Increased Heterozygosity at the Mdh-B Locus in Fish Inhabiting a Rapidly Fluctuating Thermal Environment

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Abstract
Populations of a common forage fish, red shiner Notropis lutrensis, were sampled from four localities on the Brazos River, Texas, affected by cold‐water discharge from a hydroelectric dam and from unaltered sites in the same region. Polymorphism at the Mdh‐B locus, encoding supernatant malate dehydrogenase, indicates that populations within 57 km of the dam are distinctive from other regional populations and possess a unique Mdh‐B allele, have significantly higher levels of heterozygosity at the Mdh‐B locus, represent a homogeneous set that have significantly different Mdh‐B zygotic frequencies from other regional populations, and have significantly different Mdh‐B zygotic proportions than would be expected under a Hardy‐Weinberg equilibrium. Increased levels of heterozygosity in fish within 57 km of the dam were correlated with discharge‐associated fluctuations in water temperature at sampling stations.