In horror films of the 1950s mutant insects were a nightmare. Now they are a dream that is becoming a reality, at least for those interested in studying the molecular bases of social behaviors. Two teams have just succeeded in modifying ants DNA by knocking down their olfactory system and consequently putting their social interactions in disarray. The experiments, published in August in Cell, herald the beginning of a new scientific adventure: ants are set to become the model organism for eusociality studies in the post-genomic era.
Pheromones are the primary communication medium for these insects, and many genes control their perception, but the whole gene family can be taken out by deleting a single element called Orco. Both teams directed CRISPR molecular scissors against this target but in different species. Danny Reinberg of the New York University School of Medicine and his colleagues chose the Indian ants Harpegnathos saltator, where all females are potentially fertile, not just the queen. Humble workers, who are generally sterile, can become pseudo-queens in this species and lay eggs, if appropriately stimulated. The other team, led by Daniel Kronauer of Rockefeller University, targeted raider ants native to Asia (Ooceraea biroi) which reproduce asexually, by parthenogenesis. They have no queen, and daughters are natural clones of their mothers. In both cases, the peculiarities of reproductive biology of the selected species helped create mutants having almost no smell, from mothers genetically modified with CRISPR. They exhibited bizarre and asocial behavior, such as wandering away from the colony and not foraging. The two Cell papers can be considered complementary, and suggest mechanisms common to all kinds of ants, even to all social insects, according to Reinberg who answered our questions by email.
The exploits were made possible by CRISPR’s invention in 2012, confirms the geneticist. “We started about ten years ago, and it was only possible through the passion, dedication and tremendous work of the students and postdocs involved in the project. In H. saltatory it is much harder to generate genetically modified ants, and at each step we move forward we had to overcome new challenges. The reproductive cycle takes three months, therefore to get homozygote mutant ants it takes not less than nine months”. Other scientists are eager to replicate the experiments with bees, but with ants there is still much to do. There is a CRISPR variant that doesn’t edit the string of DNA letters but changes the expression of the targeted gene by removing or adding few simple chemical groups (epigenetic markers). This is an excellent tool for studying ants, whose biology may differ a lot even when they have an identical genome. Indeed with the same DNA sequence, one can be a queen, a worker or a soldier, and Kronauer now plans to study how colonies divide their labor. Reinberg is focusing on a different issue. “H. saltator offers unique opportunities to study epigenetics. Currently, we are looking at aging, as queens live on average 10X longer than workers (all females, males in this case just provide the sperm and are very short lived). Most interestingly in all systems used to study aging, reproduction and longevity function in opposition, but in ants the phenomenon is different, they go hand by hand, so stay tuned”.