Alate susceptibility in ants.

Eddie K H Ho,Megan E Frederickson

doi:10.1002/ece3.1291

Abstract

Pathogens are predicted to pose a particular threat to eusocial insects because infections can spread rapidly in colonies with high densities of closely related individuals. In ants, there are two major castes: workers and reproductives. Sterile workers receive no direct benefit from investing in immunity, but can gain indirect fitness benefits if their immunity aids the survival of their fertile siblings. Virgin reproductives (alates), on the other hand, may be able to increase their investment in reproduction, rather than in immunity, because of the protection they receive from workers. Thus, we expect colonies to have highly immune workers, but relatively more susceptible alates. We examined the survival of workers, gynes, and males of nine ant species collected in Peru and Canada when exposed to the entomopathogenic fungus Beauveria bassiana. For the seven species in which treatment with B. bassiana increased ant mortality relative to controls, we found workers were significantly less susceptible compared with both alate sexes. Female and male alates did not differ significantly in their immunocompetence. Our results suggest that, as with other nonreproductive tasks in ant colonies like foraging and nest maintenance, workers have primary responsibility for colony immunity, allowing alates to specialize on reproduction. We highlight the importance of colony-level selection on individual immunity in ants and other eusocial organisms.

Highlights

All organisms are infected by parasites and pathogens
We found that workers were less susceptible than either gynes or males when exposed to the generalist entomopathogenic fungus B. bassiana
Fungus treatment increased mortality in seven of the nine ant species we tested, and the effect was significantly larger for gynes a 2014 The Authors

Summary

Introduction

All organisms are infected by parasites and pathogens. As a result, a role for disease has been invoked in diverse areas of evolutionary biology, including the evolution of sex (Hamilton 1980), the maintenance of genetic variation (King et al 2011), host defense dynamics (Debarre et al 2012), and social evolution (O’Donnell and Beshers 2004). The negative effect of parasites on their hosts creates strong selection for host immunity, and a large literature has investigated how host immunocompetence varies with factors such as sex (Zuk and McKean 1996; May 2007; Nunn et al 2009), age (Laughton et al.2011; Mc Namara et al 2012), body size (Moore and Wilson 2002; Hughes et al 2010), genetic diversity (O’Donnell and Beshers 2004; Ugelvig et al 2010), and life history (Stoehr and Kokko 2006; Schmidt et al 2011). (Mostly) sterile workers perform tasks such as foraging, defense, and nest maintenance, whereas gynes and males are the reproductive members of the colony that eventually emerge from the nest, mate, and establish the generation.

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Conclusion