Characterization and expression analysis of genes encoding three small heat shock proteins in the oriental armyworm,Mythimna separata (Walker)

Small heat shock proteins (sHsps) function in the response of insects to abiotic stress; however, their role in response to biotic stress has been under-investigated.Mythimna separata, the oriental armyworm, is polyphenetic and exhibits gregarious and solitary phases in response to high and low population density, respectively. In this study, three genes were identified encoding sHsps, namelyMsHsp19.7,MsHsp19.8andMsHsp21.4, and expression levels in solitary and gregariousM.separatawere compared. The deduced protein sequences of the threeMsHspshad molecular weights of 19.7, 19.8 and 21.4 kDa, respectively, and contained a conserved alpha-crystalline domain. Real-time PCR analyses revealed that the threesHspswere transcribed in all developmental stages and were dramatically up-regulated at the 6(th)larval stage in gregarious individuals. Expression of the threeMsHspswas variable in different tissues of 6(th)instar larvae, but exhibited consistent up- and down-regulation in the hindgut and Malpighian tubules of gregarious individuals, respectively. In addition,MsHsp19.7andMsHsp19.8were significantly induced when solitary forms were subjected to crowding for 36 h, but all threeMsHspswere down-regulated when gregarious forms were isolated. Our findings suggest that population density functions as a stress factor and impactsMsHspsexpression inM.separata.