Comparative cytogenetics of human chromosome 3q21.3 reveals a hot spot for ectopic recombination in hominoid evolution.

Fluorescence in situ hybridization mapping of fully integrated human BAC clones to primate chromosomes, combined with precise breakpoint localization by PCR analysis of flow-sorted chromosomes, was used to analyze the evolutionary rearrangements of the human 3q21.3-syntenic region in orangutan, siamang gibbon, and silvered-leaf monkey. Three independent evolutionary breakpoints were localized within a 230-kb segment contained in BACs RP11-93K22 and RP11-77P16. Approximately 200 kb of the human 3q21.3 sequence was not present on the homologous orangutan, siamang, and Old World monkey chromosomes, suggesting a genomic DNA insertion into the breakpoint region in the lineage leading to humans and African great apes. The breakpoints in the orangutan and siamang genomes were narrowed down to 12- and 20-kb DNA segments, respectively, which are enriched with endogenous retrovirus long terminal repeats and other repetitive elements. The inserted DNA segment represents part of an ancestral duplication. Paralogous sequence blocks were found at human 3q21, approximately 4 Mb proximal to the evolutionary breakpoint cluster region; at human 3p12.3, which contains an independent orangutan-specific breakpoint; and at the subtelomeric and pericentromeric regions of multiple human and orangutan chromosomes. The evolutionary breakpoint regions between human chromosome 3 and orangutan 2 as well their paralogous segments in the human genome coincide with breaks of chromosomal synteny in the mouse, rat, and/or chicken genomes. Collectively our data reveal reuse of the same short recombinogenic DNA segments in primate and vertebrate evolution, supporting a nonrandom breakage model of genome evolution.