Two's a crowd: phenotypic adjustments and prophylaxis in Anticarsia gemmatalis larvae are triggered by the presence of conspecifics.

Farley W S Silva,Fernando H Valicente,Daniel L Viol,Simon L Elliot,Eraldo Lima,Sirlene V Faria,Holger G Krapp

doi:10.1371/journal.pone.0061582

Abstract

Defence from parasites and pathogens involves a cost. Thus, it is expected that organisms use this only at high population densities, where the risk of pathogen transmission may be high, as proposed by the "density-dependent prophylaxis" (DDP) hypothesis. These predictions have been tested in a wide range of insects, both in comparative and experimental studies. We think it pertinent to consider a continuum between solitarious and gregarious living insects, wherein: (1) solitarious insects are those that are constitutively solitary and do not express any phenotypic plasticity, (2) the middle of the continuum is represented by insects that are subject to fluctuations in local density and show a range of facultative and plastic changes; and (3) constitutively gregarious forms live gregariously and show the gregarious phenotype even in the absence of crowding stimuli. We aimed to chart some of the intermediary continuum with an insect that presents solitarious aspects, but that is subject to fluctuations in density. Thus, Anticarsia gemmatalis (Lepidoptera: Noctuidae) larvae reared at higher densities showed changes in coloration, a greater degree of encapsulation, had higher hemocyte densities and were more resistant to Baculovirus anticarsia, but not to Bacillus thuringiensis. Meanwhile, with increased rearing density there was reduced capsule melanization. Hemocyte density was the only variable that did not vary according to larval phenotype. The observed responses were not a continuous function of larval density, but an all-or-nothing response to the presence of a conspecific. As A. gemmatalis is not known for gregarious living, yet shows these density-dependent changes, it thus seems that this plastic phenotypic adjustment may be a broader phenomenon than previously thought.

Highlights

Parasites represent an important selective force to their hosts [1]
In accordance with Sword [20], insects can be considered as living on a continuum between solitarious and gregarious lifestyles. It is mostly insects in the intermediary part of this continuum that have been used to investigate the Density-Dependent Prophylaxis hypothesis, according to which higher densities lead to elevated investment in disease resistance [7,8,11,16]
Work on species that sit at the solitarious end of the continuum tends to indicate that increased densities are a stress factor that increase susceptibility, rather than a prophylactic increase in disease resistance [5,44]

Summary

Introduction

Parasites represent an important selective force to their hosts [1]. Besides mortality, the costs of this interaction may be expressed in different ways, such as a reduction in reproductive fitness, reduced survival or inhibition of metamorphosis as seen in insects (see [2,3,4]). Insects that undergo plastic phenotypic changes when at high densities (i.e. density-dependent phase polyphenism) may invest more in prophylactic resistance mechanisms according to a predictable infection risk at moments of crowding [1]. One aspect of these phenotypic adjustments is changes in immune parameters of insects, such as an increase in hemocyte densities, capsule melanization or the encapsulation response [8,10]. These parameters, and others, can be measured in comparative studies, where species can be characterized according to typical densities and in experimental studies, where the host density is manipulated during the insect’s lifetime [11,12,13,14]

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