Geographic variation in caste ratio of trematode colonies with a division of labour reflect local adaptation.

Similarly to the division of labour in social insects, castes of morphologically distinct individuals exist within colonies of some species of parasitic trematodes. These colonies occur in the first intermediate host of the trematode's complex life cycle and are composed of clonal individuals. Individuals of the reproductive caste have significantly larger bodies while non-reproductive individuals are small and appear to be specialised for defence against co-infecting trematode colonies. In parallel with colony organisation of social insects, demographic traits such as the proportion of the small, non-reproducing individuals relative to the large, reproducing individuals and colony size are expected to vary and adjust to local conditions. In the case of colonies from geographically and potentially genetically distinct populations, this variation is hypothesised to become fixed by evolutionary divergence, as reported in social insect studies. In this study, the adaptive demography theory was further tested by looking at caste ratio and colony organisation of Philophthalmus sp. (a parasitic trematode with a recently discovered division of labour) colonies from geographically distinct populations. Results indicate that the caste ratio from geographically distinct Philophthalmus sp. colonies differs; the proportion of small, defensive individuals is higher in colonies from the location where the risk of competition is highest. This is suggestive of local adaptation, as caste ratios do not change over time under standardised laboratory conditions. This is the first evidence to support the adaptive demography theory in a species with a division of labour other than social insects.