Data from: Caste ratios affect the reproductive output of social trematode colonies

Intraspecific phenotypic diversification in social organisms often leads to formation of physical castes which are morphologically specialised for particular tasks within the colony. The optimal caste allocation theory argues that specialised morphological castes are efficient at specific tasks, and hence different caste ratios should affect the ergonomic efficiency, hence reproductive output of the colony. However, the reproductive output of different caste ratios has been documented only in few species of insects with equivocal support for the theory. The present study investigated whether the ratios of non-reproductive and reproductive morphs affect the reproductive output of a recently discovered social trematode, Philophthalmus sp., in which the non-reproductive members are hypothesized to be defensive specialists. Census of natural infections and a manipulative in-vitro experiment demonstrated a positive association between the reproductive output of trematode colonies and the ratio of non-reproductive to reproductive morphs in the presence of an intra-host trematode competitor, Maritrema novaezealandensis. On the contrary, without the competitor, reproductive output was negatively associated with the proportion of non-reproductive castes in colonies. Our findings demonstrate for the first time a clear fitness benefit associated with the non-reproductive castes in the presence of a competitor while illustrating the cost of maintaining such morphs in non-competitive situations. Although the proximate mechanisms controlling caste ratio remain unclear in this trematode system, the present study supports the prediction that the fitness of colonies is influenced by the composition of specialized functional morphs in social organisms, suggesting a potential for adaptive shifts of caste ratios over evolutionary time