Fitness benefits of a division of labour in parasitic trematode colonies with and without competition.

A reproductive division of labour has recently been discovered within polyembryonic colonies of two species of parasitic trematodes infecting snail hosts. In these colonies, one morph expands the colony through asexual reproduction while the other morph never reproduces. As in other polyembryonic species using a division of labour (parasitoid wasps, one species of sea anemone), the non-reproducing morph appears specialized for defense against competing colonies. In this study, we first assessed competition between Philophthalmus sp. (which possesses reproducing and non-reproducing morphs) and the most common co-infecting species, Maritrema novaezealandensis, by quantifying colony success within snail hosts. Colonies of either species that did not compete within their host were more successful (i.e., produced more transmission stages) than colonies that were competing in a shared host. Second, we cultured individuals of both species in vitro, alone or together, to study the interaction more closely and to measure any advantage obtained by the colony from the non-reproducing morphs. This was done by manipulating the presence and abundance of M. novaezealandensis as well as the presence of the non-reproducing 'defensive' morph. Philophthalmus sp. colonies with both reproducing and non-reproducing morphs but without M. novaezealandensis were most successful. This implies the non-reproducing morphs provide a fitness benefit to Philophthalmus sp. colonies even in the absence of competition, although the nature of this advantage remains unclear.