Banding techniques on tardigrade chromosomes: the karyotype of Macrobiotus richtersi (Eutardigrada, Macrobiotidae).

Abstract

This work represents the first attempt to define tardigrade chromosomes using banding techniques. Macrobiotus richtersi, a eutardigrade morphospecies with amphimictic diploid and thelytokous triploid cytotypes, was used as a model. Prime consideration was given to oocyte chromosomes because they are larger than those of spermatocytes and of mitotic chromosomes. With Giemsa staining, the chromatids of the 6 bivalents of the diploid cytotypes and those of the 17-18 univalents of the triploid cytotypes were very similar to each other and appeared rod- or flame-shaped. In the amphimictic strain, a chiasma was generally present in each bivalent at diplotene, whereas there were no chiasmata in the oocyte prophase chromosomes of the triploid strain. Both in diploid and triploid cytotypes, C-banding and fluorescence showed a heterochromatic centromeric band on the telomere of each chromosome oriented towards the spindle pole, indicating that all of them were acrocentric. Silver staining showed the presence of a NOR in only a pair of chromosomes, close to the centromeric C-banded site. NOR was particularly evident in the oocyte prophases. Other silver positive regions, corresponding to the kinetochore, were located on all other chromosomes on the telomeres towards the spindle pole.