CONCERTED EVOLUTION OF MEMBERS OF THE MULTISEQUENCE FAMILY CHAB4 LOCATED ON VARIOUS NONHOMOLOGOUS CHROMOSOMES

During the last years it became obvious that a lot of families of long -range repetitive DNA elements are located within the genomes of mamma ls. The principles underlying the evolution of such families, therefor e, may have a greater impact than anticipated on the evolution of the mammalian genome as a whole. One of these families, called chAB4, is r epresented with about 50 copies within the human and the chimpanzee ge nomes and with only a few copies in the genomes of gorilla, orang-utan , and gibbon. Members of chAB4 are located on 10 different human chrom osomes. FISH of chAB4-specific probes to chromosome preparations of th e great apes showed that chAB4 is located, with only one exception, at orthologous places in the human and the chimpanzee genome. About half the copies in the human genome belong to two species-specific subfami lies that evolved after the divergence of the human and the chimpanzee lineages. The analysis of chAB4-specific PCR-products derived from DN A of rodent/human cell hybrids showed that members of the two human-sp ecific subfamilies can be found on 9 of the 10 chAB4-carrying chromoso mes. Taken together, these results demonstrate that the members of DNA sequence families can evolve as a unit despite their location at mult iple sites on different chromosomes. The concerted evolution of the fa mily members is a result of frequent exchanges of DNA sequences betwee n copies located on different chromosomes. Interchromosomal exchanges apparently take place without greater alterations in chromosome struct ure.