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

Abstract

The free‐living flatworm Macrostomum lignano is used as a model in a range of research fields—including aging, bioadhesion, stem cells, and sexual selection—culminating in the 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 now impair the development of additional resources and tools. Briefly, M. lignano has undergone a whole‐genome duplication, followed by rediploidization 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 fulfills 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.

Variation in sex allocation plasticity in three closely related flatworm species

“…Macrostomum are in species that are hypodermically inseminating (e.g., M. hystrix, see Ramm, Vizoso, & Schärer, 2012;Ramm, Schlatter, Poirier, & Schärer, 2015;and M. pusillum, Giannakara & Ramm, 2017), with both species showing a similar needle-like stylet morphology that potentially facilitates self-fertilization by allowing self-injection of sperm. In contrast, the current study documents the first occurrence of self-fertilization in a reciprocally copulating species, M. mirumnovem, which has a large blunt-ending stylet (Schärer et al, 2019), and for which we currently do not understand how selfing is actually achieved.…”

“…The three study species used here are all recently described freeliving flatworms of the genus Macrostomum (Macrostomorpha, Platyhelminthes), and they include M. janickei (see also Zadesenets, Schärer, & Rubtsov, 2017;Zadesenets et al, 2016), M. cliftonensis, and M. mirumnovem (Schärer et al, 2019). Briefly, the worms used in the experiments were from laboratory cultures that were established using individuals collected from Palavas-les-Flots, near Montpellier, France, for M. janickei; from Lake Clifton, South of Perth, Western Australia, for M. cliftonensis; and from Port Phillip Bay, Queenscliff, Victoria, Australia, for M. mirumnovem (Schärer et al, 2019). Moreover, these species are all close relatives of our main model species, M. lignano, with M. janickei being the closest, as shown by molecular phylogenetic analyses (Schärer et al, 2019), using M. hystrix as an out-group species (see also Schärer, Littlewood, Waeschenbach, Yoshida, & Vizoso, 2011).…”

“…Briefly, the worms used in the experiments were from laboratory cultures that were established using individuals collected from Palavas-les-Flots, near Montpellier, France, for M. janickei; from Lake Clifton, South of Perth, Western Australia, for M. cliftonensis; and from Port Phillip Bay, Queenscliff, Victoria, Australia, for M. mirumnovem (Schärer et al, 2019). Moreover, these species are all close relatives of our main model species, M. lignano, with M. janickei being the closest, as shown by molecular phylogenetic analyses (Schärer et al, 2019), using M. hystrix as an out-group species (see also Schärer, Littlewood, Waeschenbach, Yoshida, & Vizoso, 2011). All species are kept in mass cultures in the laboratory at 20°C in Petri dishes containing 32‰ artificial seawater (ASW) and fed with the diatom Nitzschia curvilineata.…”

“…In light of an emerging comparative context, we here studied SA plasticity in three Macrostomum species, namely M. janickei, M. cliftonensis, and M. mirumnovem, which are all close relatives of the established model species M. lignano (Schärer et al, 2019). To manipulate the mating group size, we raised worms in three different social group sizes-isolated, pairs, or octets-given that social group size is a good proxy for the mating group size, as previously shown for M. lignano (Janicke et al, 2013).…”

Genome and Karyotype Reorganization after Whole Genome Duplication in Free-Living Flatworms of the Genus Macrostomum

“…Several closely related species belonging to the free-living flatworm genus Macrostomum, the subject of our study here, are a fortuitous exception to this pattern [34] (see also Figure S1). All Macrostomum species analyzed to date are characterized by relatively low chromosome numbers, ranging from generally 2n = 6 to more rarely 2n = 12 [35,36], and they have fairly small genome sizes of <1000 Mbp, going as low as 217 Mbp [34,37]. Crucially, at least three Macrostomum species have undergone recent WGD events.…”