Exploring Cold Hardiness within a Butterfly Clade: Supercooling Ability and Polyol Profiles in European Satyrinae.

Abstract

Simple SummaryIn insects distributed in temperate and cold zones, cold hardiness during overwintering crucially affects the distribution, including range shifts due to climate change. Our previous work on the genus Erebia, a cold-adapted and species-rich group of the sub-family Satyrinae (Nymphalidae), disclosed unexpected diversity of cold hardiness strategies, with closely related species surviving or not surviving freezing of larval body fluids. Asking whether this diversity is peculiar to this genus, or may be common in the Satyrinae clade, we investigated supercooling ability, contents of sugars and polyols in overwintering larvae tissues, and evolutionary signal of these traits of eight European Satyrinae species (from seven genera) and compared them with the Erebia representatives investigated earlier. We show that cold hardiness strategies are indeed diverse in the group and that high mountain and continental steppe species employ similar cryoprotection mechanisms, differing from those employed by species of more mesic environments. The cold hardiness of overwintering stages affects the distribution of temperate and cold-zone insects. Studies on Erebia, a species-rich cold-zone butterfly genus, detected unexpected diversity of cold hardiness traits. We expanded our investigation to eight Satyrinae species of seven genera. We assessed Autumn and Winter supercooling points (SCPs) and concentrations of putatively cryoprotective sugars and polyols via gas chromatography–mass spectrometry. Aphantopus hyperantus and Hipparchia semele survived freezing of body fluids; Coenonympha arcania, C. gardetta, and Melanargia galathea died prior to freezing; Maniola jurtina, Chazara briseis, and Minois dryas displayed a mixed response. SCP varied from −22 to −9 °C among species. Total sugar and polyol concentrations (TSPC) varied sixfold (2 to 12 μg × mg−1) and eightfold including the Erebia spp. results. SCP and TSPC did not correlate. Alpine Erebia spp. contained high trehalose, threitol, and erythritol; C. briseis and C. gardetta contained high ribitol and trehalose; lowland species contained high saccharose, maltose, fructose, and sorbitol. SCP, TSPC, and glycerol concentrations were affected by phylogeny. Species of mountains or steppes tend to be freeze-avoidant, overwinter as young larvae, and contain high concentrations of trehalose, while those of mesic environments tend to be freeze-tolerant, overwinter as later instars, and rely on compounds such as maltose, saccharose, and fructose.