TEMPERATURE DETERMINATION OF MALE OUTCROSSING ABILITY IN A SIMULTANEOUS HERMAPHRODITE.

Phally, a genital dimorphism found in some species of self-compatible simultaneous hermaphrodites, presents an opportunity to examine factors maintaining outcrossing within an animal species in the presence of recombination. Both aphallics and euphallics can self-fertilize but only euphallics develop a functional penis and prostate allowing them to donate sperm. Previous studies of phally in the gastropod Bulinus truncatus (Mollusca: Pulmonata) suggest that phally may be under direct genetic control in some populations and strongly influenced by environmental factors in others. Experiments reported here identify temperature as a cue affecting phally determination in two populations of B. truncatus. In both populations, a higher proportion of euphallics was produced at low temperature (22 ± 1°C) than at high temperature (30 ± 1°C). Temperatures experienced by parents did not affect the proportion of euphallics they produced. Instead, phally was sensitive to temperature during the egg stage postoviposition and during the hatchling stage; the relative influence of temperature before and after hatching varied between populations. The total number of hatchlings reaching maturity at high and low temperature did not differ, but at low temperature, snails took longer to hatch and mature, and had lower survivorship. Just as studies of environmental sex determination have shed light on selective pressures influencing sex ratio evolution, we suggest that temperature-sensitive phally determination may shed light on the selective pressures maintaining outcrossing in B. truncatus.