Diapause termination in eggs of the stonefly Arcynopteryx compacta (McLachland) in relation to dehydration and cold hardiness

Abstract

In the alpine regions of Southern Norway, eggs of the stonefly Arcynopteryx compacta are oviposited in small streams and lake outlets. During ice enclosure, eggs avoid freezing by supercooling and lose about two-thirds of their water content by shrinkage due to osmotic outflux of body water. The eggs enter diapause in autumn at an early stage of embryogenesis. The termination of diapause in 50–55% of eggs kept at 3°C for 3 or 6 months, arises from the effect of chilling on diapause development. The dehydration, however, serves as a diapause-terminating cue. For water contents down to 41%, the termination of diapause seems directly correlated with the degree of dehydration. Treatments with juvenile hormone were also effective in terminating diapause. The termination may therefore be linked to a temporary elevation in juvenile hormone titre by dehydration. Fully hydrated eggs supercool to about −26.5°C, and withstand ice enclosure for 3 days at −22°C. The elevation of supercooling point, water content and depletion of thermal hysteresis-producing proteins correspond with the initiation of postdiapause development. Also ice-enclosure for 3 days at −15 and −18°C proved fatal at this embryonic stage. The loss of cold hardiness could not be related to termination of diapause per se. Eggs remained cold hardy at 3°C as long as the postdiapause development was prevented during the thermally controlled quiescence. The physiological and biochemical mechanisms which underlie adaptation to cold, however, are specific for an early stage of embryogenesis.