Organization of rDNA spacer fragment variants among human acrocentric chromosomes in somatic cell hybrids

Although the human rDNA gene family is organized in clusters of tandem repeats on five pairs of acrocentric chromosomes, all rDNA genes have undergone a "concerted" evolution resulting in a homogeneous population of genes. Two steps are necessary for a variant to spread to all rDNA genes: dissemination of the variant to all genes in a cluster; and exchange of rDNA genes between nonhomologous chromosomes. To study the organization of rDNA genes, a restriction fragment length polymorphism in the spacer region adjacent to the 28S gene was examined in somatic cell hybrids in which individual human acrocentric chromosomes could be isolated. Human DNA cut with BamHI and analyzed by Southern hybridization yields two to four major bands that hybridize to a 32P-labeled cloned fragment of the 28S gene. Hybrids containing single human acrocentric chromosomes do not recapitulate the parental patterns, but frequently have only one of the parental bands. The data suggest that the quantitative distribution of spacer length variants differs among the human acrocentric chromosomes in hybrids. The frequently observed homogeneity of the rDNA variants on individual acrocentric chromosomes in hybrid cells may reflect the individual rDNA clusters in the parental cell or may be a result of unequal crossing over in the hybrid cell.