Understanding diversification and dispersion patterns in planarians (Tricladida, Platyhelminthes): form one to hundreds of genes

Abstract

Evolutionary Biology offers a set of methods and theories to elucidate the evolutionary processes that lead to the diversification of life. With the development of sequencing technologies, the access to genetic data has increased dramatically, and with it, the development of methods and theoretical frameworks that allow studying the evolution of any branch of the Tree of Life. Planarians (Tricladida, Platyhelminthes) are distributed across all biogeographical areas, grouping approximately 1782 species. The freshwater planarians belonging to the Dugesiidae family stand out for their high regenerative capability. This feature allows asexual reproduction by fission in several species, whose fissiparous individuals do not develop reproductive structures. Since the main systematic characters for this group are in the copulatory apparatus, the knowledge regarding the evolution of Tricladida is mostly based on genetic data, and many questions remain to be answered. This thesis aims to continue unravelling the process of diversification and dispersion of Tricladida. To this end, I worked at different taxonomic levels, to answer questions about ancient and recent diversification events. Two different approaches were used. On the one hand, specific molecular markers were used to study groups for which there is almost no phylogenetic information. On the other hand, transcriptomic data was used to study a clade that has been already analysed with few markers and for which specific questions remained unanswered. Internal relationships among the three Tricladida suborders were unveiled using fragments of the ribosomal genes 18S and 28S. The morphological hypothesis, Cavernicola is the sister group of Continenticola, was unsupported by molecular data. The resulting phylogenetic trees supported the monophyly of the Cavernicola, as well as its sister-group relationship to the Maricola. Additionally, the first molecular phylogeny of the Cavernicola suborder, including all genera described at that moment, except one was obtained. Based on these results a cautious biogeographic hypothesis was sketched, in which the Cavernicola originated, presumably before the Gondwana breakup, in a freshwater habitat and subsequently radiated and colonised both epigean and hypogean environments.

Otherwise, Girardia genus is native to the American continent but was reported in Europe in the 1920s and currently is present through all Palearctic regions. Despite their wide distribution and diversity, little is known regarding the evolutionary history of the genus. Here, the first Girardia phylogeny was obtained based on two genes, which resolved old taxonomic questions and unveiled new issues. The diversification process of Girardia, possibly started in South America with posterior colonisation of North American land masses, through different dispersion waves. Additionally, three introduced species were identified across the world, whose invasive potential, given by the fissiparity, can represent a risk for native populations. Finally, the diversification of Dugesia in the Western Mediterranean was analysed using transcriptomic data. The use of phylogenomic methods allowed obtaining a supported phylogeny of the species from this region. The species tree supported a previous biogeographical hypothesis and added new diversification events, due to the inclusion of taxa not analysed before. Furthermore, the unexpected topology of asexual individuals and their effects on the species tree inference could be reflecting the presence of long-term fissiparity in the most ancient clade of Dugesia from the Western Mediterranean. Using different genetic data sources and methodologies was possible to understand several factors that shaped the complex evolutionary process in Triclads. Processes such as new habitats adaptation, geological events, reproductive strategies, and even human intervention, have influenced the diversification and distribution of this group. Special attention deserves the fissiparity, which has played important and different roles in the evolution of freshwater planarians, leaving a genetic footprint in the asexual populations.