Data from: The first multi-gene phylogeny of the Macrostomorpha sheds light on the evolution of sexual and asexual reproduction in basal Platyhelminthes

The Macrostomorpha—an early branching and species-rich clade of free-living flatworms—is attracting interest because it contains Macrostomum lignano, a versatile model organism increasingly used in evolutionary, developmental, and molecular biology. We elucidate the macrostomorphan molecular phylogeny inferred from both nuclear (18S and 28S rDNA) and mitochondrial (16S rDNA and COI) marker genes from 40 representatives. Although our phylogeny does not recover the Macrostomorpha as a statistically supported monophyletic grouping, it (i) confirms many taxa previously proposed based on morphological evidence, (ii) permits the first placement of many families and genera, and (iii) reveals a number of unexpected placements. Specifically, Myozona and Bradynectes are outside the three classic families (Macrostomidae, Microstomidae and Dolichomacrostomidae) and the asexually fissioning Myomacrostomum belongs to a new subfamily, the Myozonariinae nov. subfam. (Dolichomacrostomidae), rather than diverging early. While this represents the first evidence for asexuality among the Dolichomacrostomidae, we show that fissioning also occurs in another Myozonariinae, Myozonaria fissipara nov. sp. Together with the placement of the (also fissioning) Microstomidae, namely as the sister taxon of Dolichomacrostomidae, this suggests that fissioning is not basal within the Macrostomorpha, but rather restricted to the new taxon Dolichomicrostomida (Dolichomacrostomidae + Microstomidae). Furthermore, our phylogeny allows new insights into the evolution of the reproductive system, as ancestral state reconstructions reveal convergent evolution of gonads, and male and female genitalia. Finally, the convergent evolution of sperm storage organs in the female genitalia appears to be linked to the widespread occurrence of hypodermic insemination among the Macrostomorpha.

大口涡虫类（Macrostomorpha）是一类早期分化、物种丰富的自由生活扁形动物支系，因其包含林氏大口虫（Macrostomum lignano）这一愈发广泛应用于进化生物学、发育生物学及分子生物学研究的多功能模式生物，而受到学界广泛关注。本研究基于40个代表类群的核基因（18S和28S核糖体DNA）与线粒体基因（16S核糖体DNA及细胞色素C氧化酶亚基I，COI）标记基因，解析了大口涡虫类的分子系统发育关系。尽管本研究的系统发育分析未能将大口涡虫类恢复为具有统计支持度的单系群，但该分析取得了三项关键成果：（i）验证了诸多此前仅基于形态学证据提出的类群；（ii）首次完成了诸多科与属的系统位置定位；（iii）揭示了若干意料之外的类群系统位置。具体而言，肌泽涡虫属（Myozona）与缓泳涡虫属（Bradynectes）并不隶属于三大经典科——大口虫科（Macrostomidae）、微口虫科（Microstomidae）与长大口虫科（Dolichomacrostomidae）；而进行无性分裂生殖的肌大口涡虫属（Myomacrostomum）应归入新亚科肌泽涡虫亚科（Myozonariinae nov. subfam.，隶属于长大口虫科），而非早期分化的支系。这不仅是长大口虫科中存在无性分裂生殖的首个实证，本研究还证实裂殖生殖同样存在于另一肌泽涡虫亚科类群——新种裂殖肌泽涡虫（Myozonaria fissipara nov. sp.）。结合（同样进行裂殖生殖的）微口虫科的系统位置（即作为长大口虫科的姊妹群），这表明裂殖生殖并非大口涡虫类的祖征，而是仅局限于新确立的长微口虫类群（Dolichomicrostomida，即长大口虫科+微口虫科）。此外，本研究的系统发育分析为生殖系统的演化提供了新视角：祖先状态重建结果显示，生殖腺及雌雄生殖器均存在趋同演化现象。最后，雌性生殖器中储精器官的趋同演化似乎与大口涡虫类中广泛存在的皮下授精（hypodermic insemination）现象密切相关。