Introgressive hybridization between anciently diverged lineages of Silene (Caryophyllaceae).

Hybridization has played a major role during the evolution of angiosperms, mediating both gene flow between already distinct species and the formation of new species. Newly formed hybrids between distantly related taxa are often sterile. For this reason, interspecific crosses resulting in fertile hybrids have rarely been described to take place after more than a few million years after divergence. We describe here the traces of a reproductively successful hybrid between two ancestral species of Silene, diverged for about six million years prior to hybridization. No extant hybrids between the two parental lineages are currently known, but introgression of the RNA polymerase gene NRPA2 provides clear evidence of a temporary and fertile hybrid. Parsimony reconciliation between gene trees and the species tree, as well as consideration of clade ages, help exclude gene paralogy and lineage sorting as alternative hypotheses. This may represent one of the most extreme cases of divergence between species prior to introgressive hybridization discovered yet, notably at a homoploid level. Although species boundaries are generally believed to be stable after millions of years of divergence, we believe that this finding may indicate that gene flow between distantly related species is merely largely undetected at present.