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
the larval environment and genotype regulate the expression of
caste-associated traits, including body size and the number of ovarioles.
However, it remains untested whether environmental variables independently
regulate caste-associated traits or if 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 interconnected
and therefore constrained in their independent responses to environmental
variation. We discuss how these results inform contemporary hypotheses for
ant caste development and evolution.