Evolution of the chAB4 multisequence family in primates.

Approximately 50 members of the primate-specific multisequence family chAB4 are located as clusters at eight different chromosomal loci within the human genome. The whole cloned region of chAB4 represents a single-copy or low-copy sequence in all nonhuman primates tested, with the exception of the chimpanzee, for which we found chAB4 copy numbers similar to those in the human. An Alu element was inserted into chAB4 after the divergence of the Old World monkeys from the hominoids but before chAB4 was amplified. The first amplification step could be dated after the great apes and the human diverged from the Old World monkeys. We have evidence that neither the copy numbers nor the chromosomal locations remained stable after this initial step and that gross alterations in the relative copy numbers of individual family members occurred even after the divergence of the human and the chimpanzee. Taken together, our data suggest that chAB4, in an evolutionary sense, is an unusually unstable sequence family.