Mixed, not stirred: Genomic data confirm the first case of interspecific hybridization in planarian triclads (Platyhelminthes: Tricladida) and raise questions about a possibly novel form of hybrid speciation

Speciation is a complex process where many evolutionary forces interplay. The Mediterranean is acknowledged as one of the most relevant biodiverse areas in the Palearctic region and researchers have long studied the species inhabiting it to pursue the goals of evolutionary biology. Here, we study a complex of freshwater flatworm species of the genus Dugesia from Corsica and Sardinia using restriction site-associated DNA sequencing (specifically, 3RAD) data to unravel their evolutionary history and tackle the processes driving it. We assess the phylogenetic relationships and population structure within the group and evaluate new species boundaries using multispecies coalescent approaches. Furthermore, we offer insights into the environmental niche model of the group and use said model to guide our sampling efforts and collect and present molecular evidence for the first time of Dugesia leporii specimens, endemic from Sardinia last spotted in 1999. Our results indicate that paleoclimatic conditions rather than microplate tectonic dynamics were likely an important driver of diversification for the Corso-Sardinian group. Furthermore, our results warrant the taxonomic re-evaluation of the group as eight primary species candidates are established based on molecular data. Our study also reveals the first case of interspecific natural hybridization reported in Dugesiidae and, to our knowledge, in Tricladida. Finally, we discuss how this hybridization might constitute a new form of hybrid speciation.