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Phosphoglucose Isomerase Polymorphism and Natural Selection in the Sand Crab, Emerita talpoida

Kendall W. Corbin
Evolution
Vol. 31, No. 2 (Jun., 1977), pp. 331-340
DOI: 10.2307/2407755
Stable URL: http://www.jstor.org/stable/2407755
Page Count: 10
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Phosphoglucose Isomerase Polymorphism and Natural Selection in the Sand Crab, Emerita talpoida
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Abstract

Horizontal starch gel electrophoresis was used to characterize the allelic variation of the phosphoglucose isomerase locus of approximately 1200 sand crabs, Emerita talpoida. Individuals were collected from 14 localities at intervals of about 100 miles along the Atlantic coast from Cape Cod, Massachusetts to the southern tip of mainland Florida. Ten PGI alleles were found in addition to a null allele category, and 24 of the 66 possible genotypes were observed. Three alleles (Pgia, Pgib, Pgic) were present at frequencies above 0.1 in all 14 localities. A comparison of the zygotic distributions with those expected on the basis of Hardy-Weinberg equilibria showed that the number of Pgia/c heterozygotes was less than expected in all localities. Overall χ2 values for deviations from expected distributions were significant in approximately 40 percent of the localities. For all localities combined the deficiency for Pgia/c was significant at the .005 level. Zygotic distributions are neither significantly different among localities nor among the size classes within localities. However, the frequencies of Pgia and Pgib are significantly related to latitude. In regression analyses, the variation in the frequencies of the two, most common PGI alleles is significantly related to extremes in temperature and salinity. Multiple regression coefficients for frequencies of Pgia and Pgib regressed against either mean low temperature or mean high salinity are significant. For the zygotic combinations, Pgia/a is significantly correlated with mean low salinity, Pgia/b is significantly related to the combination of mean high temperature and mean high salinity and Pgia/c is significantly related to mean low temperature. The differential mortality incurred by Pgia/c does not occur after the animals reach 4 mm in carapace length. The cause of this differential mortality, which apparently occurs during an earlier stage in the life cycle, is unknown. Environmental gradients are such that Pgia/a homozygotes may have a higher fitness in the northern part of the distribution and also possibly during winter months. Pgib/c heterozygotes seem most fit in the southern part of the distribution and during the summer months.

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