A phylogenetically informed search for an alternative Macrostomum model species, with notes on taxonomy, mating behavior, karyology, and genome size.

The free-living flatworm Macrostomum lignano is used as a model in a range of research fields—including ageing, bioadhesion, stem cells, and sexual selection—leading to the recent establishment of genome assemblies and transgenics. However, the Macrostomum community has run into a roadblock following the discovery of an unusual genome organization in M. lignano, which could impair the development of essential resources and tools. Briefly, M. lignano has undergone a whole-genome duplication, followed by rediploidisation into a 2n=8 karyotype (distinct from the canonical 2n=6 karyotype in the genus). Although this karyotype appears visually diploid, it is in fact a hidden tetraploid (with rarer 2n=9 and 2n=10 individuals being pentaploid and hexaploid, respectively). Here we report on a phylogenetically-informed search for close relatives of M. lignano, aimed at uncovering alternative Macrostomum models with the canonical karyotype and a simple genome organization. We taxonomically describe three new species: the first, Macrostomum janickei n. sp., is the closest known relative of M. lignano, and shares its derived genome organization; the second, Macrostomum mirumnovem n. sp., has an even more unusual genome organization, with a highly variable karyotype based on a 2n=9 base pattern; and the third, Macrostomum cliftonensis n. sp., does not only show the canonical 2n=6 karyotype, but also performs well under standard laboratory culture conditions and fulfils many other requirements. M. cliftonensis is a viable candidate for replacing M. lignano as the primary Macrostomum model, being outcrossing and having an estimated haploid genome size of only 231 Mbp.