Expression of heat shock protein-coding genes associated with anhydrobiosis in an African chironomid Polypedilum vanderplanki

Abstract

In order to survive in extreme environments, organisms need to develop special adaptations both on physiological and molecular levels. The sleeping chironomid Polypedilum vanderplanki, inhabiting temporary water pools in semi-arid regions of Africa, is the only insect to have evolutionarily acquired the ability to withstand prolonged complete desiccation at larval stage, entering a state called anhydrobiosis. Even after years in a dry state, larvae are able to revive within a short period of time, completely restoring metabolism. Because of the possible involvement of stress proteins in the preservation of biomolecules during the anhydrobiosis of the sleeping chironomid, we have analyzed the expression of genes encoding six heat shock proteins (Pv-hsp90, Pv-hsp70, Pv-hsc70, Pv-hsp60, Pv-hsp20, and Pv-p23) and one heat shock factor (Pv-hsf1) in dehydrating, rehydrating, and heat-shocked larvae. All examined genes were significantly up-regulated in the larvae upon dehydration and several patterns of expression were detected. Gene transcript of Pv-hsf1 was up-regulated within 8 h of desiccation, followed by large shock proteins expression reaching peak at 24-48 h of desiccation. Heat-shock-responsive Pv-hsp70 and Pv-hsp60 showed a two-peak expression: in dehydrating and rehydrating larvae. Both small alpha-crystallin heat shock proteins (sHSP) transcripts were accumulated in the desiccated larvae, but showed different expression profiles. Both sHSP-coding genes were found to be heat-inducible, and Pv-hsp20 was up-regulated in the larvae at the early stage of desiccation. In contrast, expression of the second transcript, corresponding to Pv-p23, was limited to the late stages of desiccation, suggesting possible involvement of this protein in the glass-state formation in anhydrobiotic larvae. We discuss possible roles of proteins encoded by these stress genes during the different stages of anhydrobiosis in P. vanderplanki.

Fig. 1

Relative mRNA expression profiles for Pv-hsf1 and Pv-hsp90 in heat-shocked (a, d) and anhydrobiotic (b, e) chironomid larvae. Values for the mRNA level of each gene were corrected with PvEf1-alpha expression level. The level of expression was calculated for each gene relative to the expression in control hydrated larvae (value = 1). Error bars represent mean value ± 95% CI for three replicates. cont. control hydrated larvae. c, e Neighbor-joining tree of the Pv-HSF1 and Pv-HSP90 amino acid sequences

Fig. 2

Relative mRNA expression profiles for Pv-hsp70, Pv-hsc70, and Pv-hsp60 in anhydrobiotic (b, e, h) and heat-shocked (a, d, g) larvae. Values for the mRNA level of each gene were corrected for expression level of PvEf1-alpha, and the relative level of expression changes for each gene was calculated using that of control hydrated larvae as standard (value = 1). Error bars represent mean value ± 95% CI for three replicates. cont. control hydrated larvae. c, f, i Neighbor-joining tree of Pv-HSC70 and Pv-HSP60 amino acid sequences

Fig. 3

Relative mRNA expression profiles for Pv-hsp20 and Pv-p23 in anhydrobiotic (b, e) and heat-shocked (a, d) larvae. Values for the mRNA level of each gene were corrected for expression level of EF1-alpha, and the relative level of expression changes for each gene was calculated using that of control hydrated larvae as standard (value = 1). Error bars represent mean value ± 95% CI for three replicates. cont. control hydrated larvae. c, f Neighbor-joining tree of the Pv-HSP20 and Pv-p23 amino acid sequences

Fig. 4

Temporal representations of anhydrobiosis stages, in which genes encoding heat shock proteins are up-regulated in the larvae of the sleeping chironomid