Tardigrades are microscopic organisms that survive DNA-damaging conditions, including high levels of radiation and desiccation, that would kill most animals. In response to irradiation, genes involved in DNA repair mechanisms such as nonhomologous end joining (NHEJ) are amplified. How tardigrades repair DNA damage through these pathways without consequences to their genome, such as large chromosomal rearrangements, is unknown. Here, we show that in early embryos of Hypsibius exemplaris, a cultured species of tardigrade, there were no nuclei with decondensed chromatin that is characteristic of interphase. Cycles of chromosome condensation and decondensation during the cell cycle are essential for proper segregation during cell division, and for gene expression and regulation during interphase. We investigated the possibility of constant chromosome condensation during the cell cycles of early tardigrade embryos. We created a timeline of fixed DNA-stained embryos throughout the early cell cycles in H. exemplaris and repeated the DNA-staining protocol in a closely related model organism, Caenorhabditis elegans, for species comparison. From the imaging data, we then quantified chromosome condensation within the nuclei of multiple embryos for both species. We also narrowed down the phases in the cell cycle where unexpected chromosome condensation occurs by performing lamin immunostaining on tardigrade embryos to visualize the nuclear envelope. Our findings show that chromosomes in H. exemplaris are more condensed than originally expected and more condensed than in another model organism, C. elegans during interphase. We speculate that the unusually condensed chromosomes we found in tardigrades might contribute to their unusual ability to survive in extreme environments.
Lillian D. Papell (Sat,) studied this question.