Altered immunity in crowded Mythimna separata is mediated by octopamine and dopamine

Hailong Kong,Lei Zhang,Zhen Tian,Cheng Wang,Xingfu Jiang,Chuanlei Dong,Yunxia Cheng,Nian Mao,Lizhi Luo

doi:10.1038/s41598-018-20711-8

Abstract

Similar to pathogenic infection, high population density alters insects’ prophylactic immunity. Density-dependent prophylaxis has been reported in many polyphenic insects, but the regulatory mechanism underlying this phenomenon remains unclear. The biogenic monoamines are known to play critical roles in mediating insect immune responses. In the current study, the immune capacity and the levels of three biogenic monoamines were investigated in the polyphenic larvae of Mythimna separata, reared at the densities of 1, 2, 5, 10, and 30 larvae per 650-mL jar. Concomitant with the increased phenoloxidase (PO) activity and total haemocyte count in the larvae at high densities (5, 10, 30 larvae/jar), the octopamine level was also increased. In contrast, the dopamine level was decreased, and the 5-hydroxytryptamine level was not significantly affected. Injection of octopamine induced significant increases in the total haemocyte count and PO activity. Conversely, epinastine, a specific antagonist of octopamine, decreased the total haemocyte count and PO activity. Another octopamine antagonist, phentolamine, inhibited the activity of PO and lysozymes. In addition, injection of dopamine induced a significant increase in PO activity and decreased the total haemocyte count and lysozyme activity. These results suggested that both octopamine and dopamine mediate the increases in total haemocyte count and PO activity in the crowded larvae.

Highlights

Insects infected with pathogens are ubiquitous in nature
Total haemocyte count was significantly affected by larval density (F = 42.39; df = 4; P < 0.01; Fig. 1B); total haemocyte count was significantly increased with increased larval density
Wilson et al.[5] found that total haemocyte count and lysozyme activity of L. migratoria were markedly higher with high population density than with low population density

Summary

Introduction

Insects infected with pathogens are ubiquitous in nature. It has been found that insects living in high-density populations have evolved adaptive prophylactic mechanisms to cope with pathogens[1]. Individuals experiencing crowded conditions are predicted to be more resistant to pathogens than those experiencing low-density conditions, through facultative allocation of physiological resources to defence mechanisms. This phenomenon, termed “density-dependent prophylaxis”[1], has been observed in many species that display phase polyphenism, including Spodoptera exempta[2], Mythimna separata[3], Spodoptera littoralis[4], Locusta migratoria[5] and Loxostege sticticalis[6]. An increased immune response is observed in insects under high-density conditions. The purpose of the present study is to examine (1) the effect of larval density on the immune response and levels of biogenic amines in M. separata and (2) the immune responses of M. separata injected with biogenic amines and their antagonists

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Conclusion