Expression patterns of a circadian clock gene are associated with age-related polyethism in harvester ants, Pogonomyrmex occidentalis.

Krista K Ingram,Scott Krummey,Michelle Leroux

doi:10.1186/1472-6785-9-7

Abstract

BackgroundRecent advances in sociogenomics allow for comparative analyses of molecular mechanisms regulating the development of social behavior. In eusocial insects, one key aspect of their sociality, the division of labor, has received the most attention. Age-related polyethism, a derived form of division of labor in ants and bees where colony tasks are allocated among distinct behavioral phenotypes, has traditionally been assumed to be a product of convergent evolution. Previous work has shown that the circadian clock is associated with the development of behavior and division of labor in honeybee societies. We cloned the ortholog of the clock gene, period, from a harvester ant (Pogonomyrmex occidentalis) and examined circadian rhythms and daily activity patterns in a species that represents an evolutionary origin of eusociality independent of the honeybee.ResultsUsing real time qPCR analyses, we determined that harvester ants have a daily cyclic expression of period and this rhythm is endogenous (free-running under dark-dark conditions). Cyclic expression of period is task-specific; foragers have strong daily fluctuations but nest workers inside the nest do not. These patterns correspond to differences in behavior as activity levels of foragers show a diurnal pattern while nest workers tend to exhibit continuous locomotor activity at lower levels. In addition, we found that foragers collected in the early fall (relative warm, long days) exhibit a delay in the nightly peak of period expression relative to foragers collected in the early spring (relative cold, short days).ConclusionThe association of period mRNA expression levels with harvester ant task behaviors suggests that the development of circadian rhythms is associated with the behavioral development of ants. Thus, the circadian clock pathway may represent a conserved 'genetic toolkit' that has facilitated the parallel evolution of age-related polyethism and task allocation in social insects.

Highlights

Recent advances in sociogenomics allow for comparative analyses of molecular mechanisms regulating the development of social behavior
PoPer expression in 12:12 LD cycle In the colonies collected in the fall (F06), foragers displayed a significant oscillation in PoPer mRNA expression with levels peaking in the late evening and the lowest levels occurring in late morning hours (Figure 1; F = 3.879, df = 5,11, p = 0.049)
In S06 colonies, the peak of mRNA expression occurs at least 4 hours earlier than the fall colonies and the lowest expression levels occur at 8:00 AM

Summary

Introduction

Recent advances in sociogenomics allow for comparative analyses of molecular mechanisms regulating the development of social behavior. Age-related polyethism, a derived form of division of labor in ants and bees where colony tasks are allocated among distinct behavioral phenotypes, has traditionally been assumed to be a product of convergent evolution. Namely the ability to characterize molecular pathways in non-model organisms, provide novel opportunities to address questions concerning the evolution of social behavior in an ecological context by comparing taxonomically relevant species (page number not for citation purposes). The Hymenoptera, including the ants, bees, and wasps, are ideally suited for comparative studies of social behavior as species in this group display a diversity of forms of social organization and represent multiple independent evolutionary origins of eusociality [4]. We have little information on whether genetic mechanisms are conserved across eusocial species or whether derived social phenotypes are a product of convergent evolution driven by alternative molecular pathways

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