I am an MSci student working on my dissertation project in the Sumner lab. My project looked at caste differences and aspects of the colony cycle in a species of Swarm-Founding wasp native to Trinidad; Metapolybia cingulata.

Insights into the determinants of caste can afford us insight into the evolution of sociality.

The division of female members of the colony into worker and queen castes is a major innovation in the evolution of the social insects, and is undeniably the key to their ecological success, allowing reproductive division of labour [2, 3]. Castes may be determined in many ways. In simple societies, castes are behaviourally determined and workers and queens do not differ much, if at all, in their morphology. In complex societies, castes are determined developmentally, and queens and workers are often morphologically very different from one another. These differences are usually based on allometry; differences in body proportions as opposed to just size [1, 2]. Yet, all members of the same colony share the same genome, which raises questions as to how such different phenotypes can develop from the same genetic template. Insights into the determinants of caste can afford us insight into the evolution of sociality.

Queen number in the Swarm-Founding wasps varies, in a process known as the colony cycle [4]. This shift in caste ratios introduces a variable dynamic to the social structure of the colony, meaning that within and between caste relationships change. Several studies have shown caste morphology to vary with the colony cycle, with particular variation seen in queens [1, 5]. The colony cycle provides an interesting opportunity to see how inter-caste relationships may change, and how differences between castes may be affected as a result of this.

Minimal published research is available on caste determination and the colony cycle in M. cingulata. However, studies on other species of swarm founding wasp have demonstrated the complexity of social structure in these social wasps [6, 7, 8]. Morphology and behaviour are both underpinned by genes and the importance of genomic and transcriptional data in these studies has become apparent over recent years. Indeed, the development of bioinformatic techniques has allowed much greater understanding as to caste and species specific patterns of gene expression, but also has allowed us to map patterns across whole lineages of social insects, allowing greater understanding of the evolution of sociality from a genetic point of view [3].

To gain as full an understanding as possible of castes and the colony cycle in M. cingulata, my project has included demographic, morphological and transcriptional data. I began by counting the numbers of queens, workers and males present in each colony by ovary dissection. Thankfully, my ten nests seemed to be at differing stages of the colony cycle, indicated by the ratio of queens to workers. Following this, I extracted the neural RNA from a randomly selected cohort of queens, workers and larvae (larvae here represent the basal level of gene expression), and sent this off to be sequenced. Neural RNA was used as it represents expressed genes (otherwise known as the transcriptome) and will, with any luck, indicate any differential patterns of gene expression that exist between the castes. I also measured a number of morphological characteristics of individuals from each colony. This will allow me to a) determine the extent to which body size and proportions differ between castes and b) to see if any existing differences vary according to the colony cycle by comparing measurements between workers and queens from different colonies.

I hope to find differential patterns of gene expression between castes. Using NCBI BLAST (an online database of DNA, RNA and protein sequences) the identity of expressed sequences can be found. I am also hoping to find morphological differences between queens and workers, these may be differences in size or shape. Previous studies have found that differences between castes tend to be proportional [1]. If such differences exist, I will look for links between my morphological and transcriptomic datasets, hoping to find correlations between the two. Using neural RNA also allows one to find genes involved in behaviour, not just morphology.

Projects such as this can afford us great insight as to the genetic and morphological determinants of castes.

Analysing caste differences at the level of genes can illuminate aspects of sociality that were previously hidden. Projects such as this can afford us great insight as to the genetic and morphological determinants of castes. They provide a good opportunity to compare the mechanisms of caste determination between different species and lineages, allowing similarities and differences to be revealed and patterns of social evolution to be mapped.

 

Laura Butters

 

References:

1. Baio, M. V, Noll, F. B., & Zucchi, R. (2003). Shape differences rather than size differences between castes in the Neotropical swarm-founding wasp Metapolybia docilis (Hymenoptera: Vespidae, Epiponini). Biomedical Central Evolutionary Biology, 9, 1–9.
2. Bell, E. & Sumner, S. (2013). Ecology and social organisation of wasps. eLS, DOI: 10.1002/9780470015902.a0023597
3. Sumner, S. (2014). The importance of genetic novelty in social evolution. Molecular Ecology, 23, 26-28.
4. West-Eberhard, M. J. (1978). Temporary queens in Metapolybia wasps: nonreproductive helpers without altruism? Science, 200, 441-443.
5. Hoffman, E. & Goodisman, M. A. D. (2007). Gene expression and the evolution of phenotypic diversity in social wasps. BMC Biology, 5, 23.
6. Hunt, J. H., Woschin, F., Henshaw, M. T., Newman, T. C., Toth, A. L. & Amdam, G. V. (2010). Differential gene expression and protein abundance evince ontogenetic bias toward castes in a primitively eusocial wasp. PLoS ONE, 5, e10674.
7. Pereboom, J. J. M., Jordan, W. C. & Sumner, S., Hammond, R. L. & Bourke, A. F. G. (2005). Differential gene expression in queen-worker caste determination in bumble-bees. Proceedings of the Royal Society of Biology, 272, 1145-1152
8. Ferreira, P. G., Patalano, S., Chauhan, R., Ffrench-constant, R., Gabaldón, T., Guigó, R., & Sumner, S. (2013) Transcriptome analyses of primitively eusocial wasps reveal novel insights into the evolution of sociality and the origin of alternative phenotypes. Genome Biology, 14, R20.