De la Rosa-Velez et aL: Genetic structure of Penaeus califomiensis and R stylirostns 



677 



phism (P<0.95) was 0.125 for P. califoniicnsis in the throe 

 populations sampleci along the Gulf of California, unlike 

 P. stylirostris that showed a clinelike pattern with increas- 

 ing polymorphism from 0.156 in the upper Gulf population 

 to 0.312 in the mouth of the Gulf The genetic diversity, 

 reported as expected heterozygosity, was quite homoge- 

 neous along the surveyed distribution of P. californiensis 

 and showed a gi-adual northern-southern increase in P. 

 stylirotitris. (Table 2). 



After the Bonferroni correction for multiple tests was 

 applied (a'=0.00142), only Est-2 was out of H-W equilib- 

 rium in at least one population of each species, and Pgm 

 and Pf-3 displayed the same pattern in P. stylirostris, all of 

 them accounting for heterozygote deficiency (Table 2). 



Four loci (Aph-3, Est-1, Est-4, and Esf-5) in the upper 

 population of P. stylirostris showed D = -1.000 values (see 

 Table 2); however, four loci had to be considered mono- 

 morphic because the polymorphism criterion applied was 

 the most conservative (Pgg; a locus is considered polymor- 

 phic only when the frequency of the most common allele 

 is <0.95). In most of these cases only one or two individu- 

 als were scored as homozygotes for the alternative allele. 

 Such a low frequency may not be significant for the evolu- 

 tionary process owing to the high probability of disappear- 

 ance through random processes in just a few generations. 



On the other hand, in spite of the high frequencies of 

 both alleles (p=0.5), only one heterozygous individual was 

 scored for the diallelic locus Aph-1 in the upper population 

 of P stylirostris, resulting in a highly significant heterozy- 

 gote deficiency. 



Standardized variance (F,,) analysis was achieved among 

 subpopulations within species (Table 3). Penaeus cali- 

 forniensis showed a lesser degree of genotypic differentia- 

 tion among its populations (F^,,=0.182) than P stylirostris 

 (F^,=0.372). This result is possibly related to the mean 

 frequency of private alleles (sensu Barton and Slatkin, 

 1986) in their populations (p=0.096 in P. californiensis; 

 and p=0.214 in P stylirostris), even though both figures 

 were notably high. Three loci accounted for differences 

 in the subpopulations of P californiensis: Aph-3. Est-1. 

 and Est-2. The higher genetic variability displayed by P 

 stylirostris produced a more complex pattern of genotypic 

 differentiation where ten loci accounted for significant 

 differences among subpopulations {Acph-3, Est-1. Est-2. 

 Est-3. Est-4. Est-5. G6pd, Lap. Pgm. and Pt-3). 



Genetic similarities among subpopulations showed a 

 similar pattern in both species (Table 4); there was a closer 

 resemblance between the middle Gulf and the mouth of 

 the Gulf populations of both species than with the upper 

 Gulf population. This pattern can be visualized in the den- 

 drogram (Fig. 2), where the upper Gulf population of both 

 species segregates from the other two, which are clustered 

 together. 



Furthermore, a different level of similarity among sub- 

 populations within species is clearly noticeable (Table 

 4). Penaeus californiensis showed a higher level of similar- 

 ity among its subpopulations (/ range: 0.988-0.997) than 

 P. stylirostris (/ range: 0.929-0.954). Genetic similarities 

 between species rendered a clustering level of 0.674 (Fig. 

 2, Table 4). 



Discussion 



Two results are of particular interest: a north-south cline- 

 like gi'adual increase in genetic variability in the more 

 coastal species and the finding of a heterogeneous distri- 

 bution of genotype frequencies in both assayed species. 

 Most of the previous studies on Penaeus species around the 

 world have depicted a general pattern of low homogeneous 

 genetic variability. For example, Mulley and Latter (1980) 

 reported heterozygosity values ranging between 0.006 and 

 0.033 in Australian penaeids (four species of Metapenaeus 

 and six of Penaeus). These data were confirmed by sub- 

 sequent studies: Richardson (1982) found an average het- 

 erozygosity of 0.028 in six populations of P latisculatus, 

 and Tam and Chu (1993) reported an observed heterozy- 

 gosity range of 0.007-0.049 in some species of Penaeus and 

 Metapenaeus from the South China Sea. Penaeus Japon- 

 icus exhibited one of the highest heterozygosity values 

 (//i,=0.047 ±0.029) among the species surveyed in that 

 study. However there are other reports of even greater 

 heterozygosity in populations of that species transported 

 to European hatcheries as broodstock {H„=0.102, Sbordoni 

 et al., 1986; //|,=0.071, Laubier et al., 1984). 



For penaeid species occurring in the western hemi- 

 sphere, Lester (1979) reported heterozygosities between 

 0.070 and 0.089 for three commercial penaeids of the Gulf 

 of Mexico. Very similar data for the same species were 

 later reported by Labacena et al. (19941. Lester (1983) 

 also studied one population of each species, P. vannamei 

 (from Chomes, Costa Rica) and P stylirostris (from Guay- 

 mas, Mexico), which dwell along the northeastern Pacific 

 coast, and reported heterozygosity values of 0.02 and 0.06, 

 respectively. Sunden and Davis (1991) reported hetero- 

 zygosity values for P. vannamei samples from Mexico, 

 Panama, Ecuador, and one farmed population at a Texas 

 hatchery which were 0.0173, 0.0172, 0.0208, and 0.0111, 

 respectively. 



Levels of genetic variability of the species that we 

 studied were not out of the range of those of previous 

 estimations. It must be noticed, in addition, that P. cali- 

 forniensis showed a lower, narrower heterozygosity range 

 (0.023 ±0.014-0.037 ±0.012) than P. stylirostris (0.038 

 ±0.021-0.086 ±0.027). The latter might be explained by 

 the different habitats that each species occupies during its 

 life cycle. This important difference may also be related to 

 the clinelike pattern of the heterozygosity values in P s(v/- 

 irostris, whereas the genetic variability of P californien- 

 sis could be evidence of the more stable oceanic conditions 

 that this species experiences during its life span. Penaeus 

 stylirostris appears to reflect the environmental variabil- 

 ity of the coastal lagoons, which it penetrates during a 

 critical stage of its life cycle. Latitudinal variability of 

 hydrological, ecological, and productivity conditions char- 

 acterize coastal lagoons along the eastern coast of the Gulf 

 of California. The upper zones coastal lagoons (Fig. 1) 

 are located in an arid region where vegetation is scarce; 

 around the lagoons, some halophytes and sea grasses pre- 

 dominate. Productivity in these basins depends almost 

 exclusively on microalgae (phytoplankton and microphy- 

 tobenthos) (Contreras, 1985). To the south, through the 



