Factors affecting sex-ratio variation and sex-determining mechanisms in the apple snail Pomacea canaliculata

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The freshwater snail Pomacea canaliculata shows a large variation in offspring sex ratio. We investigated whether this variation is caused by a small number of sex-determining genes, with two series of experiments. First, we reared F2 offspring from several crosses involving full-sib sisters with the same male, and determined sex ratios of F2. The sex ratios of F2 converged into a few values, indicating that the sex-determining genotypes of the F1 full-sib sisters were also a few. This suggests that the sex in this snail is determined by a few genes rather than polygenes. Secondly, we developed genetic markers to study linkage with the putative sex-determining genes. We found >40 microsatellite loci, and developed four useful markers. These markers were repeatable in PCR, polymorphic and followed Mendelian segregation. We conducted fragment analyses in two populations (Kumamoto and Nara) with these markers. One of them showed significantly different frequencies between males and females in both populations ; it may be in the same linkage group as a sex-determining locus.