The objectives of GreenAnt action is to gain greater insight into the molecular genetic basis underlying the greenbeard effect produced by the social supergene evolving in S. invicta. We conducted multiple complementary analyses such as gene content variation, molecular evolutionary signature and gene expression, to identify the genetic bases of the Greenbeard effect in S. invicta.
The first objective is to investigate the Supergene molecular evolution. Using an extensive dataset of sequenced haploid males from multiple socially polymorphic species carrying either the SB or the Sb haplotype, we managed to understand more in depth the recent evolutionary history of the supergene. We show that the three inversions constituting the Solenopsis social supergene emerged sequentially during the speciation process between S. invicta and S. richteri. The two first inversions arose in the ancestral population of the two species and the third one in S. richteri. Once completed, the supergene then introgressed S. invicta first, and from there the three other species. Surprisingly, the supergene introgression occurred despite hybridization being very uncommon among some of the species. In a subsequent study, we analysed the evolutionary fate of the Sb haplotype. We uncovered moderate traces of degeneracy, essentially because the Sb haplotype is rarely found at the homozygous state. Surprisingly, we also uncovered traces of recombination between the SB and the Sb haplotypes. Using both sources of information, we conducted an investigation of candidate genes associated with the phenotypes produced by the Sb haplotype. We identified 48 candidate genes.
The second objective is to conduct a gene expression analyses in organs in which the Greenbeard effect is mediated. The project is more specifically orientated toward the ability of Greenbeard-carrier workers to discriminate carrier from non-carrier queens. Recognition of Sb-queens could be mediated by the product of one or several gene(s) expressed in the antennae or in the brain. We compared patterns of gene expression in antennae and brains between workers of different genotypes, using workers brain RNA-seq data produced in the lab and antennae RNA-seq data produced by Dang et al. 2019. Such analysis allowed us to reduce even further the number of candidate genes.
Lastly, the third objective is to functionally identify the gene allowing Sb-bearer workers to discriminate egg-laying queens. We managed to adapt the CRISPR-Cas9 technique on S. invicta to transform workers. However, while transformation is successful, and allowed us to knockout one candidate gene, we met unexpected challenges which are currently addressed in the lab. Overall, this project has been successful in identifying high quality candidate genes that would be involved in the Greenbeard effect. Only a handful of genes are reuniting the conditions to be involved in the ability of Greenbeard-carrier workers to discriminate carrier from non-carrier queens.
