Rapid speciation with gene flow following the formation of Mount Etna
Environmental or geological changes can create new niches which drive ecological species divergence without the immediate cessation of gene flow. However, few such cases have been characterised. On the recently formed volcano, Mt. Etna, Senecio aethnensis and S. chrysanthemifolius inhabit contrasting environments of high and low altitude respectively. They have very distinct phenotypes, despite hybridising promiscuously, and thus may represent an important example of ecological speciation ‘in action’, possibly as a response to the rapid geological changes which Mt. Etna has recently undergone. To elucidate the species' evolutionary history, and help establish the species as study system for speciation genomics, we sequenced the transcriptomes of the two Etnean species, and the outgroup, S. vernalis, using Illumina sequencing. Despite the species' substantial phenotypic divergence, synonymous divergence between the high- and low-altitude species was low (dS = 0.016 ± 0.017 [SD]). A comparison of species divergence models with and without gene flow provided unequivocal support in favor of the former and demonstrated a recent time of species divergence (153,080 ya ± 11,470[SE]) that coincides with the growth of Mount Etna to the altitudes which separate the species today. Analysis of dN/dSrevealed wide variation in selective constraint between genes, and evidence that highly expressed genes, more ‘multifunctional’ genes and those with more paralogues were under elevated purifying selection. Taken together, these results are consistent with a model of ecological speciation, potentially as a response to the emergence of a new, high altitude niche as the volcano grew.