Phylogenetic history of the acquisition of molluscan hosts in acotylean flatworms

How symbionts acquired hosts and diversified phylogenetically during their evolutionary history is a focus of attention in many symbiotic taxa. Marine polyclad flatworms are usually free-living, but some are symbiotic, using animals as hosts. However, the history of their acquisition of symbiotic systems is not well understood. Therefore, we focused on mollusc symbiotic flatworms in the suborder Acotylea and investigated the host specificity and phylogenetic history of the acquisition of symbiosis. Field surveys revealed that symbiotic flatworms utilized certain molluscs as hosts. In particular, <i>Stylochoplana pusilla</i> and <i>Stylochoplana parasitica</i> utilized different molluscan species as hosts sympatrically. The phylogenetic analysis and the ancestral state reconstruction indicate that the mollusc symbiotic flatworms formed a monophyletic group and that their common ancestor shifted from free-living to mollusc symbiosis. These results suggest that each of the flatworms did not independently acquire a symbiotic system with molluscan hosts during its phylogenetic history, but that their common ancestor acquired a mollusc symbiotic system, which then underwent acquisition of host specificity and speciation. This study emphasizes that multiple host use can be a driving force for niche advancement and speciation in the symbionts.