Justyna M. Szamalek, Violaine Goidts, Horst Hameister, Hildegard Kehrer-Sawatzki
Department of Human Genetics, University of Ulm, Albert-Einstein-Allee, 89081 Ulm, Germany
Comparative studies of the human and chimpanzee genomes expose the full spectrum of genetic changes that accompanied evolution of these species. Regardless of the high similarity between the DNA sequence of human and chimpanzee (98.77%, when the single-nucleotide substitutions are considered), there are still many macro- and micro-differences, which remain to be clarified. At first, we provided detailed molecular characterization of the large pericentric inversions that represent the major karyotypic difference between human and chimpanzee. We showed that the breakpoints are localized within the relatively gene poor regions and none of them alters the coding sequence of a known gene. Since also no extreme expression difference of the genes neighbouring breakpoints was detected, the breakpoints as such may be regarded as neutral. Subsequently we confirmed that the two chimpanzee species Pan troglodytes and Pan paniscus share exactly the same breakpoints of the chimpanzee-specific pericentric inversions. This is informative with respect to the upper time boundary of the inversions occurrence and suggests that they have been important for the human-chimpanzee speciation. In addition to these large karyotypic differences, by genome-wide gene order comparisons, we identified numerous micro-inversions. Four out of five experimentally confirmed events were found to be polymorphic in chimpanzees or in humans. Moreover, we performed interspecies aCGH including five non-human primates and identified 14 sites of human-specific copy number differences (CNDs). Interestingly, four of them map close to the breakpoints of two human-specific inversions. We showed also that some of these CNDs are polymorphic in the human population. We conclude that the old, fixed human-chimpanzee macro-inversions may have been instrumental for the reproductive isolation during the parapatric speciation which took place about 6 Mya in East Africa. Conversely, the micro-inversions and CNDs show an ongoing dynamics and contribute to the genomic variability of humans and chimpanzees.
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