The genetic basis and ground plan of eusocial worker evolution

Lead Research Organisation: University of East Anglia
Department Name: Biological Sciences

Abstract

The major transitions are foundational to the evolution of individuality and the hierarchical structure of all life on Earth. In these events, previously independent individuals come together to form a new, higher-level cooperative group resembling an individual in its own right. Classic examples are the evolution of multicellularity and eusociality. Eusocial societies are those in which some members (workers) act altruistically towards their fellow members, as in the ants, bees and wasps. Evolutionary biology has successfully explained the ultimate reasons for transitions like the evolution of multicellularity and eusociality in terms of kin selection. But the actual genes involved in these major transitions remain very poorly known. Just as researchers need to know what genes lead a cell to become a somatic (altruistic) or germline (reproductive) cell, so do we need to know what genes lead workers in eusocial societies to be non-reproductive (altruistic) workers or reproductive workers. This project aims to fill this key gap by using new technologies to determine the genetic basis of worker altruism versus worker reproduction in a eusocial insect.

The study system will be the bumble bee, Bombus terrestris, whose workers can either be non-reproductive sterile altruists (with inactive ovaries) or reproductive egg-layers. Bumble bees are the living group most closely resembling the primitively eusocial common ancestor of the two independently-evolved advanced eusocial bee taxa, the honey bees and stingless bees. Gene expression in reproductive and non-reproductive workers has previously been profiled in honey bees, but only studies in primitively eusocial insects can provide an evolutionary perspective on the critical gene expression differences. Hence primitive eusociality in B. terrestris, and the position of B. terrestris in the bee phylogeny, make it an excellent study system for the proposed work.

Using B. terrestris, the research will (1a) determine the gene expression differences within selected tissues (brain, fat body and ovary) that underpin the differences between reproductive and non-reproductive workers, (1b) validate the most salient of these differences using independent gene expression assays, (1c) determine how key genes change expression over time as the worker reproductive phenotype arises (so separating out upstream- and downstream-acting genes in the regulatory pathways), (2) establish how reversals in workers' reproductive phenotype are tracked by gene expression changes, and (3) test a highly influential hypothesis for the evolution of the worker reproductive phenotype. This hypothesis, the reproductive ground plan hypothesis, proposes that division of labour (e.g. changes in task with age) in workers is regulated by genetic pathways co-opted from pathways regulating reproduction in females of workers' non-social ancestors (the 'ground plan'). It is important because it represents a key example of the general idea that evolutionary novelties like worker altruism can arise through the co-option of existing gene pathways.

Methods will consist of laboratory and field experiments, next-generation sequencing (RNA-Seq transcriptomics), quantitative PCR (qRT-PCR) and bioinformatics. The research is fundamental because it deals with phenomena of basic importance and broad interest in evolutionary ecology, genetics and transcriptomics. It is novel because, although a growing number of studies are elucidating molecular aspects of social evolution, an evolutionary perspective on the genetic basis of workers' loss of reproductive ability is lacking, the relationship between flexibility in the workers' reproductive phenotype and gene expression is not understood and the important reproductive ground plan hypothesis requires testing. The work will therefore substantially advance our deep understanding of the genetic foundations of eusociality and the major transitions.

Planned Impact

As well as having a substantial academic impact, this research will potentially benefit four other groups, namely commercial bumble-bee rearing companies, bee-related NGOs, policy-makers and practitioners in government departments covering biodiversity and agriculture, and the general public.

Many companies worldwide mass-rear bumble bee colonies for the pollination of glasshouse crops. These companies would potentially benefit from the project because an improved understanding of the genetic basis of reproduction and division of labour (including foraging behaviour) in bumble bee workers might aid in large-scale bee-rearing and in achieving effective, sustainable commercial pollination. The PI is committed to dialogue with the bee-rearing companies via existing projects and will use such dialogue to promote relevant results of the new research.

Several organizations, including the Bumblebee Conservation Trust, the British Beekeepers Association, the Bee Guardian Foundation and Buglife, are active in conducting and/or promoting the understanding and conservation of bees. The project would benefit from their aid in disseminating results and impact and they would benefit from having, if they chose, a science-based bee-related 'story' to publicize. The project team will send team members to the Bumblebee Working Group (BWG) meeting, a forum for UK bumble bee conservation held every 1-2 years, which is regularly attended by representatives of relevant NGOs. The PI regularly attends the BWG (and has previously hosted it). Existing contacts with the NGOs will be used for direct communication outside the BWG.

