Phylogenomic resolution of sea spider diversification through integration of multiple data classes

Despite significant advances in invertebrate phylogenomics over the past decade, the higher-level phylogeny of Pycnogonida (sea spiders) remains elusive. Due to the inaccessibility of some small-bodied lineages, few phylogenetic studies have sampled all sea spider families. Previous efforts based on a handful of genes have yielded unstable tree topologies. Here, we inferred the relationships of 89 sea spider species using targeted capture of the mitochondrial genome, 56 conserved exons, 101 ultraconserved elements, and three nuclear ribosomal genes. We inferred molecular divergence times by integrating morphological data for fossil species to calibrate 15 nodes in the arthropod tree of life. This integration of data classes resolved the basal topology of sea spiders with high support. The enigmatic family Austrodecidae was resolved as the sister group to the remaining Pycnogonida and the small-bodied family Rhynchothoracidae as the sister group of the robust-bodied family Pycnogonidae. Molecular divergence time estimation recovered a basal divergence of crown group sea spiders in the Ordovician. Comparison of diversification dynamics with other marine invertebrate taxa that originated in the Paleozoic suggests that sea spiders and some crustacean groups exhibit resilience to mass extinction episodes, relative to mollusk and echinoderm lineages.尽管过去十年间无脊椎动物系统基因组学已取得显著进展，但海蜘蛛纲（Pycnogonida）的高阶系统发育关系仍未明确。由于部分小型躯体支系难以获取，几乎没有系统发育研究能够全面采样所有海蜘蛛科类群。此前基于少量基因开展的研究，得到的系统树拓扑结构并不稳定。本研究通过靶向捕获线粒体基因组、56个保守外显子、101个超保守元件以及3个核核糖体基因，解析了89种海蜘蛛的系统发育关系。本研究整合化石物种的形态数据，对节肢动物生命树中的15个节点进行定标，以此推断分子分化时间。这种多数据类型的整合分析以高支持度解析了海蜘蛛的基部系统发育拓扑结构。此前颇具争议的澳蛛科（Austrodecidae）被确定为其余海蜘蛛纲类群的姊妹群，而小型躯体的吻胸科（Rhynchothoracidae）则为躯体粗壮的海蜘蛛科（Pycnogonidae）的姊妹群。分子分化时间估算结果显示，冠群海蜘蛛的基部分化发生在奥陶纪。将本研究的类群分化动态与其他起源于古生代的海洋无脊椎动物类群进行对比后发现：相较于软体动物和棘皮动物支系，海蜘蛛以及部分甲壳动物类群对集群灭绝事件具有更强的耐受能力。