California. 



In some populations of both 

 species, three loci {Aph-1, Est-2, and Pgm) were found to 

 be out of H-W equilibrium according to the chi-square 

 goodness-of-fit test. Two of the common causes of hetero- 

 zygote deficiency, as discussed by Zouros and Foltz ( 1984), 

 can be invoked here: 1 ) the Wahlund effect, because some 

 genetic structure has been demonstrated in both species, 

 at least in the geographic range that we studied; thus, 

 different genetic compositions taking part in reproductive 

 events, will yield such a pattern; and 2) selection against 

 heterozygotes, a hypothesis that is difficult to prove, but 

 is feasible owing to very recent environmental modifi- 

 cation (ca. half a century). Homozygous genotypes may 

 be selected if they perform better under extreme condi- 

 tions with no drastic gene erosion during the little time 

 elapsed since environmental alterations began. For exam- 

 ple, Ap/;-i displays a ca. 50'7f frequency of the two segre- 

 gating alleles, which allows us to expect high heterozygote 

 frequency. However, the sample lacked these genotypes 

 almost completely. With no additional evidence to discard 

 either of the above, a third one, i.e. the presence of a null 

 allele segregating in this population, may also be invoked. 

 Further evidence from breeding experiments among indi- 

 viduals of this population is needed to evaluate this sup- 

 position. 



Additional evidence of the divergence of populations 

 (remarkably, populations of both species dwelling in the 

 upper Gulf) is given by Nei's genetic similarity (Table 4; 

 Fig. 2). It is evident from the dendrogram that 1) there is a 

 clear subdivision in both within-populations dendrograms 

 that distinguishes the populations inhabiting the upper 

 Gulf (hence, it is not only the previously discussed char- 

 acteristic of heterozygote deficiency that segregates these 

 populations from the rest, but the distribution of their 

 genes too) and 2) a subdivision between species where the 

 degree of genetic similarity is similar to those previously 



reported in other penaeid species. Average genetic simi- 

 larities among species within Penaeus genera were 0.65 

 ±0.08, and 0.69 ±0.08 within Metapenaeus (Mulley and 

 Latter, 1980). Similar values (7=0.66) were found between 

 two forms of brachyuran freshwater crabs from South 

 Africa (Stewart and Cook, 1998). 



The characteristic genetic features we found provide 

 additional support for the recent scheme proposed by Perez- 

 Farfante and Kensley (1997). Their proposition, regarding 

 the reorganization of American shrimps in the family 

 Penaeidae, involves the promotion of an existing subgenus 

 to genus, i.e. Penaeus califomiensis would become Far- 

 fantepenaeiis califo?-niensis, and Penaeus stylirostris would 

 change to Litopenaeus stylirostris. However, more evidence 

 must be compiled to support this reclassification. 



Complexity of the Gulf of California ecosystem, as well 

 as the biological features described for each species, may 

 account for the greater degree of genetic structure in the 

 species that inhabit it. Fisheries and hatcheries manag- 

 ers can take advantage of such information, for example, 

 by defining stocks or selecting the more variable popula- 

 tions to be submitted to artificial selection programs in the 

 aquaculture scheme. 



Acknowledgments 



We thank the useful and insight comments of two anony- 

 mous reviewers which substantially improved the paper 

 M. A. Cervantes and L. M. Enriquez processed the data 

 and prepared the tables and figures. This study was par- 

 tially funded by Centro Internacional de Biologia Molecu- 

 lar y Celular, A. C. One of us (R.E.-F ) received support 

 from Consejo Nacional de Ciencia y Tecnologia during the 

 study. 



