I’m approximately one year late in posting this, but the Myrmecological News Blog interviewed Justin Walsh and Rohini Singh about “Doing an ant PhD”! The posts include photos of Rohini and Justin hard at work.
Michael’s thesis chapter on the transcriptomic basis of the convergent evolution of caste-based division of labor in pharaoh ants and honey bees is now out at Nature Communications!
We designed this large study (based on 177 RNA seq libraries) to attempt to determine the degree to which genes underlying the expression of the eusocial caste system are shared between two species (the pharaoh ant Monomorium pharaonis and the honey bee Apis mellifera), which represent two independent origins of eusociality. We collected parallel developmental time series (egg, larval, pupal samples) for reproductive-destined and worker-destined brood, as well as adult queen, worker, and male samples for both species, and we sequenced and analyzed all samples in parallel to provide an unbiased estimate of the degree of transcriptomic overlap for caste-based division of labor in pharaoh ants and honey bees.
We identified a large set of genes (~1,500 genes) with caste-biased expression that was shared between pharaoh ants and honey bees, and this shared transcriptomic architecture represented ~30% of all caste-biased genes. Most of these genes were differentially expressed in adult abdominal tissues, providing a strong transcriptomic signature of the reproductive caste system (this is not surprising given that the main physiological difference between queens and workers in both ants and honey bees is that queens reproduce and workers do not, and female reproductive organs are in the abdomen). Interestingly, we found that this ~30-35% overlap for caste-associated genes is approaching the degree of overlap (~40%) that we observed of genes differentially expressed across development for honey bees and pharaoh ants. This is striking given that development is considered to be strongly conserved, suggesting that the molecular mechanisms underlying the reproductive caste system are also strongly conserved. All of this suggests that the convergent evolution of the reproductive caste system in ants and honey bees involved the recruitment of a pre-exisiting “reproductive groundplan” of genes associated with female reproductive physiology. At the same time, we also found that the majority (~65-70%) of genes with caste-associated expression were not shared between pharaoh ants and honey bees, also highlighting that the independent evolution of the caste system in the ancestors of ants and honey bees also involved the recruitment of many lineage-specific genes and genes with lineage-specific expression patterns. Overall, our results highlight the importance of both conserved and lineage-specific genes for the evolution of phenotypic innovations such as caste-based division of labor.
The first manuscript from Rohini’s thesis is now on biorxiv, showing how the bacterial endysombiont Wolbachia increases reproductive investment (i.e. queen production) and accelerates the life cycle of pharaoh ant colonies. This study uses two assays to show that Wolbachia seems to have a positive effect on pharaoh ant colonies (infected colonies grow faster and produce more queens), so that Wolbachia infection might be expected to quickly sweep through pharaoh ant populations. This positive effect of Wolbachia infection on host colony growth and reproduction is likely particularly beneficial in rapidly growing populations. Notably, pharaoh ants are considered to be one of the most successful and widespread invasive ants, and perhaps Wolbachia contributes to the pharaoh ant’s invasiveness.
After more than 10 years of hard work establishing the pharaoh ant mapping population by systematically intercrossing 8 initial parental lineages, we have completed the first manuscript using the heterogeneous stock mapping population. In the manuscript now on biorxiv, “Ant collective behavior is heritable and shaped by selection“, Justin quantified variation for three collective behaviors, as well as caste ratio, sex ratio, and body size for three replicate sub-colonies of 81 distinct colony genotypes of known pedigree to estimate heritability and genetic correlations for the traits, as well as the strength and pattern of selection acting on the traits.
I am currently on sabbatical as part of a syngenomics focus group at the Wissenschaftskolleg zu Berlin (Berlin Institute of Advanced Studies).
(me with Mike Wade, Jason Wolf, and Judie Bronstein)
Michael led a study on the transcriptomic basis and evolution of the pharaoh ant “social interactome”, i.e. the set of genes expressed in interacting worker nurses and developing larvae that affects larval development, which is now available on bioRxiv .
Michael led a study on the genomic basis of the convergent evolution of reproductive caste and worker age polyethism in honey bees and pharaoh ants, which is now available on bioRxiv: https://www.biorxiv.org/content/early/2018/10/26/454645 .
Our previous manuscripts on bioRxiv have now been published!
Justin, Michael, Adrian, and Ben have submitted a manuscript showing behavioral and transcriptomic specialization of pharaoh ant nurses based on the age of larvae they feed: https://doi.org/10.1101/218834.
Michael and Jessica have submitted a manuscript showing how pharaoh ant colonies regulate caste and colony demography by culling sexual brood: https://doi.org/10.1101/211573
Our paper ” Genomic signature of kin selection in an ant with obligately sterile workers” is finally accepted at Molecular Biology and Evolution!
Michael’s first paper in the lab and the first from a PhD student in the lab!