Data from: Inclusive taxon sampling suggests a single, stepwise origin of ectolecithality in Platyhelminthes

Ectolecithality is a form of oogenesis unique within Metazoa but common in Platyhelminthes, in which nearly-yolkless oocytes and tightly associated yolk cells are deposited together in egg capsules. Despite profound impacts on the embryogenesis and morphology of its beneficiaries, the origins of this developmental phenomenon remain obscure. Traditionally, all ectolecithal flatworms were grouped in a clade called Neoophora. However, there are also morphological arguments for multiple origins of ectolecithality, and Neoophora has to date seen little support from molecular phylogenetic research, largely due to gaps in taxon sampling. Accordingly, we present a molecular phylogeny focused on resolving the deepest divergences among the free-living Platyhelminthes. Species were chosen to completely span the diversity of all major endo- and ectolecithal clades, including several aberrant species of uncertain systematic affinity, and additionally a thorough sampling of the “lecithoepitheliate” higher taxa Prorhynchida and Gnosonesimida, respectively under- and unrepresented in phylogenies to date. Our analyses validate the monophyly of all classical higher platyhelminth taxa, and also resolve a clade possessing distinct yolk-cell and oocyte generating organs (which we name Euneoophora new taxon). Furthermore, implied-weights parsimony and Bayesian mixture model analyses suggest common ancestry of this clade with the lecithoepitheliates, implying that these taxa may retain a primitive form of ectolecithality. This topology thus corroborates the classical hypothesis of homology between yolk cells and oocytes in all Neoophora, and should serve to guide future evolutionary research on this unique developmental innovation in Platyhelminthes.

外卵黄型发生（ectolecithality）是后生动物（Metazoa）中特有的、但在扁形动物门（Platyhelminthes）中广泛存在的一种卵子发生形式，指几乎无卵黄的卵母细胞与紧密结合的卵黄细胞一同被置入卵荚中的生殖模式。尽管该生殖模式对其承载类群的胚胎发生与形态特征具有深远影响，但这一发育现象的起源至今仍不明确。传统上，所有具有外卵黄型发生的扁形动物被归入一个被称为新卵群（Neoophora）的支系。然而，也有形态学证据支持外卵黄型发生存在多次起源的假说，且迄今分子系统发育研究几乎未为新卵群（Neoophora）提供支持，这在很大程度上源于类群采样的缺失。据此，本研究构建了一项分子系统发育分析，旨在解析自由生活扁形动物门内最深层次的分化关系。本研究选取的物种全面覆盖了所有主要内卵黄型与外卵黄型支系的多样性，包括数个系统发育归属不明的异常类群；此外，我们还对迄今系统发育研究中采样不足的卵黄上皮类（lecithoepitheliate）高级类群——前吻目（Prorhynchida），以及完全未被纳入过系统发育分析的格诺松目（Gnosonesimida）进行了充分采样。本研究的分析结果验证了所有经典扁形动物高级分类群的单系性，同时还解析出一个拥有独立卵黄细胞与卵母细胞生成器官的支系，我们将其命名为新分类群优新卵群（Euneoophora）。此外，隐含权重简约法（implied-weights parsimony）与贝叶斯混合模型（Bayesian mixture model）分析结果显示，该支系与卵黄上皮类拥有共同祖先，这意味着这些类群可能保留了外卵黄型发生的原始形式。该系统发育拓扑结构因此验证了所有新卵群中卵黄细胞与卵母细胞同源性的经典假说，并将为后续扁形动物门内这一独特发育创新的进化研究提供指引。