Comparative analysis of Alu insertion sequences in the APP 5' flanking region in humans and other primates

Nonadaptive processes in primate and human evolution

Evolutionary biology has tended to focus on adaptive evolution by positive selection as the primum mobile of evolutionary trajectories in species while underestimating the importance of nonadaptive evolutionary processes. In this review, I describe evidence that suggests that primate and human evolution has been strongly influenced by nonadaptive processes, particularly random genetic drift and mutation. This is evidenced by three fundamental effects: a relative relaxation of selective constraints (i.e., purifying selection), a relative increase in the fixation of slightly deleterious mutations, and a general reduction in the efficacy of positive selection. These effects are observed in protein-coding, regulatory regions, and in gene expression data, as well as in an augmentation of fixation of large-scale mutations, including duplicated genes, mobile genetic elements, and nuclear mitochondrial DNA. The evidence suggests a general population-level explanation such as a reduction in effective population size (N(e)). This would have tipped the balance between the evolutionary forces of natural selection and random genetic drift toward genetic drift for variants having small selective effects. After describing these proximate effects, I describe the potential consequences of these effects for primate and human evolution. For example, an increase in the fixation of slightly deleterious mutations could potentially have led to an increase in the fixation rate of compensatory mutations that act to suppress the effects of slightly deleterious substitutions. The potential consequences of compensatory evolution for the evolution of novel gene functions and in potentially confounding the detection of positively selected genes are explored. The consequences of the passive accumulation of large-scale genomic mutations by genetic drift are unclear, though evidence suggests that new gene copies as well as insertions of transposable elements into genes can potentially lead to adaptive phenotypes. Finally, because a decrease in selective constraint at the genetic level is expected to have effects at the morphological level, I review studies that compare rates of morphological change in various mammalian and island populations where N(e) is reduced. Furthermore, I discuss evidence that suggests that craniofacial morphology in the Homo lineage has shifted from an evolutionary rate constrained by purifying selection toward a neutral evolutionary rate.

Alu sequence variants of the BPY2 gene in proven fertile and infertile men with Sertoli cell-only phenotype

OBJECTIVE

The basic protein on Y chromosome, 2 gene (BPY2) is implicated in the spermatogenic process. Three copies (paralogs) of the BPY2 sequence lie in the AZFc region, within huge palindromic repeats consisting of Alu sequences located in 5' flanking regions. Our objective was to screen the single nucleotide variation of BPY2 gene paralogously.

METHODS

Mutation screening of the exons and 5' flanking region of the BPY2 genes was carried out by polymerase chain reaction (PCR) and sequencing in 106 infertile patients with a Sertoli cell-only (SCO) phenotype and in 126 fertile men.

RESULTS

No sequence variation was detected in any of the BPY2 exons. Paralogous A/A/A and mixed A/G genotypes are referred to the promoter of the gene. The G/G/G genotype (wild type) was observed at the rates of 89.6% (95/106) in the SCO subjects and 96.0% (121/126) in the proven-fertile subjects. The G/G/G plus mixed A/G paralogous variants and the A/A/A paralogous variant were detected in 96.2% (102/106) and 3.8% (4/106) of infertile patients with SCO, respectively. The G/G/G plus mixed A/G paralogous variants were detected in 100% (126/126) of the fertile controls, with no instances of the A/A/A phenotype, and this difference was statistically significant (P = 0.038, Fisher's exact test).

CONCLUSION

The A/A/A genotype may be associated with the SCO phenotype. The alteration of all three copies of BPY2 within the AZFc region on the Y chromosome may affect spermatogenic process.

Transposable elements as a significant source of transcription regulating signals

Transposable elements (TEs) are major components of eukaryotic genomes, contributing about 50% to the size of mammalian genomes. TEs serve as recombination hot spots and may acquire specific cellular functions, such as controlling protein translation and gene transcription. The latter is the subject of the analysis presented. We scanned TE sequences located in promoter regions of all annotated genes in the human genome for their content in potential transcription regulating signals. All investigated signals are likely to be over-represented in at least one TE class, which shows that TEs have an important potential to contribute to pre-transcriptional gene regulation, especially by moving transcriptional signals within the genome and thus potentially leading to new gene expression patterns. We also found that some TE classes are more likely than others to carry transcription regulating signals, which can explain why they have different retention rates in regions neighboring genes.

Positive selection in MAOA gene is human exclusive: determination of the putative amino acid change selected in the human lineage

Monoamine oxidase A (MAOA) is the X-linked gene responsible for deamination and subsequent degradation of several neurotransmitters and other amines. Among other activities, the gene has been shown to play a role in locomotion, circadian rhythm, and pain sensitivity and to have a critical influence on behavior and cognition. Previous studies have reported a non-neutral evolution of the gene attributable to positive selection in the human lineage. To determine whether this selection was human-exclusive or shared with other species, we performed a population genetic analysis of the pattern of nucleotide variation in non-human species, including bonobo, chimpanzee, gorilla, and orangutan. Footprints of positive selection were absent in all analyzed species, suggesting that positive selection has been recent and unique to humans. To determine which human-unique genetic changes could have been responsible for this differential evolution, the coding region of the gene was compared between human, chimpanzee, and gorilla. Only one human exclusive non-conservative change is present in the gene: Glu151Lys. This human substitution affects protein dimerization according to a three-dimensional structural model that predicts a non-negligible functional shift. This is the only candidate position at present to have been selected to fixation in humans during an episode of positive selection. Divergence analysis among species has shown that, even under positive selection in the human lineage, the MAOA gene did not experience accelerated evolution in any of the analyzed lineages, and that tools such as K(a)/ K(s) would not have detected the selective history of the gene.