A Comparison of Genetic Variation in Two Endemic Thermal Spring Isopods,Thermosphaeroma thermophilumandT. milleri(Crustacea - Isopoda: Sphaeromatidae)

Populations with reduced gene flow and restricted population size are expected to show reduced genetic variation. Using starch gel electrophoresis, we examined allozyme variation at 12 loci in two species of freshwater, sphaeromatid isopods. Thermosphaeroma thermophilum , an endangered species, inhabits a single thermal spring in central New Mexico, USA; and T. milleri , inhabits a more complex thermal spring system in northern Chihuahua, México. We found no significant differences in allelic variation between the sexes within each species. Between species, electromorphs at each locus differed significantly in both number and moiety on the gel, with T. milleri showing greater polymorphism and greater heterozygosity than T. thermophilum . Nei’s unbiased genetic distance, calculated using the nine loci common to both populations ( D = 0.75), was consistent with morphological classification of T. thermophilum and T. milleri as separate species, as well as with molecular analyses suggesting that these populations have been separated since the late Cretaceous (88 myr). Moreover, consistent with the theoretical expectation that small, isolated populations will exhibit reduced genetic variation, T. thermophilum , an endangered species, exhibited significantly less genetic variation than the more numerous and less confined T. milleri . We compare our results with other recent studies using this approach to understand the population genetics of natural populations.