1 Otago University, New Zealand
DiscussionSocial organisation involving division of labour is a phenomenon we generally associate with humans and colonial insects like ants, bees and termites. One of the most remarkable discoveries regarding parasite biology in recent years has been the recognition that this same kind of division of labour occurs in clonal colonies of trematode parasites within their snail intermediate hosts. In several trematode species, the life stages (rediae) occurring in snails come in two morphologically and functionally distinct forms: a reproductive caste ensuring the production of infective stages (cercariae) released from the snail, and a soldier caste consisting of much smaller and more mobile individuals that defend the colony, by attacking and killing trematodes of other species that attempt to colonise the same snail. I will first present an overview of this division of labour, summarise its phylogenetic occurrence among trematode taxa, and propose a simple evolutionary model to explain its stepwise evolution from an ancestral state where all rediae are identical. Then, I will discuss our recent in-vivo and in-vitro experiments exploring adaptive responses in the social organisation of trematodes. These experiments investigated whether trematode colonies adjust their caste ratio (number of soldiers versus number of reproductives) in the face of competition from other trematodes or other external threats. Also, we investigated phenotypic plasticity at the individual level, to test whether members of one caste can pick up the functions of the other caste when the latter's numbers are low. Overall, these studies reveal a level of complex social organisation comparable to that in higher organisms.