Chromosome c-banding and Ag-NOR pattern in tardigrades

The first data on tardigrade chromosomes were derived from histological sections (Henneke 1911; von Wenck 1914). Specific studies were performed in the early seventies on animals stained in toto with acetic lactic orcein and squashed (Figs. 3-4; Bertolani 1971, 1975, 1982). Besides the definition of the chromosome number of several species (often n = 5 or n = 6), the main results were the identification of the polyploidy (triploidy and tetraploidy) and the definition of the cytology in the oocyte maturation of the parthenogenetic animals. Chromosomes always appeared small, without an evident centromere and similar to each other in the same plate and among the species. Oocyte chromosomes were clearly larger than those of the spermatocytes and of the mitotic divisions. More recently, Giemsa staining was applied to the eutardigrade Xerobiotus pseudohufelandi, in which diploid, triploid and tetraploid cytotypes were identified (Rebecchi, 1991). Triploidy and tetraploidy in tardigrades had been confirmed on the basis of the DNA content (Bertolani et al. 1987, 1994). Giemsa staining provides good details of the chromosome shape and confirms that M. richtersi is characterized by a chromosome set made up of very similar elements. Sex chromosomes are not recognizable.The kind of chromosome arrangement along the spindle fibers and the presence of a heterochromatic region on a telomere, evidenced by C-banding, allow us to conclude that all chromosomes of M. richtersi are acrocentric.There is only one NOR, localized on one extremity of one chromosome pair. It is evident in the oocyte prophases. In the oocyte metaphases the NOR could correspond to the most intense terminal dots that often characterize one of the six bivalents. As in other animals, the NOR coincides or is just adjacent the C-band site. The silver-positive regions located on one of the telomeres of all the other chromosomes of M. richtersi should correspond to the kynetochore, whereas the fainter regions located on the other telomere of all five bivalents resemble the “telochore” evidenced in the grasshoppers (Suja and Rufas 1994).