Tese de mestrado, Biologia Evolutiva e do Desenvolvimento, Universidade de Lisboa, Faculdade de Ciências, 2020Hybridization is a process known to occur in various animal and plant taxa, leading to the combination of genetic material from previously isolated gene pools. While hybridization can lead to favourable outcomes, such as adaptive introgression, reconstructing speciation and hybridization histories is essential to better understand such outcomes. In Southern Finland, the distributions of the wood ant species Formica polyctena and F. aquilonia overlap and these species interbreed, producing viable and stable hybrid offspring. Whether these hybrid populations have a single origin or were formed through independent hybridization events remained an open question. In this project, we used genome-wide polymorphism data to study speciation and hybridization between F. polyctena and F. aquilonia. We characterized the genetic diversity and differentiation of populations of F. polyctena and F. aquilonia sampled across Europe, and hybrid populations from Finland. We modelled the demographic history of parental species and inferred their relationship with hybrid populations using the site frequency spectrum. To reconstruct the speciation history between F. polyctena and F. aquilonia, we considered alternative models of divergence, with and without gene flow. Across comparisons with different pairs of populations, we found that divergence between these species started in the Pleistocene, with continuous asymmetric gene flow from F. aquilonia into F. polyctena. The genomic patterns consistent with asymmetric migration could not be explained by gene flow from unsampled species, more closely related to F. polyctena or F. aquilonia. To reconstruct the hybridization history in Finland, we tested alternative secondary contact and admixture scenarios. Our results confirm that the three Finnish populations studied likely arose due to hybridization between F. polyctena and F. aquilonia. Our estimates indicate a higher contribution from F. polyctena into all hybrids (0.55 to 0.65 depending on the population), and strongly suggest that the two genetic lineages in Långholmen, the most extensively studied population of F. polyctena x F. aquilonia hybrids, share the same origin. It is, however, unclear whether this is the case for the remaining hybrid populations. This is the first study modelling the demographic history to elucidate the speciation of rufa group species. This allows us to provide insight into speciation with gene flow in eusocial haplodiploid organisms. Our findings concerning admixture between F. polyctena and F. aquilonia expand on the current knowledge on hybridization in the rufa group and will be useful to interpret the observed patterns of variation in F. polyctena x F. aquilonia hybrid genomes
