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Molecular biology and evolution [journal]
- Cover. [Journal Article]
- Mol Biol Evol 2013 Jun; 30(6):i1.
- Artificial selection on brain expressed genes during the domestication of dog. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 May 8.
Domesticated dogs have many unique behaviors not found in gray wolves that have augmented their interaction and communication with humans. The genetic basis of such unique behaviors in dogs remains poorly understood. We found that genes within regions highly differentiated between outbred Chinese native dogs and wolves show high bias for expression localized to brain tissues, particularly the prefrontal cortex, a specific region responsible for complex cognitive behaviors. In contrast, candidate genes showing high population differentiation between Chinese native dogs and German Shepherd dogs did not demonstrate significant expression bias. These observations indicate that these candidate genes highly expressed in the brain have rapidly evolved. This rapid evolution was probably driven by artificial selection during the primary transition from wolves to ancient dogs, and was consistent with the evolution of dog-specific characteristics, such as behavior transformation, for thousands of years.
- Transcription Start Site Evolution in Drosophila. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 May 10.
Transcription start site (TSS) evolution remains largely undescribed in Drosophila, likely due to limited annotations in non-melanogaster species. In this study, we introduce a concise new method that selectively sequences from the 5' end of mRNA and used it to identify TSS in four Drosophila species, including: D. melanogaster, D. simulans, D. sechellia, and D. pseudoobscura. To locate TSS and their associated promoters, we chose a threshold that removed 99% of our modeled background noise. For verification, we compared our results in D. melanogaster with known annotations, published 5' RACE data, and with RNAseq from the same mRNA pool. Then, we paired 2849 D. melanogaster TSS with its closest equivalent TSS in each species (likely to be its true ortholog) using the available multiple sequence alignments. Most of the D. melanogaster TSS were successfully paired with an ortholog in each species (83%, 86%, and 55% for D. simulans, D. sechellia, and D. pseudoobscura, respectively). Based on the number and distribution of reads mapped at each TSS, we also estimated promoter-specific expression and TSS peak shape, respectively. Among paired TSS orthologs, the location and promoter activity were largely conserved. TSS location appears important as promoter-specific expression and TSS peak shape were more frequently divergent among TSS that had moved. Unpaired TSS were surprisingly common in D. pseudoobscura. An increased mutation rate upstream of TSS might explain this pattern. We found an enrichment of ribosomal protein genes among diverged TSS, suggesting that TSS evolution is not uniform across the genome.
- Subfunctionalization via adaptive evolution influenced by genomic context: the case of histone chaperones ASF1a and ASF1b. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 May 2.
Gene duplication is regarded as the main source of adaptive functional novelty in eukaryotes. Processes such as neo- and sub-functionalization impact the evolution of paralogous proteins where functional divergence is frequently key to retain the gene copies. Here we examined anti-silencing function 1 (ASF1), a conserved eukaryotic H3-H4 histone chaperone, involved in histone dynamics during replication, transcription, and DNA repair. While yeast feature a single ASF1 protein, two paralogs exist in most vertebrates, termed ASF1a and ASF1b, with distinct cellular roles in mammals. To explain this division of tasks, we integrated evolutionary and comparative genomic analyses with biochemical and structural approaches. First, we show that a duplication event at the ancestor of jawed vertebrates, followed by ASF1a relocation into an intron of MCM9 gene at the ancestor of tetrapods, provided a different genomic environment for each paralog with marked differences of GC content and DNA replication timing. Second, we found signatures of positive selection in the N- and C-terminal regions of ASF1a and ASF1b. Third, we demonstrate that regions outside the primary interaction surface are key for the preferential interactions of the human paralogs with distinct H3-H4 chaperones. Based on these data, we propose that ASF1 experienced subfunctionalization shaped by the adaptation of the genes to their respective genomic context, reflecting a case of genomic-context driven escape from adaptive conflict.
- Genome analysis of Elysia chlorotica egg DNA provides no evidence for horizontal gene transfer into the germ line of this kleptoplastic mollusc. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 May 2.
