ORDERED TANDEM ARRANGEMENT OF CHROMOSOMES IN THE SPERM HEADS OF MONOTREME MAMMALS

A very old unanswered question in classical cytology is whether chromo somes are arranged randomly in sperm or whether they occupy specific p ositions. Even with modern methods of chromosome painting, it is diffi cult to resolve this question for the very condensed and almost spheri cal sperm head of most mammals. We have taken advantage of the unusual fibrillar sperm head of monotreme mammals (echidna and platypus) to e xamine the position of chromosome landmarks in a two-dimensional array . We used fluorescence and radioactive in situ hybridization to telome ric, rDNA, acid unique sequences to show that chromosomes are arranged tandemly and in a defined order in the sperm nucleus.