Cannon Johanna T, Kocot Kevin M, Waits Damien S, Weese David A, Swalla Billie J, Santos Scott R, Halanych Kenneth M
Department of Biological Sciences and Molette Biology Laboratory for Environmental and Climate Change Studies, Auburn University, Auburn, AL 36849, USA; Department of Zoology, Naturhistoriska Riksmuseet, 104 05 Stockholm, Sweden; Friday Harbor Laboratories, University of Washington, 620 University Road, Friday Harbor, WA 98250, USA.
Department of Biological Sciences and Molette Biology Laboratory for Environmental and Climate Change Studies, Auburn University, Auburn, AL 36849, USA; School of Biological Sciences, University of Queensland, Brisbane, QLD 4072, Australia; Friday Harbor Laboratories, University of Washington, 620 University Road, Friday Harbor, WA 98250, USA.
Curr Biol. 2014 Dec 1;24(23):2827-32. doi: 10.1016/j.cub.2014.10.016. Epub 2014 Nov 6.
Ambulacraria, comprising Hemichordata and Echinodermata, is closely related to Chordata, making it integral to understanding chordate origins and polarizing chordate molecular and morphological characters. Unfortunately, relationships within Hemichordata and Echinodermata have remained unresolved, compromising our ability to extrapolate findings from the most closely related molecular and developmental models outside of Chordata (e.g., the acorn worms Saccoglossus kowalevskii and Ptychodera flava and the sea urchin Strongylocentrotus purpuratus). To resolve long-standing phylogenetic issues within Ambulacraria, we sequenced transcriptomes for 14 hemichordates as well as 8 echinoderms and complemented these with existing data for a total of 33 ambulacrarian operational taxonomic units (OTUs). Examination of leaf stability values revealed rhabdopleurid pterobranchs and the enteropneust Stereobalanus canadensis were unstable in placement; therefore, analyses were also run without these taxa. Analyses of 185 genes resulted in reciprocal monophyly of Enteropneusta and Pterobranchia, placed the deep-sea family Torquaratoridae within Ptychoderidae, and confirmed the position of ophiuroid brittle stars as sister to asteroid sea stars (the Asterozoa hypothesis). These results are consistent with earlier perspectives concerning plesiomorphies of Ambulacraria, including pharyngeal gill slits, a single axocoel, and paired hydrocoels and somatocoels. The resolved ambulacrarian phylogeny will help clarify the early evolution of chordate characteristics and has implications for our understanding of major fossil groups, including graptolites and somasteroideans.
包括半索动物门和棘皮动物门的步带动物门与脊索动物门密切相关，这使其对于理解脊索动物的起源以及区分脊索动物的分子和形态特征不可或缺。不幸的是，半索动物门和棘皮动物门内部的关系仍未解决，这削弱了我们从脊索动物门之外最密切相关的分子和发育模型（例如柱头虫Saccoglossus kowalevskii和Ptychodera flava以及海胆Strongylocentrotus purpuratus）推断研究结果的能力。为了解决步带动物门中长期存在的系统发育问题，我们对14种半索动物以及8种棘皮动物的转录组进行了测序，并将这些数据与现有数据相结合，总共得到了33个步带动物门的操作分类单元（OTU）。对叶稳定性值的检查表明，杆壁虫类羽鳃类动物和肠鳃类动物Stereobalanus canadensis在分类位置上不稳定；因此，分析也在没有这些分类群的情况下进行。对185个基因的分析导致肠鳃纲和羽鳃纲的相互单系性，将深海的Torquaratoridae科置于Ptychoderidae科之内，并确认了蛇尾纲脆星作为海星纲海星姐妹群的位置（海星假说）。这些结果与早期关于步带动物门原始特征的观点一致，包括咽鳃裂、单个轴体腔以及成对的水管腔和体腔。解析后的步带动物门系统发育将有助于阐明脊索动物特征的早期演化，并对我们理解包括笔石和桨海星在内的主要化石类群具有启示意义。
