Comprehensive Description

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Comprehensive Description

Hypsibius dujardini is a member of the phylum Tardigrada, a group of microscopic animals sometimes known as "water bears". This species, named after the French biologist Félix Dujardin, is found in moss and in freshwater habitats and has a cosmopolitan (world-wide) distribution. It is at least largely parthenogenetic (i.e., progeny develop from unfertilized eggs), with females laying eggs that undergo meiosis and then restore a diploid chromosome number by reduplicating chromosomes (rather than by fertilization). However, males have also been described, suggesting that some populations of this species may reproduce sexually. Like other tardigrades, H. dujardini can shut down their metabolism almost entirely and survive long periods in an extremely desiccated state. In this state, tardigrades are highly resistent to extremes of temperature, pressure, and radiation. Desiccated tardigrades sent into space survived exposure to space vacuum and cosmic radiation with no significant effect on survival; exposure to unfiltered solar radiation had a strong negative effect on survival, but some individuals survived even the combined efects of space vacuum and cosmic and solar radiation. (Gabriel et al. 2007; Jönsson et al. 2008)

A broad fundamental question for biologists interested in animal development is how morphological diversity arises through the evolution of developmental mechanisms. Gabriel et al. (2007) suggested that H. dujardini exhibits a variety of features that make it an excellent organism for investigating questions relating to the evolution of development. Tardigrades belong to the same animal superclade (Ecdysozoa) as two of the best studied models for investigating animal development, the fruit fly Drosophila melanogaster (an arthropod) and the nematode worm Caenorhabditis elegans. Thus, background and tools developed for studying these other model systems could leverage investigations of H. dujardini. Gabriel et al. showed that H. dujardini can be cultured continuously for decades (long-term culture of tardigrades has been a challenge historically) and that it has a short generation time, 13 to 14 days at room temperature. Conveniently, it can be cryopreserved for storage at -80° C and thawed months later with a fairly high survival rate. It also has a compact genome (plans to sequence the complete genome of this tardigrade were approved in 2007). The haploid genome size was estimated at ~75 Mb. Twenty other tardigrade species have been assessed for genome size to date, but none have exhibited a genome quite this compact (range: 80 Mb to 800 Mb). The H. dujardini genome is among the smallest so far identified for animals, being less than half the size of the D. melanogaster genome and three-quarters as large as that of C. elegans. The only metazoan genome sizes reported to exceed this level of compactness are found in some nematodes (as small as ∼30 Mb), the placozoan Trichoplax adhaerens (40 Mb), gastrotrichs (≥50 Mb), sponges (≥60 Mb), some polychaete annelids (≥60 Mb), and the larvacean Oikopleura dioica (70 Mb). Both adults and embryos of H. dujardini are optically clear, with much of their anatomy visible by light microscopy, making it possible to follow cell divisions by optically sectioning live embryos using differential interference contrast (DIC) microscopy. Gabriel et al. found that the embryos of H. dujardini have a stereotyped cleavage pattern with asymmetric cell divisions, nuclear migrations, and cell migrations occurring in reproducible patterns and presented a cell lineage of the early embryo and an embryonic staging series. (Gabriel et al. 2007 and references therein)


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© Shapiro, Leo

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