It is a pleasure to announce that the winner of the 2019 Best Paper Award is “Vitellogenin and vitellogenin-like gene expression patterns in relation to caste and task in the ant Formica fusca” by Claire Morandin, Anna Hietala and Heikki Helanterä at the University of Helsinki, Finland. Here is the journey of how this project came to be, in Claire’s words.
When I started my Ph.D. in early 2011, Vitellogenin (Vg) was the “hit” gene that everyone was interested in. Vg is one of the most studied genes involved in the division of labour across social insects. My first Ph.D. project was to look at Vg expression patterns across multiple Formica ant species, but upon constructing the first Formica transcriptome (Dhaygude et al., 2017), we surprisingly came across not only one Vg gene, but four. To understand these new Vg-like-genes (as we named them), we performed gene expression analyses, protein modelling, and evolutionary analyses, and found that these three new homologues were the result of ancient duplications. They partly differ in their conserved protein domains and have undergone rapid evolution after duplications. Furthermore, their expression patterns and thus their likely roles in social regulation were not consistent across the seven Formica species we looked at, providing important new insights into the complexity of insect social behaviour and gene expression variation amongst even closely related species.
Understandably, we got really excited about these Vg-like genes and knew we needed to look deeper into their roles. In species with multiple conventional Vg (such as Solenopsis invicta), the multiple copies show sub-caste and task-related expression patterns, potentially linked to the loss of reproductive constraints and evolution of new functions for the duplicated copies (Wurm et al., 2011). We then questioned if similar caste- and/or task-related expression differences would likewise have emerged during the ancient duplication events between/among the conventional Vg and the three Vg-like genes. Hence, to begin to comprehend the role of Vg homologs and their potential involvement in division of labour, we designed a study that would allow us to investigate the relationships of expression patterns not only between castes (queens vs. workers), but also between different tasks (nurses vs. foragers), between colonies with and without a queen, and between several points in time.
At the same time, Anna started her master thesis with us. It was a challenging and stimulating project that involved a lot of field and lab work. Once the snow melted, we went out around Tvärminne Zoological Station in the south of Finland and collected dozens of F. fusca colonies. The time frame was tight, as we needed to find them after they came out of hibernation, but before they got too active, in order to catch the entire colony (or most of it), the queens included. As we needed colonies with several queens, we could not afford to miss queens while collecting. Back in the lab, the colonies were carefully sorted to find and count all of the queens (that’s a tedious job that involves going through a bucketful or two of nest material and soil for each nest, and hundreds or even a few thousand workers that do not appreciate your efforts). Experimental nests were established in plastic trays with a feeding platform so we could differentiate nurses and foragers. The experiment lasted for 20 days; every five days we collected nurses, queens, and foragers from each nest for gene expression analyses and checked whether queens or workers had been laying eggs. After that, we brought back the samples to the University of Helsinki for gene expression analysis. Anna extracted RNA from more than 500 individuals and performed qPCR analysis (needless to say it was a challenging task for someone who has never done any lab work after one or two basic courses, but she managed brilliantly). Apart from the wet lab work, Anna also dissected the ovaries of a few hundred workers to see whether queenlessness incites ovary development in workers.
Our results showed that each of these genes had a unique caste-specific expression pattern in F. fusca. Expectedly, we found a significant caste and worker task-related increase for the conventional Vg. We found that task (nurses vs. foragers) was the only factor that explained expression variation among workers in any of the studied genes and that removing the queens did affect expression, despite the fact that the proportion of fertile nurses increased significantly. As in previous studies (Kohlmeier et al., 2018; Salmela et al., 2016), our results are consistent with the idea that Vg-like-A may be involved in worker behaviour, and Vg-like-B in stress resistance in ants, while Vg-like-C displayed a consistent forager-biased expression pattern (just like in other social insects (Harrison et al., 2015), suggesting that Vg-like-C might have sub functionalized to a completely different role.
With this project, we aimed to get a clearer picture of the roles of these newly found Vg-like genes for caste differentiation. We still do not know their precise roles, and for example, tissue-specific expression analyses would be an important next step, but at least now we know that their expression patterns are consistent with roles in the division of labour separate from the conventional Vg. We hope this study will spark further interests in these really interesting Vg homologues, and hopefully, at some point we will find out exactly what these genes are doing at a molecular level.
As many projects do, this one took its time as well, and there were challenges along the way. For example, extra effort was needed from Anna who had to write the thesis in English and not in her native Finnish as Claire as her supervisor needed to understand it as well. It was a long journey from the beginning to Anna’s MSc thesis, and then publication in Insectes Sociaux. After all the work for this study, the award means a lot to all of us!
Dhaygude, K., Trontti, K., Paviala, J., Morandin, C., Wheat, C., Sundström, L., & Helanterä, H. (2017). Transcriptome sequencing reveals high isoform diversity in the ant Formica exsecta. PeerJ, 2017(11), 1–31. https://doi.org/10.7717/peerj.3998
Harrison, M. C., Hammond, R. L., & Mallon, E. B. (2015). Reproductive workers show queenlike gene expression in an intermediately eusocial insect, the buff-tailed bumble bee Bombus terrestris. Molecular Ecology, 24(24), 3043–3063. https://doi.org/10.1111/mec.13215
Kohlmeier, P., Feldmeyer, B., & Foitzik, S. (2018). Vitellogenin-like A–associated shifts in social cue responsiveness regulate behavioral task specialization in an ant. PLoS Biology, 16(6), 1–26. https://doi.org/10.1371/journal.pbio.2005747
Salmela, H., Stark, T., Stucki, D., Fuchs, S., Freitak, D., Dey, A., … Sundström, L. (2016). Ancient duplications have led to functional divergence of vitellogenin-like genes potentially involved in inflammation and oxidative stress in honey bees. Genome Biology and Evolution, 8(3), 495–506. https://doi.org/10.1093/gbe/evw014
Wurm, Y., Wang, J., Riba-Grognuz, O., Corona, M., Nygaard, S., Hunt, B. G., … Keller, L. (2011). The genome of the fire ant Solenopsis invicta. Proceedings of the National Academy of Sciences of the United States of America, 108(14), 5679–5684. https://doi.org/10.1073/pnas.1009690108