Tardigrada, a phylum of meiofaunal organisms, have been at the center of discussions of the evolution of Metazoa, the biology of survival in extreme environments, and the role of horizontal gene transfer in animal evolution. Tardigrada are placed as sisters to Arthropoda and Onychophora (velvet worms) in the superphylum Panarthropoda by morphological analyses, but many molecular phylogenies fail to recover this relationship. This tension between molecular and morphological understanding may be very revealing of the mode and patterns of evolution of major groups. Limnoterrestrial tardigrades display extreme cryptobiotic abilities, including anhydrobiosis and cryobiosis, as do bdelloid rotifers, nematodes, and other animals of the water film. These extremophile behaviors challenge understanding of normal, aqueous physiology: how does a multicellular organism avoid lethal cellular collapse in the absence of liquid water? Meiofaunal species have been reported to have elevated levels of horizontal gene transfer (HGT) events, but how important this is in evolution, and particularly in the evolution of extremophile physiology, is unclear. To address these questions, we resequenced and reassembled the genome of H. dujardini, a limnoterrestrial tardigrade that can undergo anhydrobiosis only after extensive pre-exposure to drying conditions, and compared it to the genome of R. varieornatus, a related species with tolerance to rapid desiccation. The 2 species had contrasting gene expression responses to anhydrobiosis, with major transcriptional change in H. dujardini but limited regulation in R. varieornatus. We identified few horizontally transferred genes, but some of these were shown to be involved in entry into anhydrobiosis. Whole-genome molecular phylogenies supported a Tardigrada+Nematoda relationship over Tardigrada+Arthropoda, but rare genomic changes tended to support Tardigrada+Arthropoda.

Tardigrada是一种系膜生物的门，一直在讨论后生动物的进化，在极端环境中生存的生物学以及水平基因转移在动物进化中的作用。通过形态学分析，将塔氏纲动物作为节肢动物纲上节肢动物和节肢动物（天鹅绒蠕虫）的姐妹，但是许多分子系统发育未能恢复这种关系。分子和形态学理解之间的这种张力可能非常揭示了主要群体进化的模式和模式。线虫的陆栖纲动物显示出极高的隐生能力，包括脱水生物和低温生物，以及b类轮虫，线虫和其他水膜动物也是如此。这些极端微生物的行为挑战了对正常水生理的理解：在缺少液态水的情况下，多细胞生物如何避免致命的细胞崩溃？据报道，淡金鸡物种的水平基因转移（HGT）事件水平升高，但是尚不清楚这在进化中，特别是在极端微生物生理学的进化中有多重要。为了解决这些问题，我们重新测序并重新组装了^ h。dujardini是一种线虫性的斜纹动物纲纲动物，仅在广泛预暴露于干燥条件后才能发生脱水生物，并将其与R的基因组进行比较。varieornatus，对快速干燥具有耐受性的相关物种。这两个物种对脱水生物具有相反的基因表达反应，在H中有主要的转录变化。dujardini，但R中的法规有限。水母。我们确定了很少水平转移的基因，但其中一些已显示参与脱水生物的发生。全基因组分子系统发育支持Tardigrada + Arthropoda的Tardigrada + Nematoda关系，但是罕见的基因组变化倾向于支持Tardigrada + Arthropoda。