STABILITY AND GENETIC-BASIS OF VARIABILITY OF PHALLY POLYMORPHISM IN NATURAL-POPULATIONS OF THE SELF-FERTILE FRESH-WATER SNAIL BULINUS-TRUNCATUS

We investigated the genetic variability for phally polymorphism within and between natural populations of the hermaphrodite self-fertile fre shwater snail Bulinus truncatus. Phally polymorphism is characterized by the co-occurrence in natural populations of regular hermaphrodite i ndividuals (euphallic) and individuals deprived of the male copulatory organ (aphallic). The two morphs can both self-fertilize and outcross . However, aphallic individuals cannot outcross as males. We examined the variation of the aphally ratio in 22 natural populations from Nige r over two successive years. During the second years, populations were sampled three times at 3 week intervals. The aphally ratio was highly variable among populations at a given sampling data and remained rela tively stable over time. For 10 of these populations, one population f rom Corsica and two from Sardinia, we also estimated the between- and within-population variability, analysing the aphally ratio of 346 fami lies under laboratory conditions. The aphally ratio varied significant ly among populations and was highly correlated with the aphally ratio of the natural populations. Some within-population variability, associ ated with a high value of the broad sense heritability, was observed i n four populations out of 13. In these populations, aphallic individua ls produced significantly more aphallic offspring than euphallic indiv iduals. Our results indicate a strong genetic basis for aphally, with large genetic differences among populations and some genetic variabili ty for aphally within populations. We discuss the adaptive and stochas tic factors that may shape the distribution of the genetic variability for aphally.