Chitin Synthases Are Critical for Reproduction, Molting, and Digestion in the Salmon Louse (Lepeophtheirus salmonis).

Hulda Harðardóttir,Frank Nilsen,Rune Male,Sussie Dalvin

doi:10.3390/life11010047

Abstract

Chitin synthase (CHS) is a large transmembrane enzyme that polymerizes Uridine diphosphate N-acetylglucosamine into chitin. The genomes of insects often encode two chitin synthases, CHS1 and CHS2. Their functional roles have been investigated in several insects: CHS1 is mainly responsible for synthesizing chitin in the cuticle and CHS2 in the midgut. Lepeophtheirus salmonis is an ectoparasitic copepod on salmonid fish, which causes significant economic losses in aquaculture. In the present study, the tissue-specific localization, expression, and functional role of L. salmonis chitin synthases, LsCHS1 and LsCHS2, were investigated. The expressions of LsCHS1 and LsCHS2 were found in oocytes, ovaries, intestine, and integument. Wheat germ agglutinin (WGA) chitin staining signals were detected in ovaries, oocytes, intestine, cuticle, and intestine in adult female L. salmonis. The functional roles of the LsCHSs were investigated using RNA interference (RNAi) to silence the expression of LsCHS1 and LsCHS2. Knockdown of LsCHS1 in pre-adult I lice resulted in lethal phenotypes with cuticle deformation and deformation of ovaries and oocytes in adult lice. RNAi knockdown of LsCHS2 in adult female L. salmonis affected digestion, damaged the gut microvilli, reduced muscular tissues around the gut, and affected offspring. The results demonstrate that both LsCHS1 and LsCHS2 are important for the survival and reproduction in L. salmonis.

Highlights

The salmon louse, Lepeophtheirus salmonis, a parasitic copepod on salmonid fish, is the most commonly found fish louse on salmonids in the Atlantic Ocean [1]
Expression of LsCHS1 in adult female L. salmonis was found in all organs and tissue types analyzed: Integument, oocytes, ovaries, fertilized eggs, and intestine (Figure 2)
LsCHS2 was detected in all tissues and organs tested apart from the fertilized eggs

Summary

Introduction

The salmon louse, Lepeophtheirus salmonis, a parasitic copepod on salmonid fish, is the most commonly found fish louse on salmonids in the Atlantic Ocean [1]. Salmon lice produced on farmed fish spread to wild fish and pose an ecological challenge [9,10,11]. The life cycle of L. salmonis consists of eight stages, each separated by a molt [12]. The copepodid is the infective stage and becomes parasitic after attachment to a host. The last five life stages (chalimus I and II, pre-adult I and II, and adult stage) are parasitic. The final and last molt is to the adult stage, after which the female undergoes additional growth in the genital segment and abdomen, defined in six maturition stages (T1–T6), during which reproduction is initiated [13]. Males attach spermatophores to the posterior end of the genital segment of females, and eggs are fertilized externally as they are extruded from the genital segment

Methods

Results

Conclusion