The evolution and consequences of simultaneous hermaphroditism in the freshwater mussel genus Utterbackia (Bivalvia: Unionidae).

Abstract

The rare occurrences of simultaneous hermaphroditism in the predominantly gonochoric freshwater mussels offer an opportunity for increased understanding of the ecological, genetic, historical, and morphological correlates involved in the origin and maintenance of hermaphroditism. The evolution of simultaneous hermaphroditism and breeding system variation was investigated in the freshwater mussel genus Anodonta. Comparisons of allozymes and morphology were used to generate a phylogenetic hypothesis for the species of Anodonta in eastern North America in order to determine the sister taxon of the simultaneous hermaphrodite, A. imbecillis. This analysis suggests the following relationships: (((((A. cataracta, A. gibbosa) (A. lacustris, A. grandis)) A. fragilis)(A. imbecillis, A. peggyae)) (A. cygnea (A. kennerlyi (A. implicata (A. "couperiana" (A. couperiana, A. suborbiculata)))))). From this hypothesis, Anodonta (s.l.) comprises three monophyletic groups of species each of which is herein recognized at the generic rank: Pyganodon cataracta, P. gibbosa, P. lacustris, P. grandis, P. fragilis; Utterbackia imbecillis, U. peggyae; Anodonta (s.s.) cygnea, A. kennerlyi, A. implicata, A. "couperiana", A. couperiana, A. suborbiculata. Allozyme analysis was used to estimate the phylogenetic relationships among 52 populations of U. imbecillis and U. peggyae. The resultant hypothesis (((U. imbecillis, U. sp.) U. peggyae)) suggests that U. peggyae (s.l.) is actually composed of two distinct lineages: U. peggyae (s.s.), from the panhandle of Florida and U. sp. from the Florida peninsula. Evidence is presented that is consonant with a hybrid origin for U. imbecillis. The uniformly hermaphroditic condition of U. imbecillis is consistent with the hypothesis of a transition from gonochorism to hermaphroditism soon after the origin of U. imbecillis. Low within-population levels of allozymic variation suggest that self-fertilization is common in U. imbecillis. However, the relative amount of self-fertilization may vary among populations. Among-population levels of allozymic variation were much higher than expected. This result may be due to relatively high levels of genetic drift in U. imbecillis brought about by relatively small effective population size. The relatively small effective population size is likely a consequence of (1) the inbreeding caused by high levels of self-fertilization and (2) the colonizing ability of U. imbecillis.