Transcriptomes resolve phylogenetic relationships and reveal undescribed diversity in taildropper slugs (Genus Prophysaon )

The temperate rainforests of the Pacific Northwest of North America harbour many endemic taxa whose evolutionary histories have been shaped by major climatic and geologic events. The enigmatic taildropper slugs (genus Prophysaon) are one example, notable for their ability to autotomize their tails to escape predators. Despite extensive work uncovering the evolutionary histories of individual lineages, relationships among the species of Prophysaon remain poorly understood due to insufficient molecular data. To address this, we collected transcriptomes for six of the nine currently accepted species of Prophysaon. Using these data, we were able to resolve species’ relationships, calling into question the existing subgeneric classification based on morphology. We also detected undescribed phenotypic diversity within the P. andersonii–P. foliolatum species complex, with molecular data supporting the genetic distinctness of two populations exhibiting distinct phenotypes. Finally, our transcriptomic data suggest a moderate role of introgression in shaping the evolutionary history of Prophysaon. Here, we synonymize the subgenus Mimetarion with nominotypical Prophysaon, providing a stable framework for further revision. Future work should investigate whether the undescribed diversity detected here represents species-level differentiation.