The sea slug Elysia chlorotica offers a unique opportunity to study the evolution of a novel function (photosynthesis) in a complex multicellular host. E. chlorotica harvests plastids (absent of nuclei) from its heterokont algal prey, Vaucheria litorea. The 'stolen' plastids are maintained for several months in cells of the digestive tract and are essential for animal development. The basis of long-term maintenance of photosynthesis in this sea slug was thought to be explained by extensive horizontal gene transfer (HGT) from the nucleus of the alga to the animal nucleus, followed by expression of algal genes in the gut to provide essential plastid-destined proteins. Early studies of target genes and proteins supported the HGT hypothesis, but more recent genome-wide data provide conflicting results. Here we generated significant genome data from the E. chlorotica germ line (egg DNA) and from V. litorea to test the HGT hypothesis. Our comprehensive analyses fail to provide evidence for alga-derived HGT into the germ line of the sea slug. PCR analyses of genomic DNA and cDNA from different individual E. chlorotica suggest however that algal nuclear genes (or gene fragments) are present in the adult slug. We suggest these nucleic acids may derive from and/or reside in extra-chromosomal DNAs that are made available to the animal through contact with the alga. These data resolve a long-standing issue and suggest that HGT is not the primary reason underlying long-term maintenance of photosynthesis in E. chlorotica. Therefore, sea slug photosynthesis is sustained in as yet unexplained ways that do not appear to endanger the animal germ line through the introduction of dozens of foreign genes.
- Conservation and Functional Element Discovery in 20 Angiosperm Plant Genomes. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 May 2.
Here we describe the construction of a phylogenetically-deep, whole-genome alignment of 20 flowering plants, along with an analysis of plant genome conservation. Each included angiosperm genome was aligned to a reference genome, A. thaliana, using the LASTZ/MULTIZ paradigm and tools from the UCSC Genome Browser source code. In addition to the multiple alignment, we created a local genome browser displaying multiple tracks of newly-generated genome annotation, as well as annotation sourced from published data of other research groups. An investigation into A. thaliana gene features present in the aligned A. lyrata genome revealed better conservation of start codons, stop codons, and splice sites within our alignments (51% of features from A. thaliana conserved without interruption in A. lyrata) when compared to previous publicly-available plant pairwise alignments (34% of features conserved). The detailed view of conservation across angiosperms revealed high coding-sequence conservation, but also a large set of previously-uncharacterized intergenic conservation. From this, we annotated the collection of conserved features, revealing dozens of putative non-coding RNAs, including some with recorded small RNA expression. Comparing conservation between kingdoms revealed a faster decay of vertebrate genome features when compared to angiosperm genomes. Finally, conserved sequences were searched for folding RNA features, including but not limited to ncRNA genes. Among these, we highlight a double hairpin in the 5' UTR of the PRIN2 gene, and a putative ncRNA with homology targeting the LAF3 protein.
- Analysis of piRNA-mediated silencing of active TEs in Drosophila melanogaster suggests limits on the evolution of host genome defense. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 Apr 26.
The piRNA pathway defends animal genomes against the harmful consequences of TE infection by imposing small-RNA mediated silencing. Because silencing is targeted by TE-derived piRNAs, piRNA production is posited to be central to the evolution of genome defense. We harnessed genomic data sets from Drosophila melanogaster, including genome-wide measures of piRNA, mRNA, and genomic abundance, along with estimates of age structure and risk of ectopic recombination, to address fundamental questions about the functional and evolutionary relationships between TE families and their regulatory piRNAs. We demonstrate that mRNA transcript abundance, robustness of "ping-pong" amplification, and representation in piRNA clusters together explain the majority of variation in piRNA abundance between TE families, providing the first robust statistical support for the prevailing model of piRNA biogenesis. Intriguingly, we also discover that the most transpositionally active TE families, with the greatest capacity to induce harmful mutations or disrupt gametogenesis, are not necessarily the most abundant among piRNAs. Rather, the level of piRNA targeting is largely independent of recent transposition rate for active TE families, but is rapidly lost for inactive TEs. These observations are consistent with population genetic theory that suggests a limited selective advantage for host repression of transposition. Additionally, we find no evidence that piRNA targeting responds to selection against a second major cost of TE infection: ectopic recombination between TE insertions. Our observations confirm the pivotal role of piRNA-mediated silencing in defending the genome against selfish transposition, yet also suggest limits to the optimization of host genome defense.
