For Queen and Country : Reproductive and Non-Reproductive Division of Labour in the Primitively Eusocial Wasp Ropalidia Cyathiformis
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Division of labour is a hallmark of eusocial insects and is believed to be a major factor in their evolutionary success and ecological dominance. Division of labour can be of two kinds – reproductive division of labour where a minority of individuals are egg-layers or reproductives (kings and queens) and the majority are workers or non-reproductives involved mostly in non-reproductive tasks of the colony (workers). Kings/queens and workers are often referred to as separate castes within a social insect colony. There may be further non-reproductive division of labour within the worker caste, based on their morphology or age. In primitively eusocial organisms there is no morphological caste differentiation between the egg-layers and non-egg-layers resulting in greater flexibility in the social roles of individuals within a colony. This creates a very interesting scenario to study the mechanism of division of labour. Moreover our knowledge regarding division of labour especially non-reproductive division of labour is very limited for primitively eusocial organisms. In this thesis I have studied division of labour in a primitively eusocial wasp species, Ropalidia cyathiformis. R. cyathiformis is a tropical primitively eusocial wasp with a perennial nesting cycle. This species usually has a single dominant queen and several workers. I studied reproductive and non-reproductive division of labour, as well as the role of dominance behaviour in the regulation of both reproductive and non-reproductive activities. In addition to this I have also compared my findings with what is already known in the well-studied congeneric species, Ropalidia marginata. Reproductive division of labour To understand reproductive division of labour in R. cyathiformis, I studied queen succession, by experimentally removing the queen. When the queen was removed, one and only one individual increased her aggression and became the new queen of the colony, unchallenged by any other worker. Such a successor was referred to as a potential queen (PQ) until she lays her first egg. By removing the queen and successive PQs, I showed that there is not just one successor but a strict reproductive hierarchy of up to 3 PQs, who succeed the queen one after the other. Of many variables tested, I found that only the frequency of dominance behaviour was a significant predictor of whether or not an individual is part of the reproductive hierarchy and also of her position in the hierarchy. Dominance behaviour however does not perfectly predict the position of an individual in the reproductive hierarchy because I showed that an average of three more dominant individuals, are bypassed when an individual becomes the next queen or PQ. This was in contrast to the reproductive hierarchy in the congeneric Ropalidia marginata, where age rather than dominance behaviour was a predictor (though imperfect once again) of an individual’s position in the queue. Taken together, my results suggest that (a) these two sister species have evolved two rather different mechanisms of reproductive caste differentiation, (b) that neither of them strictly conform either to the so called “temperate” or “tropical” patterns of queen succession seen in most other species studied so far. Non-reproductive division of labour As mentioned above, non-reproductive division of labour in eusocial insects is based on either the morphology or the age of the individuals within the colony. Since there is no morphological castes present in primitively eusocial species, I focussed on the effect of age on division of labour in R. cyathiformis. I analysed the frequency as well as the probability of performance of four functionally significant tasks namely, two intranidal tasks – feed larva and build as well as two extranidal tasks – bring food and bring building material. I measured absolute as well as relative ages of the wasps. I found that there is an effect of age on division of labour. Age of first performance of the tasks indicated a clear sequence for the initiation of the tasks with intranidal tasks initiated before extranidal tasks. The frequency of task performance (FTP) and absolute age better explained the variation in the data as compared to probability of task performance (PTP) and relative age. This was in contrast to the pattern of age polyethism found in the congeneric species, Ropalidia marginata, where PTP and relative age better explains the variation in the data. This suggests a more flexible age-dependant division of labour in R. marginata and a rigid age polyethism in R cyathiformis. In addition I found that there was no clear-cut partitioning of the intranidal and extranidal tasks in R. cyathiformis, whereas in R. marginata, it has been shown that the frequency of the intranidal tasks decline with age while that of extranidal tasks increase with age. When taken together, I could say that R. marginata has a more strongly developed age polyethism as compared to R. cyathiformis. This study also shows an evolution of age polyethism with R. cyathiformis behaving more like a typical primitively eusocial species while R. marginata more like a highly eusocial species. Role of dominance behaviour in reproductive and non-reproductive division of labour When reproductive regulation in R. cyathiformis was studied, I found that queens of this species target the potential queen (PQ) by showing the maximum frequency per hour of dominance behaviour to the PQ. The PQs on the other hand seem to show the maximum amount of dominance behaviour towards newborns (wasps of age class 0-5 days). Queens seem to regulate only reproductive activities and not the non-reproductive activities as there was no difference in the frequency of both feed larva and bring food behaviour in the colony even after removing the queen. It also appears that dominance behaviour is not used to signal hunger or regulate foraging as there was no significant correlation between the frequency per hour of bring food behaviour and dominance behaviour received. Moreover the foragers do not receive more aggression than other wasps in the colony from either the queen, PQ or intranidal workers. I also found a significant positive correlation between the frequency per hour of bring food behaviour and feed larva behaviour implying that foraging might be a self-regulated process in this species. Hence in R. cyathiformis it appears that dominance behaviour is used only for regulation of reproductive division of labour and not for regulation of non-reproductive division of labour. This was in contrast to the congeneric species, R. marginata where the opposite has been shown to be true; the reproductive regulation is achieved by means of pheromones produced by the queen and work organisation follows a decentralised self-organised manner with intranidal workers signalling or regulating foragers using dominance behaviour. Comparison with Ropalidia marginata Ropalidia cyathiformis and Ropalidia marginata, although congeneric species co-existing in the same habitat, have evolved very different mechanisms for division of labour. R. marginata exhibiting features such as 1) presence of a docile queen 2) reproductive regulation by means of pheromones 3) strongly developed and flexible age polyethism 4) decentralised work organisation seem to be more similar to a highly eusocial organism than to a primitively eusocial species. R. cyathiformis on the other hand seems to exhibit several features typical to a primitively eusocial species, such as 1) presence of a dominant queen 2) reproductive regulation by physical means 3) relatively weak and rigid age polyethism 4) self-regulatory method of work organisation. Hence the two species seem to be at two different stages of evolution with R. marginata appearing to be intermediate between primitively and highly eusocial species.
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