Evolution, not transgenerational plasticity, explains the divergence of acorn ant thermal tolerance across an urban-rural temperature cline

Disentangling the mechanisms of phenotypic shifts in response to environmental change is critical, and although studies increasingly disentangle phenotypic plasticity from evolutionary change, few explore the potential role for transgenerational plasticity in this context. Here, we evaluate the potential role that transgenerational plasticity plays in phenotypic divergence of acorn ants in response to urbanization. F2 generation worker ants (offspring of lab-born queens) exhibited similar divergence among urban and rural populations as F1 generation worker ants (offspring of field-born queens) indicating that evolutionary differentiation rather than transgenerational plasticity was responsible for shifts towards higher heat tolerance and diminished cold tolerance in urban acorn ants. Hybrid matings between urban and rural populations provided further insight into the genetic architecture of thermal adaptation. Heat tolerance of hybrids more resembled the urban-urban pure type, whereas cold tolerance of hybrids more resembled the rural-rural pure type. As a consequence, thermal tolerance traits in this system appear to be influenced by dominance rather than being purely additive traits, and heat and cold tolerance might be determined by separate genes. Though transgenerational plasticity does not explain divergence of acorn ant thermal tolerance, its role in divergence of other traits and across other urbanization gradients merits further study.