Natural Selection in the Sow Bug Killer Spider Dysdera crocata
Authors:Amy de Harde, Kayla Saunders
Mentor:Dr. Martina Ramirez, Professor, Loyola Marymount University
In 2009-2010, we determined the phosphoglucomutase (PGM) genotypes for a large sample (n = 333) of D. crocata from Kenneth Hahn State Recreation Area, Los Angeles, CA. We found that there was a significant reduction or absence of heterozygotes at this locus at 9 of 12 sites in the park which were sampled. This may be due to selection against heterozygotes. To determine if this is the case, we then documented the representation of heterozygotes and homozygotes among life stages in a new sample of D. crocata (n = 339) collected in summer 2010. This analysis has shown that while PGM genotype frequencies did not violate Hardy-Weinberg expectations for either adult females or the oldest juveniles, there were significant violations in the form of a reduction or absence of heterozygotes for adult males and younger juveniles. Thus, for juveniles, there is an excess of homozygotes that diminishes as they age, resulting in genotype ratios that match Hardy Weinberg expectations. As for adult spiders, while females are also in conformance with such expectations, adult males are exclusively homozygotes. While this outcome suggests that natural selection is operating at the PGM locus in D. crocata, it is unclear what mechanisms would simultaneously act to disfavor or favor heterozygotes, depending on the life stage and/or gender.