SUMMARY Tardigrades are microscopic animals that survive a remarkable array of stresses, including desiccation. How tardigrades survive desiccation has remained a mystery for more than 250 years. Trehalose, a disaccharide essential for several organisms to survive drying, is detected at low levels or not at all in some tardigrade species, indicating that tardigrades possess potentially novel mechanisms for surviving desiccation. Here we show that tardigrade-specific intrinsically disordered proteins (TDPs) are essential for desiccation tolerance. TDP genes are constitutively expressed at high levels or induced during desiccation in multiple tardigrade species. TDPs are required for tardigrade desiccation tolerance, and these genes are sufficient to increase desiccation tolerance when expressed in heterologous systems. TDPs form non-crystalline amorphous solids (vitrify) upon desiccation, and this vitrified state mirrors their protective capabilities. Our study identifies TDPs as functional mediators of tardigrade desiccation tolerance, expanding our knowledge of the roles and diversity of disordered proteins involved in stress tolerance.

The superphylum Panarthropoda (Arthropoda, Onychophora, and Tardigrada) exhibits a remarkable diversity of segment morphologies, enabling these animals to occupy diverse ecological niches. The molecular identities of these segments are specified by Hox genes and other axis patterning genes during development [1, 2]. Comparisons of molecular segment identities between arthropod and onychophoran species have yielded important insights into the origins and diversification of their body plans [3-9]. However, the relationship of the segments of tardigrades to those of arthropods and onychophorans has remained enigmatic [10, 11], limiting our understanding of early panarthropod body plan diversification. Here, we reveal molecular identities for all of the segments of a tardigrade. Based on our analysis, we conclude that tardigrades have lost a large intermediate region of the body axis-a region corresponding to the entire thorax and most of the abdomen of insects-and that they have lost the Hox genes that originally specified this region. Our data suggest that nearly the entire tardigrade body axis is homologous to just the head region of arthropods. Based on our results, we reconstruct a last common ancestor of Panarthropoda that had a relatively elongate body plan like most arthropods and onychophorans, rather than a compact, tardigrade-like body plan. These results demonstrate that the body plan of an animal phylum can originate by the loss of a large part of the body.

Within the framework of a DNA barcoding project on tardigrade species, a study was carried out on Macrobiotus hufelandi C.A.S. Schultze 1834, the first formally described tardigrade species. We used samples collected from the type locality and additional material from other European sites containing species of the “M. hufelandi group”. The study was performed by integrating morphological, karyological and molecular (mt-DNA cox1) information and comparing these data with morphological data from the type material. Several species from this group were found in the type locality of M. hufelandi (near Freiburg, Black Forest, Germany) and these were all barcoded. One was M. hufelandi, the other two were: Macrobiotus sandrae Bertolani & Rebecchi 1993 (originally described from the same locality), and Macrobiotus vladimiri Bertolani, Biserov, Rebecchi & Cesari in press (type locality Andalo, Italy), all with interspecific genetic distances of more than 19%. A fourth cryptic species, which had the same morphology as M. hufelandi but a genetic distance of 6.7%, was not described as a new taxon but named M. cf. hufelandi sp.1 for this study. Macrobiotus sandrae and M. vladimiri were also present (and barcoded) in Italy (Alps). Additional individuals (animals and eggs) were also found, and barcoded, in Italy (Apennines) and Switzerland that belonged to the haplogroup Macrobiotus cf. hufelandi sp. 1. These data together with other recent studies on tardigrade DNA barcoding represent a starting point for further studies on tardigrade biogeography, phylogeography and diversity.

Global climate change has become an important issue, particularly for organisms living in\ud the Antarctic region, as the predicted temperature increase can affect their life history traits. The reproductive mode and life history traits of one of the most widespread species of tardigrades in Antarctica were analyzed. Specimens of the eutardigrade Acutuncus antarcticus from a temporary freshwater pond at Victoria Land (Antarctica) were individually\ud cultured. This species reproduced continuously by thelytokous meiotic parthenogenesis. Its life cycle was short (60–90 days) and the reproductive output was low, with a short generation time (25–26 days). The maternal effect can be responsible of the phenotypic\ud plasticity observed in life history traits of the three analyzed generations that may be seen as a bethedging strategy, as also observed in other animals inhabiting stochastic environments. These traits, along with the cryptobiotic capability of A. antarcticus, are advantageous for exploiting the conditions suitable for growth and reproduction during the short Antarctic\ud summer, and can explain its wide distribution on the Antarctic continent. These results open new avenues of research for determining the role of bet-hedging strategy in organisms living in unpredictable environments

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