Static allometries of caste-associated traits vary with genotype but not environment in the clonal raider ant

Polyphenic traits in animals often exhibit nonlinear scaling with body size. Static allometries (i.e., scaling relationships) themselves can exhibit plasticity, such that individuals of the same size and genotype differ in body proportions across different environments. In ants, both larval environment and genotype regulate the expression of caste-associated traits, including body size and ovariole number. However, it remains untested whether caste-associated traits are independently regulated by environmental variables or whether they covary due to coupled developmental mechanisms. If caste traits are regulated independently, developmental plasticity should affect both trait expression and the scaling relationships between traits. Using the clonal raider ant, Ooceraea biroi, we tested this by manipulating the rearing environment of genetically identical larvae. We found that caregiver genotype, temperature, and food quantity influenced caste morphology strictly in tandem with body size, producing similar static allometries across rearing conditions (i.e., no allometric plasticity was detected). In contrast, clonal genotypes differed in average body size and their static allometries. Thus, size-matched individuals of the same genotype from different rearing environments exhibited no differences in mean caste trait expression, while those of different genotypes did. This absence of plasticity in the static allometries of different caste traits suggests that they are developmentally coupled due to systemic regulatory factors. Our findings contrast with reports of allometric plasticity in other insects, suggesting that ant caste traits are exceptionally integrated and therefore constrained in their independent responses to environmental variation. We discuss how these results inform contemporary hypotheses for ant caste development and evolution.