Reciprocal chromosome painting shows that genomic rearrangement between rat and mouse proceeds ten times faster than between humans and cats

Reciprocal chromosome painting between mouse and rat using complete chromos ome probe sets of both species permitted us to assign the chromosomal homol ogy between these rodents. The comparative gene mapping data and chromosome painting have a better than 90% correspondence. The reciprocal painting re sults graphically show that mouse and rat have strikingly different karyoty pes. At least 14 translocations have occurred in the 10-20 million years of evolution that separates these two species. The evolutionary rate of chrom osome translocations between these two rodents appears to be up to 10 times greater than that found between humans and cats, or between humans and chi mpanzees, where over the last 5-6 million years just one translocation has occurred. Outgroup comparison shows that the mouse genome has incorporated at least three times the amount of interchromosomal rearrangements compared to the rat genome. The utility of chromosome painting was also illustrated by the assignment of two new chromosome homologies between rat and mouse u nsuspected by gene mapping: between mouse 11 and rat 20 and between mouse 1 7 and rat 6. We conclude that reciprocal chromosome painting is a powerful method, which can be used with confidence to chart the genome and predict t he chromosome location of genes. Reciprocal painting combined with gene map ping data will allow the construction of large-scale comparative chromosome maps between placental mammals and perhaps other animals.