Cloning and expression analysis of six small heat shock protein genes in the common cutworm, Spodoptera litura

Small heat shock proteins (sHsps) are probably the most diverse in structure and function among the various superfamilies of stress proteins. To explore the diverse functions of insect sHsps, six sHsp cDNAs were cloned from the midgut cDNA library of Spodoptera litura, and a phylogenetic tree was constructed based on the conserved α-crystalline domains. The expression patterns in different developmental stages and tissues, as well as in response to both thermal and 20-hydroxyecdysone (20E) induction, were studied by real-time quantitative PCR. Based on sequence characteristics and phylogenetic relationships, the six SlHsps were classified into three independent groups: BmHsp20.4 like proteins (SlHsp19.7, 20.4, 20.7, 20.8), BmHsp26.6 like protein (SlHsp20), and BmHsp21.4 like protein (SlHsp21.4). All the SlHsps showed highest expression in the Malpighian tubules. The four BmHsp20.4 like protein genes were up-regulated by thermal stress and showed expression variation with development. SlHsp20 exhibited lower expression levels in both egg and larval stages than in pupal and adult stages. SlHsp21.4 retained a constant expression level during all life stages. The expression of both SlHsp20.4 and SlHsp20.8 was significantly up-regulated by 20E. These results indicate that sHsps play diverse functions in S. litura: the BmHsp20.4 like proteins are involved in both thermal adaptation and development; SlHsp20 does not respond to temperature stress but possibly plays a role in metamorphosis; SlHsp21.4 may have no direct relationship with either thermal response or development.