Ontogenetic reaction norm for binary traits: the timing of phallus development in the snail Bulinus truncatus

The ontogenetic trajectory of plastic binary traits may provide valuable insights into their evolutionary rate of change. In this paper, the timing of the plastic response of a temperature-dependent sexual polymorphism, aphally, is investigated in the freshwater snail Bulinus truncatus. Aphally is defined as the loss of the male copulatory organ in otherwise hermaphroditic animals. Individuals from two inbred lines were switched at various times during their early development between 25 and 30 degrees C, and their phally status ascertained, in order to evaluate the parameters characterising the ontogenetic reaction norm of aphally to temperature. A series of nested models including parameters for the onset, offset, and the intensity of the response to temperature were fitted to the data, allowing for a wide range of reaction norms. One genotype did not show any variation in aphally ratio with switching temperature, while a switch-point model (onset and offset corresponding to the same developmental point in time) best fitted the second genotype. The results suggest that the plasticity of aphally is expressed before eggs hatch. Their consequences on the evolution of aphally are discussed. More generally, the methodology proposed here can be used to analyse variation in ontogenetic parameters of discrete traits.