Plasticity and associated epigenetic mechanisms play a role in thermal evolution during range expansion

Due to global change many species are shifting their distribution and are
thereby confronted with novel thermal conditions at the moving range
edges. Especially during the initial phases of exposure to a new
environment, it has been hypothesized that plasticity and associated
epigenetic mechanisms enable species to cope with environmental change. We
tested this idea by capitalizing on the well-documented southward range
expansion of the damselfly Ischnura elegans from France into Spain where
the species has invaded warmer regions in the 1950s in eastern Spain (old
edge region) and in the 2010s in central Spain (new edge region). Using a
common garden experiment at rearing temperatures matching the ancestral
and invaded thermal regimes, we tested for evolutionary changes in
(thermal plasticity in) larval life history and heat tolerance in these
expansion zones. Through the use of de- and hypermethylating agents we
tested whether epigenetic mechanisms play a role in enabling heat
tolerance during expansion. We used the phenotype of the native sister
species in Spain, I. graellsii, as proxy for the locally adapted
phenotype. New edge populations converged towards the phenotype of the
native species through plastic thermal responses in life history and heat
tolerance while old edge populations (partly) constitutively evolved a
faster life history and higher heat tolerance than the core populations,
thereby matching the native species. Only the heat tolerance of new edge
populations increased significantly when exposed to the hypermethylating
agent. This suggests that the DNA methylation machinery is more amenable
to perturbation at the new edge and shows it is able to play a role in
achieving a higher heat tolerance. Our results show that both (evolved)
plasticity as well as associated epigenetic mechanisms are initially
important when facing new thermal regimes but that their importance
diminishes with time.