THE GENETIC AND ENVIRONMENTAL DETERMINATION OF PHALLY POLYMORPHISM INTHE FRESH-WATER SNAIL BULINUS-TRUNCATUS

In some species of self-fertile pulmonate snails, two sexual morphs co -occur in natural populations: regular individuals and aphallic indivi duals that cannot transmit sperm to other snails. Purely aphallic popu lations therefore reproduce obligatorily by selfing. Understanding the evolution of aphally and selfing in these snails requires a precise k nowledge of phally determination. In this paper, we investigate the ge netic and environmental determination of aphally in Bulinus truncatus by a survey of the family (offspring) aphally ratio of 233 individuals originating from seven natural populations and a study of the reactio n norm of the family aphally ratio to temperature using 60 individuals from 10 selfed lineages of one population. Our results indicate a hig h genetic variability for the determination of aphally between populat ions and within some populations, associated with a high level of gene tic determination. Our second experiment indicates a significant tempe rature and lineage effect though no interaction between these two effe cts. We discuss our results in the framework of threshold models devel oped for dimorphic traits with polygenic inheritance. We propose that the sexual morph of an individual at a given temperature is determined by a temperature threshold value depending on both the individual gen otype and probabilistic processes.