Understanding the mechanisms used by insects to survive stress is critical for understanding adaptations to the environment and may suggest future tools for pest management. To this end, two very different flies from different parts of the world were examined to determine how they deal with environmental stressors in their habitat. The insects examined include the fruit fly Rhagoletis pomonella and the Antarctic midge, Belgica antarctica, two species that spend most of their immature period in the soil.
The apple maggot, Rhagoletis pomonella, is a major pest of domesticated apples. It can spend most of the year (~11 months) in the soil in a deep diapause. During this time, the pupae are vulnerable to the low temperatures of winter as well as the warm temperatures of early summer. In late summer, adults emerge and oviposit into apples, where the larvae spend several weeks. The larvae then drop to the soil and pupariate. Infested apples were also placed in the field for a 24 hr period during August, and the heat shock proteins were monitored. During this time, the core temperature of the apples exceeded 40°C for at least 2 hrs per day. Hsp70 was mildly upregulated in the field at 9:30 am and upregulation was maximal at 5:30 pm. By 11 pm Hsp expression again declined and by 6 am the next day there was no signal. Hsp90 was again constitutively expressed during the entire day, but a strong upregulation was also noted at 5:30 pm. Hsp expression was also noted during diapause. Diapausing pupae were sampled at 30, 60, 90, 120, 150 and 250 days post pupariation. Hsp70 was expressed throughout diapause but was further upregulated at 120 d (deep diapause). Hsp90 was constitutively expressed during diapause.
Larvae of the Antarctic midge, Belgica antarctica, spend nearly 11 months each year encased in a matrix of substrate and ice. During that time the larvae are exposed to the dehydrating conditions of being surrounded by ice, but they also experience numerous freeze and thaw cycles. Those conditions can lead to the generation of oxygen radicals, and thus we monitored the expression of antioxidant enzymes and heat shock proteins. We subjected the larvae to heat shock, freezing, anoxia, direct sunlight and ultraviolet radiation, five of the main stressors that the larvae normally confront. We also evaluated the antioxidant capacity of the larvae and the adults, and monitored markers of oxidative damage. We found that SOD was constitutively expressed by larvae, but not adults, and this expression was not further upregulated by any of the treatments, suggesting maximal threshold levels of expression for this gene (i.e. overexpression). Catalase was expressed in both the larvae and the adults, but larval expression was far stronger. Catalase, a small heat shock protein and Hsp70 were strongly upregulated in response to sunlight, as well as to ultraviolet radiation A (UVA).
The dehydration tolerance of both organisms was also studied and the expression of heat shock proteins and antioxidants enzyme was altered by water loss.