Autophagic activity in the midgut cells of three arachnids responds selectively to different modes of overwintering in caves.

Autophagy is a highly conserved metabolic process that regulates cellular homeostasis and energy supply by degrading dysfunctional and excess cell constituents and reserve materials into products that are reused in metabolic and biosynthetic pathways. Macroautophagy is the best studied form of autophagy in invertebrates. Starvation is a common stress factor triggering autophagy in overwintering animals. In arachnids, the midgut diverticula cells perform many vital metabolic functions and are therefore critically involved in the response to starvation. Here we studied macroautophagy in three species which apply different modes for overwintering in caves: the harvestmen Gyas annulatus in diapause, Amilenus aurantiacus with ongoing ontogenesis under fasting conditions, and the spider Meta menardi, which feeds opportunistically even in winter. The main goal was to find eventual qualitative and quantitative differences in autophagic processes by inspecting TEM micrographs. In all three species, the rates of midgut epithelial cells with autophagic structures gradually increased during overwintering, but were significantly lower in G. annulatus in the middle and at the end of overwintering than in the other two species, owing to metabolic activity having been more suppressed. Decomposition of mitochondria and glycogen took place in autophagic structures in all three species. Moreover, spherite disintegration in A. aurantiacus and a special form of lipid disintegration through "lipid bubbly structures" in M. menardi indicate the crucial involvment of selective autophagy, while no specific autophagy was observed in G. annulatus. We conclude that autophagic activities support overwintering in different ways in the species studied.