Abstract
The chromosome number of the hexaploid species Xenopus ruwenzoriensis is 108. The chromosomes fall into sets of six usually similar chromosomes (hextets) which can be classified into the morphological groups characteristic for the genus Xenopus. However, the three distinct secondary constrictions, typical for X. ruwenzoriensis, appear on two homologs only within a particular hextet, unmasking an underlying heterogeneity. The secondary constrictions on chromosome pair 11 show chromosomal association and are considered to represent the nucleolar organizer regions. There is no evidence of dimorphic sex chromosomes. Plates of spermatocyte metaphase I generally show bivalents, but single large multivalents have also been observed. In the majority of cases, 54 dyads are found in second spermatocyte metaphases. X. ruwenzoriensis is, therefore, either an ancient autohexaploid or, more likely, an allopolyploid of more recent origin.
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