Rapid evolution of major histocompatibility complex class I genes in primates generates new disease alleles in humans via hitchhiking diversity.

A plausible explanation for many MHC-linked diseases is lacking. Sequencing of the MHC class I region (coding units or full contigs) in several human and nonhuman primate haplotypes allowed an analysis of single nucleotide variations (SNV) across this entire segment. This diversity was not evenly distributed. It was rather concentrated within two gene-rich clusters. These were each centered, but importantly not limited to, the antigen-presenting HLA-A and HLA-B/-C loci. Rapid evolution of MHC-I alleles, as evidenced by an unusually high number of haplotype-specific (hs) and hypervariable (hv) (which could not be traced to a single species or haplotype) SNVs within the classical MHC-I, seems to have not only hitchhiked alleles within nearby genes, but also hitchhiked deleterious mutations in these same unrelated loci. The overrepresentation of a fraction of these hvSNV (hv1SNV) along with hsSNV, as compared to those that appear to have been maintained throughout primate evolution (trans-species diversity; tsSNV; included within hv2SNV) tends to establish that the majority of the MHC polymorphism is de novo (species specific). This is most likely reminiscent of the fact that these hsSNV and hv1SNV have been selected in adaptation to the constantly evolving microbial antigenic repertoire.

Effect of Wild and Cultivated Rice Genotypes on Rhizosphere Bacterial Community Composition.

BACKGROUND: Deposition and secretion from roots influences the composition of the microbial communities surrounding them in the rhizosphere, and microbial activities influence the growth and health of the plant. Different host plant genotypes result in differences in those microbial communities. Crop genomes may have a narrow genetic base because of bottlenecks that occurred when domesticated crops were derived from small populations within the progenitor species. Desirable traits influencing root-associated microbial communities might therefore have been lost in the transition from wild species to modern cultivars. To investigate the diversity of bacterial communities associated with wild and cultivated rice, we surveyed a series of plant species and cultivars spanning the Oryza genus, growing them in the same nutrient-poor soil and assessing the bacterial composition of their rhizospheres and the surrounding soil using 16S rDNA sequencing. RESULTS: Root-associated bacterial communities showed small but significant differences dependent on the plant genotype. We found that differences between bacteria associated with differing plant genotypes were only weakly correlated with the phylogenetic distance between the Oryza wild species and cultivars. In ordination plots, domesticated and wild samples could be separated on the basis of their associated bacterial communities. Taxa of the Anaerolineae were overrepresented in wild samples compared to domesticated ones. Certain methanotrophs were overrepresented in the earliest diverged part of the Oryza genus. CONCLUSIONS: Bacterial populations associated with the rhizosphere of wild rice species displayed differences with those associated with cultivars, suggesting that root traits selected in domestication could have significant influence on the rhizosphere microbiota composition. Variation within the genus seems to influence the representation of methanotrophs. This suggests that greenhouse emissions from paddy fields could be altered by manipulating plant genotypes through the introgression of wild rice genetic material.

Evolutionary Transitions of MicroRNA-Target Pairs.

How newly generated microRNA (miRNA) genes are integrated into gene regulatory networks during evolution is fundamental in understanding the molecular and evolutionary bases of robustness and plasticity in gene regulation. A recent model proposed that after the birth of a miRNA, the miRNA is generally integrated into the network by decreasing the number of target genes during evolution. However, this decreasing model remains to be carefully examined by considering in vivo conditions. In this study, we therefore compared the number of target genes among miRNAs with different ages, combining experiments with bioinformatics predictions. First, we focused on three Drosophila miRNAs with different ages. As a result, we found that an older miRNA has a greater number of target genes than a younger miRNA, suggesting the increasing number of targets for each miRNA during evolution (increasing model). To further confirm our results, we also predicted all target genes for all miRNAs in D. melanogaster, considering co-expression of miRNAs and mRNAs in vivo The results obtained also do not support the decreasing model but are reasonably consistent with the increasing model of miRNA-target pairs. Furthermore, our large-scale analyses of currently available experimental data of miRNA-target pairs also showed a weak but the same trend in humans. These results indicate that the current decreasing model of miRNA-target pairs should be reconsidered and the increasing model may be more appropriate to explain the evolutionary transitions of miRNA-target pairs in many organisms.

Comparative sequencing of human and chimpanzee MHC class I regions unveils insertions/deletions as the major path to genomic divergence.

Despite their high degree of genomic similarity, reminiscent of their relatively recent separation from each other ( approximately 6 million years ago), the molecular basis of traits unique to humans vs. their closest relative, the chimpanzee, is largely unknown. This report describes a large-scale single-contig comparison between human and chimpanzee genomes via the sequence analysis of almost one-half of the immunologically critical MHC. This 1,750,601-bp stretch of DNA, which encompasses the entire class I along with the telomeric part of the MHC class III regions, corresponds to an orthologous 1,870,955 bp of the human HLA region. Sequence analysis confirms the existence of a high degree of sequence similarity between the two species. However, and importantly, this 98.6% sequence identity drops to only 86.7% taking into account the multiple insertions/deletions (indels) dispersed throughout the region. This is functionally exemplified by a large deletion of 95 kb between the virtual locations of human MICA and MICB genes, which results in a single hybrid chimpanzee MIC gene, in a segment of the MHC genetically linked to species-specific handling of several viral infections (HIV/SIV, hepatitis B and C) as well as susceptibility to various autoimmune diseases. Finally, if generalized, these data suggest that evolution may have used the mechanistically more drastic indels instead of the more subtle single-nucleotide substitutions for shaping the recently emerged primate species.