Correlation between mating system and distribution of genetic variation inUtterbackia (Bivalvia : Unionidae)

Variation in mating systems (e.g., dioecy versus hermaphroditism, self-fert ilization versus cross-fertilization) has been shown to affect the distribu tion of genetic variation in plants. However, the paucity of this type of v ariation in closely related taxa has hampered similar evaluations in animal s. The freshwater bivalve genus Utterbackia (Unionidae: Anodontinae), curre ntly comprising three nominal and one undescribed species, contains gonocho ric (dioecious) as well as hermaphroditic species and thus is a model anima l system for examining mating system evolution and its effects on the distr ibution of genetic variation. Comparisons of the level of within- and among -population allozymic variation (at 9 putative genetic loci) in the simulta neous hermaphrodites Utterbackia imbecillis (23 populations, 331 individual s) and U. "imbecillis" (4 populations, 51 individuals), with those of the g onochoric U. peggyae (7 populations, 99 individuals) and U. peninsularis (6 populations, 77 individuals) allowed inferences to be made regarding (1) t he mating system of U. imbecillis and U. "imbecillis" and (2) the populatio n genetic structure of these four species. The low levels of within-populat ion variation and marked heterozygote deficiency observed in U. imbecillis and U. "imbecillis" relative to that in U. peggyae and U. peninsularis, sug gest that there is a high degree of self-fertilization in both hermaphrodit ic species. However, the among-population variation in the level of heteroz ygote deficiency (Selander D range: -0.181 to -1.000) is consistent with th e hypothesis that the relative amounts of cross-fertilization and self-fert ilization vary among populations of U. imbecillis and U. "imbecillis." The hypothesis of high levels of self-fertilization in U. imbecillis and U. "im becillis" is consonant with the presumed high colonization potential of the former species. The estimates of F-ST obtained for the four species of Utt erbackia suggest a very high level of among-population genetic differentiat ion (mean F-ST range: 0.218-0.818). This observation is quire unexpected fo r an able colonizer such as U. imbecillis (mean F-ST = 0.818) unless detail ed knowledge of this species' mating system is considered. The combined imp act of self-fertilization and founder events in the hermaphroditic Utterbac kia species likely potentiates among-population genetic differentiation whi ch increases F-ST values. These results suggest that the mating system and distribution of genetic variation in unionid populations should be carefull y evaluated prior to the enactment of conservation initiatives.