The pattern of terminal DNA synthesis of the sex chromosomes of the Syrian hamster was studied by means of tritiated-thymidine incorporation and autoradiography. In the female, an entire X-chromosome and the long arm of the other X-chromosome underwent late replication. In the male, one of the largest submetacentric chromosomes and the long arm of the X-chromosome underwent late replication. It was presumed that this uniquely late-replicating large submetacentric chromosome was the Y-chromosome. The pattern of late replication among the autosomes was similar in both sexes and included approximately half the smallest pair of median metacentric chromosomes and the short arms of many of the larger submetacentric chromosomes. The asynchronous duplication pattern of the sex chromosomes observed in the Syrian hamster bore a striking resemblance to that previously reported in established cell lines derived from the Chinese hamster. This similarity attests to the close taxonomic relationship of these two species and is not inconsistent with the hypothesis of polyploid evolution of the Syrian hamster.
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