Tardigrada

Abstract: The complete 18S rDNA gene sequence of Macrobiotus group hufelandi (Tardigrada) was obtained and aligned with 18S rDNA and rRNA gene sequences of 24 metazoans (mainly protostomes) Discrete character (maximum-parsimony) and distance (neighbor-joining) methods were used to infer their phylogeny The evolution of bootstrap proportions with sequence length (pattern of resolved nodes, PRN) was studied to test the resolution of the nodes in neighbor-joining trees The results show that arthropods are monophyletic Tardigrades represent the sister group of arthropods (in parsimony analyses) or they are related with crustaceans (distance analysis and PRN) Arthropoda are divided into two main evolutionary lines, the Hexapoda + Crustacea line (weakly supported), and the Myriapoda + Chelicerata line The Hexapoda + Crustacea line includes Pentastomida, but the internal resolution is far from clear The Insecta (Ectognatha) are monophyletic, but no evidence for the monophyly of Hexapoda is found The Chelicerata are a monophyletic group and the Myriapoda cluster close to Arachnida Overall, the results obtained represent the first molecular evidence for a Tardigrada + Arthropoda clade In addition, the congruence between molecular phylogenies of the Arthropoda from other authors and this obtained here indicates the need to review those obtained solely on morphological characters

Abstract: 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.

Abstract: This paper is the second monograph of nine that describes the global records of limno-terrestrial water bears (Tardigrada). Here, we provide a comprehensive list of non-marine tardigrades recorded from South America, providing an updated and revised taxonomy accompanied by geographic co-ordinates, habitat, and biogeographic comments. It is hoped this work will serve as a reference point and background for further zoogeographical and taxonomical studies.