Temperature-Determined Outcrossing Ability in Wild Populations of a Simultaneous Hermaphrodite Snail

Phally, a genital polymorphism found in some species of self—compatible simultaneous hermaphrodites, presents an opportunity to examine environmental and genetic influences on mating system traits. Both aphallics and euphallics can self—fertilize, but aphallics cannot donate sperm because they do not develop a functional penis and prostate. Previous studies of the freshwater gastropod Bulinus truncatus (Mollusca: Pulmonata) suggest that in at least some laboratory populations, phally is determined by temperature during egg and hatchling stages, with colder temperatures favoring the development of euphally. Here we provide evidence for temperature—influenced phally determination in natural populations of B. truncatus in northern Nigeria. The prevalence of euphally across 49 populations ranged from 0 to 81%. Levels of euphally increased significantly both within and across sites as temperatures decreased during the dry—cool season. Across sites, time of year (date) was the strongest single predictor of proportion of euphallics. When the effects of time of year were controlled for, there was a significant negative correlation between temperature during phally development and proportion of euphallics. In addition, maximum water temperature during the 1st wk of the study predicted proportions of euphallics in the next snail generation. A laboratory study of snails from three of these populations demonstrated that temperature sensitivities differed between populations: one population that remained 100% aphallic over the course of field observations showed only low levels of temperature sensitivity in the laboratory, while a population that tripled in proportion of euphallics in the field showed strong levels of temperature sensitivity. Furthermore, individuals within populations varied in their temperature sensitivities. These results are discussed in light of the adaptive significance of labile phally determination.