Does endopolyploidy affect caste, body size, and societal role 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.  While endoreduplication is an important process in the terminal differentiation of some specialized cell types, often providing increases in cell size and metabolism, the direct effects of increasing nuclear ploidy on cell function are not well resolved.  Here we seek to determine 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.  Cell cycle values, interpreted as the mean number of endocycles performed by each cell in the sample and thus a measure of overall endoreduplication, of whole body and body segment samples were compared across castes and female body sizes.  Among females, endoreduplication is positively related with size within the worker caste, but is not related to caste generally.  Additionally, abdomens have the greatest endoreduplication of all body segments regardless of caste or size.  We also note that males, having derived from haploid unfertilized eggs in the haplo-diploid sex determination system, may compensate for their haploid origin by performing an additional endocycle relative to females.  These results suggest that endoreduplication plays an important role in both size determination among workers of eusocial insects and the development of some segment-specific tissues.