Abstract
In recent decades the diploid numbers recorded in the New World marsupials have been widely discussed in the context of the processes of karyotype evolution in these mammals. While Interstitial Telomeric Sequences (ITS) have long been interpreted as remnants of chromosomal fusion, the biological role of these features, together with their intraspecific variation, has raised a number of questions. In the present study, we analyzed the karyotype of 11 species of Amazonian didelphids, comparing the distribution of the heterochromatin with that of the telomeric signals, and found that, in six species, the ITS coincided with the blocks of heterochromatin. While ITS were found in the X chromosomes of all Marmosa murina individuals, they were variable in all the other species, representing a specific character of each lineage. Our results support the conclusion that ITS may not always be a consequence of chromosomal rearrangements, and that the mechanisms that produce them are still unclear.