- Evidence for Polygenic Adaptation to Pathogens in the Human Genome. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 Apr 26.
Most approaches aiming at finding genes involved in adaptive events have focused on the detection of outlier loci, which resulted in the discovery of individually 'significant' genes with strong effects. However, a collection of small effect mutations could have a large effect on a given biological pathway that includes many genes, and such a polygenic mode of adaptation has not been systematically investigated in humans. We propose here to evidence polygenic selection by detecting signals of adaptation at the pathway or gene set level instead of analyzing single independent genes. Using a gene-set enrichment test to identify genome-wide signals of adaptation among human populations, we find that most pathways globally enriched for signals of positive selection are either directly or indirectly involved in immune response. We also find evidence for long-distance genotypic linkage disequilibrium, suggesting functional epistatic interactions between members of the same pathway. Our results show that past interactions with pathogens have elicited widespread and coordinated genomic responses, and suggest that adaptation to pathogens can be considered as a primary example of polygenic selection.
- Accelerated evolution after gene duplication: a time-dependent process affecting just one copy. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 Apr 26.
Gene duplication is widely regarded as a major mechanism modelling genome evolution and function. However, the mechanisms that drive the evolution of the two, initially redundant, gene copies are still ill-defined. Many gene duplicates experience evolutionary rate acceleration, but the relative contribution of positive selection and random drift to the retention and subsequent evolution of gene duplicates, and for how long the molecular clock may be distorted by these processes, remains unclear. Focusing on rodent genes which duplicated before and after the mouse and rat split, we find significantly increased sequence divergence after duplication in only one of the copies, which in nearly all cases corresponds to the novel daughter copy, independent of the mechanism of duplication. We observe that the evolutionary rate of the accelerated copy, measured as the ratio of non-synonymous to synonymous substitutions, is on average 5 fold higher in the period spanning 4 to 12 My after the duplication than it was before the duplication. This increase can be explained, at least in part, by the action of positive selection according to the results of the maximum likelihood based branch-site test. Subsequently, the rate decelerates until purifying selection completely returns to preduplication levels. Reversion to the original rates has already been accomplished 40.5 My after the duplication event, corresponding to a genetic distance of about 0.28 synonymous substitutions per site. Differences in tissue gene expression patterns parallel those of substitution rates, reinforcing the role of neofunctionalization in explaining the evolution of young gene duplicates.
- Population genomics and transcriptional consequences of regulatory motif variation in globally diverse Saccharomyces cerevisiae strains. [JOURNAL ARTICLE]
- Mol Biol Evol 2013 Apr 25.
Noncoding genetic variation is known to significantly influence gene expression levels in a growing number of specific cases; however, the patterns of genome-wide noncoding variation present within populations, the evolutionary forces acting on noncoding variants, and the relative effects of regulatory polymorphisms on transcript abundance are not well characterized. Here, we address these questions by analyzing patterns of regulatory variation in motifs for 177 DNA binding proteins in 37 strains of S. cerevisiae. Between S. cerevisiae strains, we found considerable polymorphism in regulatory motifs across strains (mean π=0.005) as well as diversity in regulatory motifs (mean 0.91 motifs differences per regulatory region). Population genetics analyses reveal that motifs are under purifying selection, and there is considerable heterogeneity in the magnitude of selection across different motifs. Finally, we obtained RNA-Seq data in 22 strains and identified 49 polymorphic DNA sequence motifs in 30 distinct genes that are significantly associated with transcriptional differences between strains. In 22 of these genes, there was a single polymorphic motif associated with expression in the upstream region. Our results provide comprehensive insights into the evolutionary trajectory of regulatory variation in yeast and the characteristics of a compendium of regulatory alleles.