Can endopolyploidy explain body size variation within and between castes in ants?

Endoreduplication is the process by which the nuclear genome is repeatedly replicated without mitotic cell division, resulting in nuclei that contain numerous additional genome copies. Endoreduplication occurs widely throughout Eucarya and is particularly common in angiosperms and insects. Although endoreduplication is an important process in the terminal differentiation of some specialized cell types, and often increases cell size and metabolism, the direct effects of increasing nuclear ploidy on cell function are not well resolved. Here, we examine if endoreduplication may play a role in body size and/or caste differentiation in ants. Nuclear ploidy was measured by flow cytometry of whole individuals (providing the basis for overall body size patterns) and individual body segments for multiple polymorphic ant species. We used cell cycle values, interpreted as the mean number of endocycles performed by each cell in the sample, as our measure of overall endoreduplication. Among females of four polymorphic ant species, endoreduplication was positively related with size within the worker caste, but was not related to caste generally in two species where we also examined queens. Additionally, abdomens had the greatest endoreduplication of all body parts regardless of caste or size. We also found that males, having derived from haploid unfertilized eggs, had the highest rates of endoreduplication and may compensate for their haploid origin by performing an additional endocycle relative to females. These results suggest that endoreduplication may play a role in body size variation in eusocial insects and the development of some segment-specific tissues.