Transcriptional Profiling of Midgut Immunity Response and Degeneration in the Wandering Silkworm, Bombyx mori

Qiuyun Xu,Brenda T Beerntsen,Peng Zhang,Jingmin Guan,Erjun Ling,Xuquan Li,Jie Zhang,Qimiao Shao,Bing Yang,Chengshu Wang,Anrui Lu,Guohua Xiao,Christian Schönbach

doi:10.1371/journal.pone.0043769

Abstract

BackgroundLepidoptera insects have a novel development process comprising several metamorphic stages during their life cycle compared with vertebrate animals. Unlike most Lepidoptera insects that live on nectar during the adult stage, the Bombyx mori silkworm adults do not eat anything and die after egg-laying. In addition, the midguts of Lepidoptera insects produce antimicrobial proteins during the wandering stage when the larval tissues undergo numerous changes. The exact mechanisms responsible for these phenomena remain unclear.Principal FindingsWe used the silkworm as a model and performed genome-wide transcriptional profiling of the midgut between the feeding stage and the wandering stage. Many genes concerned with metabolism, digestion, and ion and small molecule transportation were down-regulated during the wandering stage, indicating that the wandering stage midgut loses its normal functions. Microarray profiling, qRT-PCR and western blot proved the production of antimicrobial proteins (peptides) in the midgut during the wandering stage. Different genes of the immune deficiency (Imd) pathway were up-regulated during the wandering stage. However, some key genes belonging to the Toll pathway showed no change in their transcription levels. Unlike butterfly (Pachliopta aristolochiae), the midgut of silkworm moth has a layer of cells, indicating that the development of midgut since the wandering stage is not usual. Cell division in the midgut was observed only for a short time during the wandering stage. However, there was extensive cell apoptosis before pupation. The imbalance of cell division and apoptosis probably drives the continuous degeneration of the midgut in the silkworm since the wandering stage.ConclusionsThis study provided an insight into the mechanism of the degeneration of the silkworm midgut and the production of innate immunity-related proteins during the wandering stage. The imbalance of cell division and apoptosis induces irreversible degeneration of the midgut. The Imd pathway probably regulates the production of antimicrobial peptides in the midgut during the wandering stage.

Highlights

Insects such as Drosophila melanogaster live on rotten fruit and food containing many microbes, yet they still survive
This study provided an insight into the mechanism of the degeneration of the silkworm midgut and the production of innate immunity-related proteins during the wandering stage
The immune deficiency (Imd) pathway probably regulates the production of antimicrobial peptides in the midgut during the wandering stage

Summary

Introduction

Insects such as Drosophila melanogaster live on rotten fruit and food containing many microbes, yet they still survive. Under the delicate control of the midgut innate immune system, the pathogenic microbes can be eliminated with minimal disruption to commensal and mutualistic bacteria [1,2]. These insects have evolved an effective defense system, which has become a research focus. The insect gut is a continuous tube that starts from the mouth and ends at the anus. It is composed of three parts: the foregut, midgut, and hindgut.

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