LEARNED , J.L.; SHUSTER, S.M; Northern Arizona University: Geographic isolation and genetic differentiation among nine populations of the freshwater isopod, Thermosphaeroma subequalum (Cole and Bane)
The eight described species of Thermosphaeroma isopods inhabit warm springs in Mexico, Texas, and New Mexico. Thermosphaeroma subequalum, the easternmost described species of the genus, is found in nine hot springs located on both sides of the Rio Grande River over a distance of approximately 150 kilometers. To determine if the nine populations of T. subequalum are isolated within individual springs or represent one continuous population, we examined genetic variation at 8 allozyme loci using starch gel electrophoresis. We then performed a series of analyses including basic descriptive statistics, Wright�s F-statistics, and a regression analysis using Nei�s unbiased genetic distance (1978) as well as geographical distance for all pairs of populations. Our calculated F-statistics suggest that gene flow is restricted among populations of T. subequalum; the relative magnitudes of FIT (0.319) and FIS (0.175) indicated a deficit of heterozygotes while a moderately high FST value (0.175) indicated significant differentiation among populations. The correlation between genetic and geographical distance was significant and positive suggesting that isolation by distance is likely to be a major barrier to gene flow for this species.
