G-banded karyotype of T. aculeatus. Top: the male has 27 chromosome pairs and 9 unpaired chromosomes. Three kinds of information are given next to the chromosomes. Chromosomes 3, 6, and X5 contain the NOR regions. Certain chromosomes have specific regions represented by colored bars on the left of the chromosomes, 'w' means that the region is relatively under-represented (see text). The numbers on the right refer to platypus chromosome paints that hybridized to the indicated regions. Middle: the pairing regions of the nine sex chromosomes determined by chromosome painting on mitotic preparations. Those of Y3 with X3 could not be determined in mitotic metaphases. Bottom: G-banded female karyotype of T. aculeatus. The female has 32 chromosome pairs and no unpaired chromosomes.