Takahiro Kikawada

Factors Inducing Successful Anhydrobiosis in the African Chironomid Polypedilum vanderplanki: Significance of the Larval Tubular Nest

Abstract The African chironomid Polypedilum vanderplanki exhibits anhydrobiosis, i.e., the larvae can survive complete desiccation. Recovery rate and trehalose content were investigated in larvae desiccated slowly or at a rate more than 3 times faster. Upon slow desiccation (evaporation rate 0.22 ml day−1) larvae synthesized 38 μg trehalose/individual before complete desiccation, and all of them recovered after rehydration, whereas larvae that were dehydrated quickly (evaporation rate 0.75 ml day−1) accumulated only 6.8 μg trehalose/individual and none of them revived after rehydration. In the pools that are their natural habitat P. vanderplanki larvae make tubes by incorporating detritus or soil with their sticky saliva. This tubular structure is a physical barrier not only to protect the larva from natural enemies but also induces successful anhydrobiosis by reducing the dehydration rate. When larvae were dehydrated with 100 μl distilled water (DW) in soil tubes, they accumulated 37 μg trehalose/individual and more than half of them could revive after rehydration, whereas larvae without tubes accumulated lower level of trehalose and none recovered after rehydration.

Involvement of Heat Shock Proteins in Invertebrate Anhydrobiosis

The anhydrobiosis is a unique state notable for a complete lack of detectable metabolic activity and an ability to withstand extreme stresses. It is induced, as its name suggests, by severe water loss, extending up to complete desiccation. Despite of severe stress, organisms entering anhydrobiosis normally retain their viability. This phenomena is ensured by a variety of protective mechanisms, including expression of protective proteins. The latter include Heat shock proteins (Hsp), which have long been recognized for their importance in a wide range of stress protection mechanisms. This chapter summarizes the theory and available experimental data, both suggesting the importance of Hsp in invertebrate anhydrobiosis. However, most of experimental data are the results of expression studies. We show that they are insufficient to make robust conclusions on the role of Hsp in anhydrobiosis. To date, only two robust evidences based on loss of-function-experiments are available, leaving for the future research the complete elucidation of Hsp function in anhydrobiosis.

The Antioxidant System in the Anhydrobiotic Midge as an Essential, Adaptive Mechanism for Desiccation Survival

One of the major damaging factors for living organisms experiencing water insufficiency is oxidative stress. Loss of water causes a dramatic increase in the production of reactive oxygen species (ROS). Thus, the ability for some organisms to survive almost complete desiccation (called anhydrobiosis) is tightly related to the ability to overcome extraordinary oxidative stress. The most complex anhydrobiotic organism known is the larva of the chironomid Polypedilum vanderplanki. Its antioxidant system shows remarkable features, such as an expansion of antioxidant genes, their overexpression, as well as the absence or low expression of enzymes required for the synthesis of ascorbate and glutathione and their antioxidant function. In this chapter, we summarize existing data about the antioxidant system of this insect, which is able to cope with substantial oxidative damage, even in an intracellular environment that is severely disturbed due to water loss.