MHC VARIATION IN THE ENDANGERED GILA-TOPMINNOW

Sequence variation at a major histocompatibility complex (MHC) gene, a ssumed to be involved in parasite and pathogen resistance, was examine d in the endangered Gila topminnow (Poeciliopis o. occidentalis), from the four watersheds where they remain in the United States. This is t he first estimate of variation in MHC genes in natural populations of an endangered species. The population that has experienced the most bo ttlenecks historically was monomorphic for MHC variation. Another popu lation, which earlier had been found to be the only population polymor phic for allozymes, had five MHC alleles, four different from those fo und in the other populations. Overall, nine different alleles were fou nd. The four populations were highly divergent at MHC with four of the six population pairs not sharing any alleles. However, the magnitude of differentiation between populations on the amino-acid level varied fivefold for the populations that shared no alleles. Using single-stra nded conformational polymorphism (SSCP), these alleles segregated cons istently with Mendelian expectations in families. Because of the high genetic differentiation between these populations for a potentially ad aptive gene, we recommend that the four watersheds be examined further for separate conservation and management.