Immunity for nothing and the eggs for free: Apparent lack of both physiological trade-offs and terminal reproductive investment in female crickets (Gryllus texensis).

Atsushi Miyashita,Shelley A Adamo,Ting Yat Marco Lee,Russell Easy,Laura E Mcmillan

doi:10.1371/journal.pone.0209957

Abstract

Should females alter their reproductive strategy when attacked by pathogens? Two hypotheses provide opposite predictions. Terminal reproductive investment theory predicts that reproduction should increase when the risk of death increases. However, physiological trade-offs between reproduction and immune function might be expected to produce a decrease in reproduction during a robust immune response. There is evidence for both hypotheses. We examine whether age determines the effect of an immune challenge on reproductive strategy in long-winged females of the Texas field cricket, Gryllus texensis, when fed an ecologically valid (i.e. limited) diet. The limited diet reduced reproductive output. However, even under resource-limited conditions, immune challenge had no effect on the reproductive output of young or middle-aged females. Both reproductive output and immune function (lysozyme-like activity and phenoloxidase (PO) activity) increased with age, which is contrary to both hypotheses. We hypothesize that PO activity is pleiotropic and represents an investment in both reproduction and immune function. Three proPO genes (identified in a published RNA-seq dataset (transcriptome)) were expressed either in the fat body or the ovaries (supporting the hypothesis that PO is bifunctional). The possible bifunctionality of PO suggests that it may not be an appropriate immune measure for studies on immune/reproductive trade-offs. This study also suggests that the threshold for terminal reproductive investment may not decrease prior to senescence in some species.

Highlights

(3) Genes for different phenoloxidase activity (PO) were expressed in both fat body and ovaries (Fig 5), suggesting that PO in the hemolymph may be involved in both reproduction and immune function in crickets
Females in this study were short on resources; the females on the food limited diet laid fewer eggs than those fed ad lib (Fig 2). These results suggest that resource-strapped females funnel resources into both egg production and immune function at the end of life, which is inconsistent with both a physiological trade-off [2] and the terminal investment hypothesis [14]
Given that No Treatment Control (NTC) (No treatment control) and Control (Blood collection only) groups showed comparable reproductive output, the reproductive cost of handling and bleeding appears to be too low to have had an effect on reproductive output

Summary

Introduction

How resources are allocated across growth, somatic maintenance, and reproduction is an important determinant of fitness. Reproduction and immunity are two metabolically expensive traits [1]. Activation of an immune response can lead to physiological trade-offs, resulting in declines in reproduction Immunity for nothing and the eggs for free. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Methods

Results

Conclusion