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Abstract

Subtelomeric duplications of an obscure tubulin "genic" segment located near the telomere of human chromosome 4q35 have occurred at different evolutionary time points within the last 25 million years of the catarrhine (i.e., hominoid and Old World monkey) evolution. The analyses of these segments reported here indicate an exceptional level of evolutionary instability. Substantial intra- and interspecific differences in copy number and distribution are observed among cercopithecoid (Old World monkey) and hominoid genomes. Characterization of the hominoid duplicated segments reveals a strong positional bias within pericentromeric and subtelomeric regions of the genome. On the basis of phylogenetic analysis from predicted proteins and comparisons of nucleotide-substitution rates, we present evidence of a conserved b-tubulin gene among the duplications. Remarkably, the evolutionary conservation has occurred in a nonorthologous fashion, such that the functional copy has shifted its positional context between hominoids and cercopithecoids. We propose that, in a chimpanzee-human common ancestor, one of the paralogous copies assumed the original function, whereas the ancestral copy acquired mutations and eventually became silenced. Our analysis emphasizes the dynamic nature of duplication-mediated genome evolution and the delicate balance between gene acquisition and silencing.