Reciprocal chromosome painting shows that squirrels, unlike murid rodents, have a highly conserved genome organization.

We present the first report of reciprocal chromosome painting between humans and a rodent. Gene mapping and sequencing data lead to the generalization that rodent genomes are highly rearranged. In contrast, our results show a surprising conservation of genome structure between humans and squirrels. The synteny of 12 human chromosomes was entirely conserved (5, 6, 9, 11, 13-15, 17, 18, 20, 21, and X). Of the 12 syntenic associations of human chromosomes present in the squirrel, six are well-known ancestral eutherian associations (3/21, 4/8, 7/16, 12/22, 14/15, 16/19). Apparently, few derived translocations characterize the evolutionary origin of the rodents. One association (10p/1qter) may be a cladistic marker for the cohort Glires, linking rodents and lagomorphs.