POLLUTION-TOLERANT ALLELE IN FINGERNAIL CLAMS (MUSCULIUM-TRANSVERSUM)

For nearly 50 years, the fingernail clam (Musculium transversum) was b elieved to be virtually eliminated from the Illinois River. In 1991, w orkers began finding substantial populations of M. transversum in the Illinois River including several beds in and around the highly pollute d Chicago Sanitary District. In order to determine if populations of M . transversum from polluted sites exhibited any genetic response to th e high levels of toxins and to examine the genetic structure of severa l populations of M. transversum for any changes due to the population crash, starch-gel electrophoresis was performed on M. transversum from three Illinois River localities and four Mississippi River basin loca tions. The sampled populations produced an inbreeding coefficient (F-I S) of 0.929, indicating that the populations were highly inbred. The r esults of a suspected founder effect due to a bottleneck was suggested by an F-ST = 0.442. The isozyme Glucose-6-phosphate isomerase-2 (Gpi- 2) produced allelic frequency patterns that were consistent with expec ted patterns of a pollution-tolerant allele. Polluted sites exhibited elevated frequencies of Gpi-2(100) whereas nonpolluted sites exhibited elevated frequencies of Gpi-2(74). This frequency pattern suggested t hat natural selection was occurring in populations under severe toxic pressures, leading to an increase in the frequency of the allele Gpi-2 (100) Therefore, Gpi-2(100) is a possible pollution-tolerant mutation in M. transversum.