Ecdysteroid and chitinase fluctuations in the western tarnished plant bug (Lygus hesperus) prior to molt indicate roles in development

Authors: Brent, Colin; WANG, MEIXIAN - Zhejiang University; MIAO, YUN-GEN - Zhejiang University; Hull, Joe

Submitted to: Archives of Insect Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2016
Publication Date: 6/15/2016
Citation: Brent, C.S., Wang, M., Miao, Y., Hull, J.J. 2016. Ecdysteroid and chitinase fluctuations in the western tarnished plant bug (Lygus hesperus) prior to molt indicate roles in development. Archives of Insect Biochemistry and Physiology. 91(5):108-126.

Interpretive Summary: Increasing body size typical of insect growth and development is accompanied by periodic shedding of the cuticle (molting), during which the old cuticle is replaced by a new, larger cuticle. Basic mechanisms involved in molting, such as biochemical messengers (hormones) or chemicals involved in breaking down the old cuticle (chitinases), have been studied in a few model insects. However, the processes controlling molting in many important pest insects, such as the western tarnish plant bug (Lygus bug), have not been studied. Detailed understanding of these mechanisms in Lygus bugs would facilitate development of novel, environmentally friendly control methods. ARS scientists at Maricopa, AZ showed that amounts of a key hormone varied in a predictable pattern as immature Lygus bugs developed from one molt to the next. When these hormonal patterns were disrupted by application of synthetic hormone, molting was delayed and many of the insects died. In addition, ten genes that produce chitinases were identified, and activity of four of these genes followed a pattern consistent with the hormone that controls molting. These are the first reports of the hormonal and genetic control of molting in Lygus bugs. The results provide biochemical targets for additional study, and for potential development of novel, genetic-based control tactics.

Technical Abstract: Vital physiological processes that drive the insect molt represent areas of interest for the development of alternative control strategies. The western tarnished plant bug (Lygus hesperus Knight) is a pest of numerous agronomic and horticultural crops but the development of novel control approaches is impeded by limited knowledge of the mechanisms regulating its molt. To address this deficiency, we examined the fundamental relationship underlying the hormonal and molecular components of ecdysis. At 27 degrees C L. hesperus exhibits a temporally controlled nymph-adult molt that occurs about 4 d after the final nymph-nymph molt with ecdysteroid levels peaking 2 d prior to the final molt. Application of exogenous ecdysteroids when endogenous levels had decreased disrupted the nymphal-adult molt, with treated animals exhibiting an inability to escape the old exoskeleton and resulting in mortality compared to controls. Using available transcriptomic data, we identified ten chitinase-like sequences (LhCht), eight of which had protein motifs consistent with chitinases. Phylogenetic analyses revealed orthologous relationships to chitinases critical to molting in other insects. RT-PCR based transcript profiling revealed that expression changes to four of the LhChts was coordinated with the molt period and ecdysteroid levels. Collectively, our results support a role for ecdysteroid regulation of the L. hesperus molt and suggest that cuticle clearance is mediated by LhCht orthologs of chitinases that are essential to the molt process. These results provide the initial hormonal and molecular basis for future studies to investigate the specific roles of these components in molting.