Can Ectoparasite Phylogenetics Shed Light on Host Evolution? The <i>Batracobdella</i> Leeches and <i>Speleomantes</i> Salamanders' System

Understanding species' evolutionary history is essential for comprehending trait diversity. Evolutionary relationships among many species, however, have conflicted phylogenetic inferences due to evolutionary discordances and need additional sources of information. Phylogenetic relationships of symbionts can inform about the evolution of their hosts. In this study, we reconstructed the evolutionary relationships among Sardinian salamander ectoparasitic leeches to resolve evolutionary discordances among their hosts, the Sardinian <i>Speleomantes</i> salamander species, and proposed novel evolutionary hypotheses that account for host-parasite evolutionary processes. We inferred the most up-to-date phylogeny for <i>Batracobdella</i> leeches and uncovered high levels of genetic diversity in <i>B. algira</i>, a species commonly known as the amphibian leech, revealing three divergent lineages. Leech phylogeny complemented previous information about Sardinian <i>Speleomantes</i> evolutionary history. Our comparison of this parasite phylogeny with the most recent inferred salamander tree revealed potential processes of host-parasite co-evolution and a host-switching event in the studied system. In both trees, a clade comprising species with the most northeastern distributions in Sardinia (<i>S. flavus</i>, <i>S. imperialis</i>, and <i>S. supramontis</i> and their respective leeches) was recovered. The phylogenetic position of the <i>S. genei</i> ectoparasitic leech was incongruent with that of its host in previous studies, which recovered <i>S. genei</i> as sister to the other Sardinian <i>Speleomantes</i>. Disentangling the evolutionary processes underlying these host-parasite interactions is important for understanding not only the evolution of these parasitic leeches but also that of their amphibian hosts.