Effect of low temperatures on the intracellular symbionts of the grain aphid Sitobion avenae (F.) (Hem., Aphididae)

Abstract

Aphids depend to a high degree on prokaryotic symbionts within specialized cells (mycetocytes) to maintain normal growth, development and reproduction. Aphids that overwinter anholocyclically (i.e. as active stages) have a limited cold tolerance (compared to the egg stage) with an LT 50 of −14.6°C, about 12° above the supercooling (freezing) temperature. This paper investigates the possibility that this pre-freeze mortality in aphids is related to cold-induced damage to the symbionts leading to a fatal injury in some aphids and reduced longevity and fecundity in the survivors. Aphids were cooled to −10°C (producing approx. 50% mortality) and −17.5°C (100% mortality) and fixed immediately after cooling or 6 or 24 h later. The structures of the symbionts in these specimens were compared to control (uncooled) aphids by transmission electron microscopy. Cytoplasmic clearing was observed in symbionts of aphids fixed immediately after cooling to −10 and −17.5°C. In aphids fixed 6 and 24 h after cooling, the clearing effect was not as apparent. Thus there is some correlation between low temperature and damage to symbionts, which is most obvious in aphids fixed immediately after cooling. The reduced incidence of damaged symbionts in aphids fixed up to 24 h after cold exposure may be indicative of a repair mechanism.