Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Robert U. "Bud" Fischer


The study was conducted to determine the effects of lowered genetic variance in the Western mosquitofish (Gambusia affinis) on the physiological parameters of critical thermal maximum and minimum pH. Mosquitofish were obtained from two small central Illinois ponds, Lost Pond and Shadow Pond, by seines and dip nets. Once caught, fish were returned to the laboratory and acclimated for a minimum of five days at a temperature of 27°C and a pH of 8.25. At the beginning of each experimental run, fish were subjected to the physiological experiments for determination of critical thermal maximum and minimum pH, and then measured for length and weight. At the completion of each trial, mosquitofish were frozen at -70°C for later electrophoretic analysis. The electrophoretic analysis at eighteen loci using two buffers revealed that Lost Pond mosquitofish had a significantly higher mean heterozygosity of 0.118 ± 0.046 (F1,128 = 9.64, P = 0.002) than Shadow Pond mosquitofish 0.065 ± 0.027. The Lost Pond mosquitofish population had a mean number of alleles per locus of 1.4 ± 0.1 and six polymorphic loci (G3pdh, Mpi, Pgdh, Ldh-1, Ldh-2, Np), while the Shadow Pond population had a mean number of alleles per locus of 1.6 ± 0.1 and three polymorphic loci (Mpi, Ldh-1, Ldh-2). The physiological experiments show that Shadow Pond mosquitofish had a significantly higher mean CTMax of 42.2 ± 0.16°C (Z = 4.46, P < 0.0000) than Lost Pond mosquitofish 40.7 ± 0.26°C, while Lost Pond mosquitofish exhibited a significantly lower mean minimum pH of 1.42 ± 0.025 (Z = 3.57, P = 0.0002) than Shadow Pond mosquitofish 1.82 ± 0.025. An analysis of covariance showed that structural size (F = 0.048, P = 0.827) and heterozygosity (F = 0.500, P = 0.685) did not significantly affect CTMax but weight (F = 6.241, P = 0.018) and population (F = 5.30, P = 0.028) had a significant effect. An ANCOVA on minimum pH showed no significant effect of weight (F = 1.237, P = 0.274) or heterozygosity (F = 0.749, P = 0.530), a marginal affect of structural size (F = 3.385, P = 0.075) and a significant effect of population (F = 72.652, P < .0001). Thus, because CTMax and minimum pH for both populations of mosquitofish from the two sites was not correlated with heterozygosity, we may be observing physiological or genetic adaptations in response to local environmental conditions.