The determination of the chimpanzee genome sequence provides a means to study both structural and functional aspects of the evolution of the human genome. Here we compare humans and chimpanzees with respect to differences in expression levels and protein-coding sequences for genes active in brain, heart, liver, kidney, and testis. We find that the patterns of differences in gene expression and gene sequences are markedly similar. In particular, there is a gradation of selective constraints among the tissues so that the brain shows the least differences between the species whereas liver shows the most. Furthermore, expression levels as well as amino acid sequences of genes active in more tissues have diverged less between the species than have genes active in fewer tissues. In general, these patterns are consistent with a model of neutral evolution with negative selection. However, for X-chromosomal genes expressed in testis, patterns suggestive of positive selection on sequence changes as well as expression changes are seen. Furthermore, although genes expressed in the brain have changed less than have genes expressed in other tissues, in agreement with previous work we find that genes active in brain have accumulated more changes on the human than on the chimpanzee lineage.

In development, timing is of the utmost importance, and the timing of developmental processes often changes as organisms evolve. In human evolution, developmental retardation, or neoteny, has been proposed as a possible mechanism that contributed to the rise of many human-specific features, including an increase in brain size and the emergence of human-specific cognitive traits. We analyzed mRNA expression in the prefrontal cortex of humans, chimpanzees, and rhesus macaques to determine whether human-specific neotenic changes are present at the gene expression level. We show that the brain transcriptome is dramatically remodeled during postnatal development and that developmental changes in the human brain are indeed delayed relative to other primates. This delay is not uniform across the human transcriptome but affects a specific subset of genes that play a potential role in neural development.human evolution ͉ brain development ͉ gene expression ͉ heterochrony ͉ chimpanzee

Previous studies have shown two homologous chromodomain modules in the HP1 and Polycomb proteins exhibit discriminatory binding to related methyllysine residues (embedded in ARKS motifs) of the histone H3 tail. Methylated ARK(S/T) motifs have recently been identified in other chromatin factors (e.g. linker histone H1.4 and lysine methyltransferase G9a). These are thought to function as peripheral docking sites for the HP1 chromodomain. In vertebrates, HP1-like chromodomains are also present in the chromodomain Y chromosome (CDY) family of proteins adjacent to a putative catalytic motif. The human genome encodes three CDY family proteins, CDY, CDYL, and CDYL2. These have putative functions ranging from establishment of histone H4 acetylation during spermiogenesis to regulation of transcription co-repressor complexes. To delineate the biochemical functions of the CDY family chromodomains, we analyzed their specificity of methyllysine recognition. We detected substantial differences among these factors. The CDY chromodomain exhibits discriminatory binding to lysinemethylated ARK(S/T) motifs, whereas the CDYL2 chromodomain binds with comparable strength to multiple ARK(S/T) motifs. Interestingly, subtle amino acid changes in the CDYL chromodomain prohibit such binding interactions in vitro and in vivo. However, point mutations can rescue binding. In support of the in vitro binding properties of the chromodomains, the full-length CDY family proteins exhibit substantial variability in chromatin localization. Our studies underscore the significance of subtle sequence differences in a conserved signaling module for diverse epigenetic regulatory pathways.The human Y chromosome has been thoroughly sequenced and compared with partially sequenced Y chromosomes of chimpanzee and mouse (1-3). The Y chromosomes are believed to be enriched in genes essential for spermatogenesis and testis development. Interstitial Y chromosome deletions are associated with spermatogenic failure and male infertility (1,4,5). One gene that is present in multiple copies on the human Y chromosome is CDY, 5 which exhibits testis-specific expression (1). Interestingly, the mouse Y chromosome does not encode CDY, suggesting a developmentally advanced usage of CDY in primates (3).The human CDY gene seems to be derived from the autosomal homologs CDYL or CDYL2 (Fig. 1, A and B) (6). CDYL is ubiquitously expressed, whereas CDYL2 exhibits selective expression in tissues of testis, prostate, spleen, and leukocytes (6). The mouse genome also encodes related CDYL and CDYL2 genes (Fig. 1B). The presence of CDY-like genes appears to be a hallmark of echinoderm and vertebrate genomes. In sea urchin and chicken genomes we found only one CDY-like gene that corresponds to mammalian CDYL2 (Fig. 1B).CDY family proteins have two conserved domains implicated in histone modification and recognition; that is, a chromodomain followed by an enoyl-coenzyme A hydratase/isomerase (ECH) putative catalytic domain (Fig. 1A). Previously, it was shown that the chromodomain of hum...