High divergence in primate-specific duplicated regions: human and chimpanzee chorionic gonadotropin beta genes.
BACKGROUND
Low nucleotide divergence between human and chimpanzee does not sufficiently explain the species-specific morphological, physiological and behavioral traits. As gene duplication is a major prerequisite for the emergence of new genes and novel biological processes, comparative studies of human and chimpanzee duplicated genes may assist in understanding the mechanisms behind primate evolution. We addressed the divergence between human and chimpanzee duplicated genomic regions by using Luteinizing Hormone Beta (LHB)/Chorionic Gonadotropin Beta (CGB) gene cluster as a model. The placental CGB genes that are essential for implantation have evolved from an ancestral pituitary LHB gene by duplications in the primate lineage.


RESULTS
We shotgun sequenced and compared the human (45,165 bp) and chimpanzee (39,876 bp) LHB/CGB regions and hereby present evidence for structural variation resulting in discordant number of CGB genes (6 in human, 5 in chimpanzee). The scenario of species-specific parallel duplications was supported (i) as the most parsimonious solution requiring the least rearrangement events to explain the interspecies structural differences; (ii) by the phylogenetic trees constructed with fragments of intergenic regions; (iii) by the sequence similarity calculations. Across the orthologous regions of LHB/CGB cluster, substitutions and indels contributed approximately equally to the interspecies divergence and the distribution of nucleotide identity was correlated with the regional repeat content. Intraspecies gene conversion may have shaped the LHB/CGB gene cluster. The substitution divergence (1.8-2.59%) exceeded two-three fold the estimates for single-copy loci and the fraction of transversional mutations was increased compared to the unique sequences (43% versus approximately 30%). Despite the high sequence identity among LHB/CGB genes, there are signs of functional differentiation among the gene copies. Estimates for dn/ds rate ratio suggested a purifying selection on LHB and CGB8, and a positive evolution of CGB1.


CONCLUSIONS
If generalized, our data suggests that in addition to species-specific deletions and duplications, parallel duplication events may have contributed to genetic differences separating humans from their closest relatives. Compared to unique genomic segments, duplicated regions are characterized by high divergence promoted by intraspecies gene conversion and species-specific chromosomal rearrangements, including the alterations in gene copy number.
Hallast
Pille
P
Department of Biotechnology, Institute of Molecular and Cell Biology, University of Tartu, Riia 23, 51010 Tartu, Estonia. pilleh@ut.ee
Saarela
Janna
J
Palotie
Aarno
A
Laan
Maris
M
D000818:Animals; D006063:Chorionic Gonadotropin / Q000235:genetics*; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000235:genetics*; D019143:Evolution, Molecular; D005785:Gene Conversion / Q000235:genetics; D020440:Gene Duplication; D014644:Genetic Variation; D006801:Humans; D008969:Molecular Sequence Data; D005810:Multigene Family; D002679:Pan troglodytes / Q000235:genetics*; D010802:Phylogeny; D017354:Point Mutation; D017385:Sequence Homology
D006063:Chorionic Gonadotropin; D018997:Chorionic Gonadotropin, beta Subunit, Human