Effect of inbreeding on sex ratio in the apple snail Pomacea canaliculata

Female-biased sex ratios are adaptive in populations founded by a small number of individuals and are mainly due to local mate competition (the haystack model). However, little empirical support for this theory exists and, with the exception of terrestrial vertebrates and arthropods, very little is known about the possible mechanisms for biased sex ratios under this model in animals. The highly invasive freshwater snail Pomacea canaliculata usually reproduces in small temporary water bodies and is characterized by genetically based variable brood sex ratios. We conducted a mating experiment to test the prediction that sex ratio is biased towards females in inbred populations. Inbred lines (pairing of a male and a female from the same brood) and outcrossed lines (pairing of a male and a female from different broods) were reared in the lab, and sex ratios were compared between these two breeding types for three generations (the F-1 generation was produced by outcrossing only). As predicted, the sex ratios of the inbred lines showed greater bias towards females (average proportion of males per generation was 0.38-0.40) than the outcrossed lines (0.45-0.55). The female-biased sex ratios of P. canaliculata may facilitate rapid population growth and may thus enhance the invasive capacity of this snail. Female-biased sex ratios under metapopulation structures, as predicted by the haystack models, may be more common than previously considered.