Policy-makers and practitioners in government departments covering biodiversity and agriculture have an interest in understanding the biology of insect pollinators in its broadest sense. This group might benefit from the evidence base that the project will provide as regards bumble bee genetics and workers' reproductive biology. The PI will seek opportunities to present relevant results to policy-makers and practitioners, building on contacts made at two 2012 workshops (UK government's Science and Innovation Network workshop of experts on 'Insect Pollinators: From Research to Policy', London, February 2012; Insect Pollinators Initiative midterm workshop, York, November 2012). The PI has a record of seeking to maximise the impacts of his research, for example through applying genetic census techniques that he and colleagues have developed for wild bumble bee populations to projects exploring the population biology of declining bumble bee species or testing the effectiveness of agri-environment schemes for bumble bees in the UK.

Bumble bees are immensely popular organisms with the public, and there is widespread affection for them and appreciation of their pollinating role. In addition, ongoing publicity over declines of honey bees and wild bees has increased awareness of the need to conserve bee populations. The public would benefit from learning the results of the project to satisfy the clear appetite that therefore exists for scientific information on the biology of bees. The project team will engage the public with the results of the research via press releases, dedicated project webpages and events such as exhibitions and public talks. The PI has wide experience of dealing with the media and a demonstrable commitment to interacting with the public in the manner proposed.

Publications

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Description The primary goal of the research was to profile, using RNA-seq, gene expression differences that underpin the differences between reproductive and non-reproductive workers in the bumble bee Bombus terrestris, so as to advance our understanding of the genetic basis of eusocial evolution. Our main findings are as follows:

1. We isolated differentially expressed genes (DEGs) in three tissues across age-matched ovary-active versus inactive workers of B. terrestris. From Gene Ontology (GO) analyses of these genes, we found that ovary-active workers primarily upregulate gene pathways associated with reproductive physiology and either do not differentially alter or down-regulate those associated with brain function and behaviour. Our other findings were:

(a) By also characterising gene expression in newly-laid eggs in B. terrestris workers, we found that many DEGs in ovaries of ovary-active versus inactive workers were attributable to gene expression in eggs. This novel finding affects conclusions of previous studies finding DEGs in eusocial insect worker ovaries.
(b) Differentially expressed novel transcripts were comparatively scarce and were likely to produce non-coding RNAs rather than proteins. This suggested that most genes underpinning worker reproductivity in B. terrestris are conserved across the Hymenoptera and other insects.
(c) From a comparative analysis of DEGs associated with worker reproductivity in B. terrestris and the advanced eusocial honey bee Apis mellifera, there was significant overlap between DEGs in B. terrestris and A. mellifera workers at the gene level but little overlap at the GO level.

2. We isolated DEGs associated with the process of ovary activation using RNA-seq profiling of the same three worker tissues across a 7-day time-course in which workers were manipulated to activate their ovaries.

3. We tested the hypothesis that ovary-active workers resemble queens in their gene expression profiles by determining the level of similarity in the expression patterns of novel transcripts in ovary-active workers versus mature and virgin queens, with the queen patterns being determined by qRT-PCR. We found that 50% of differentially expressed novel transcripts had expression patterns unique to workers, but 50% showed expression patterns shared with queens, suggesting that ovary-activation in workers is underpinned by both novel and conserved genes.

This study represents one of the most comprehensive of DEGs associated with worker reproductivity in eusocial Hymenoptera. Our findings, which are currently being prepared for publication, add considerably to our fundamental understanding of the molecular basis of worker reproductivity and, by extension, the loss of reproductivity during eusocial evolution.
Exploitation Route Many companies worldwide mass-rear colonies of Bombus terrestris and, increasingly, other Bombus species, for sale as pollinators of commercial glasshouse crops. Given worker bees make the most efficient and frequent pollinators, and ovary-active workers generally forage less, an improved understanding of the molecular basis of worker reproductivity in bumble bee colonies might help maximize the effectiveness of pollination by commercially-reared colonies. By isolating genes associated with worker reproductivity in B. terrestris, our work provides a step along the path to manipulating colonies in order achieve this.
Sectors Agriculture, Food and Drink,Environment

 
Description Poster display at UEA Open Day 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Many members of the public viewed a poster displaying our IPI project work and expressed interest in the methods and conclusions.

n/a
Year(s) Of Engagement Activity 2014,2015