Colonizing polar environments: thermal niche evolution in Collembola

Temperature is a primary driver in defining the ecophysiological activity
and performance of ectotherms. Thus, thermal tolerance limits have a
profound effect in determining geographic ranges. In regions with extreme
cold temperatures, lower thermal limits of species are a key physiological
trait for survival. Moreover, thermal niche breadth also plays an
important role in allowing organisms to withstand climatic variability and
confers species with broader potential to establish in new regions. Here
we study the evolution of thermal tolerance limits among Collembola
(Arthropoda) and explore how they are affected by the colonization of
polar environments. In addition, we test the hypothesis that globally
invasive species are more eurythermal than non-invasive ones. Critical
thermal limits (CTmin and CTmax), classic measurements of thermal
tolerance, were compiled from the literature and complemented with
experimental assays for springtail species. Genetic data of the
mitochondrial gene cytochrome oxidase subunit 1 (COI) was used to assemble
a phylogeny. Our results show that polar springtails have lower CTmin and
lower CTmax compared to species from temperate and tropical regions,
consistent with the Polar pressure hypothesis. We found no phylogenetic
signal for CTmax, but low values of phylogenetic signal for CTmin.
Globally invasive species do not have significantly broader thermal
tolerance breadth (CTrange) than non-invasive ones, thus not supporting
the predictions of the Eurythermality hypothesis. We conclude that polar
springtails have evolved their thermal niches in order to adapt to
extremely cold environments, which has led to decreasing both upper and
lower thermal tolerance limits.