Chimeric Genes as a Source of Rapid Evolution in Drosophila melanogaster

Chimeric genes form through the combination of portions of existing coding sequences to create a new open reading frame. These new genes can create novel protein structures that are likely to serve as a strong source of novelty upon which selection can act. We have identified 14 chimeric genes that formed through DNA-level mutations in Drosophila melanogaster, and we investigate expression profiles, domain structures, and population genetics for each of these genes to examine their potential to effect adaptive evolution. We find that chimeric gene formation commonly produces mid-domain breaks and unites portions of wholly unrelated peptides, creating novel protein structures that are entirely distinct from other constructs in the genome. These new genes are often involved in selective sweeps. We further find a disparity between chimeric genes that have recently formed and swept to fixation versus chimeric genes that have been preserved over long periods of time, suggesting that preservation and adaptation are distinct processes. Finally, we demonstrate that chimeric gene formation can produce qualitative expression changes that are difficult to mimic through duplicate gene formation, and that extremely young chimeric genes (d S < 0.03) are more likely to be associated with selective sweeps than duplicate genes of the same age. Hence, chimeric genes can serve as an exceptional source of genetic novelty that can have a profound influence on adaptive evolution in D. melanogaster.Organismic and Evolutionary Biolog

Revised Annotations, Sex-Biased Expression, and Lineage-Specific Genes in the Drosophila melanogaster group

Here, we provide revised gene models for D. ananassae, D. yakuba, and D. simulans, which include UTRs and empirically verified intron-exon boundaries, as well as ortholog groups identified using a fuzzy reciprocal-best-hit blast comparison. Using these revised annotations, we perform differential expression testing using the cufflinks suite to provide a broad overview of differential expression between reproductive tissues and the carcass. We identify thousands of genes that are differentially expressed across tissues in D. yakuba and D. simulans, with roughly 60% agreement in expression patterns of orthologs in D. yakuba and D. simulans. We identify several cases of putative polycistronic transcripts, pointing to a combination of transcriptional read-through in the genome as well as putative gene fusion and fission events across taxa. We furthermore identify hundreds of lineage specific genes in each species with no blast hits among transcripts of any other Drosophila species, which are candidates for neofunctionalized proteins and a potential source of genetic novelty.Comment: Revised manuscript, also available online preprint at G3: Genes, Genomes, Genetics. Gene models, ortholog calls, and tissue specific expression results are available at http://github.com/ThorntonLab/GFF or the UCSC browser on the Thornton Lab public track hub at http://genome.ucsc.ed

Strong, Recent Selective Sweeps Reshape Genetic Diversity in Freshwater Bivalve \u3ci\u3eMegalonaias nervosa\u3c/i\u3e

Freshwater Unionid bivalves have recently faced ecological upheaval through pollution, barriers to dispersal, harvesting, and changes in fish–host prevalence. Currently, over 70% of species in North America are threatened, endangered or extinct. To characterize the genetic response to recent selective pressures, we collected population genetic data for one successful bivalve species, Megalonaias nervosa. We identify megabase-sized regions that are nearly monomorphic across the population, signals of strong, recent selection reshaping diversity across 73 Mb total. These signatures of selection are greater than is commonly seen in population genetic models. We observe 102 duplicate genes with high dN/dS on terminal branches among regions with sweeps, suggesting that gene duplication is a causative mechanism of recent adaptation in M. nervosa. Genes in sweeps reflect functional classes important for Unionid survival, including anticoagulation genes important for fish host parasitization, detox genes, mitochondria management, and shell formation. We identify sweeps in regions with no known functional impacts, suggesting mechanisms of adaptation that deserve greater attention in future work on species survival. In contrast, polymorphic transposable elements (TEs) appear to be detrimental and underrepresented among regions with sweeps. TE site frequency spectra are skewed toward singleton variants, and TEs among regions with sweeps are present at low frequency. Our work suggests that duplicate genes are an essential source of genetic novelty that has helped this species succeed in environments where others have struggled. These results suggest that gene duplications deserve greater attention in non-model population genomics, especially in species that have recently faced sudden environmental challenges

Strong, recent selective sweeps reshape genetic diversity in freshwater bivalve Megalonaias nervosa

Freshwater Unionid bivalves have recently faced ecological upheaval through pollution, barriers to dispersal, human harvesting, and changes in fish-host prevalence. Currently, over 70% of species are threatened, endangered or extinct. To characterize the genetic response to these recent selective pressures, we collected population genetic data for one successful bivalve species, Megalonaias nervosa. We identify megabase sized regions that are nearly monomorphic across the population, a signal of strong, recent selection reshaping genetic diversity. These signatures of selection encompass a total of 73Mb, greater response to selection than is commonly seen in population genetic models. We observe 102 duplicate genes with high dN/dS on terminal branches among regions with sweeps, suggesting that gene duplication is a causative mechanism of recent adaptation in M. nervosa. Genes in sweeps reflect functional classes known to be important for Unionid survival, including anticoagulation genes important for fish host parasitization, detox genes, mitochondria management, and shell formation. We identify selective sweeps in regions with no known functional impacts, suggesting mechanisms of adaptation that deserve greater attention in future work on species survival. In contrast, polymorphic transposable element insertions appear to be detrimental and underrepresented among regions with sweeps. TE site frequency spectra are skewed toward singleton variants, and TEs among regions with sweeps are present only at low frequency. Our work suggests that duplicate genes are an essential source of genetic novelty that has helped this successful species succeed in environments where others have struggled. These results suggest that gene duplications deserve greater attention in non-model population genomics, especially in species that have recently faced sudden environmental challenges.Comment: 6 figures, 4 supplementary tables, 31 pages tota

Landscape of standing variation for tandem duplications in Drosophila yakuba and Drosophila simulans

We have used whole genome paired-end Illumina sequence data to identify tandem duplications in 20 isofemale lines of D. yakuba, and 20 isofemale lines of D. simulans and performed genome wide validation with PacBio long molecule sequencing. We identify 1,415 tandem duplications that are segregating in D. yakuba as well as 975 duplications in D. simulans, indicating greater variation in D. yakuba. Additionally, we observe high rates of secondary deletions at duplicated sites, with 8% of duplicated sites in D. simulans and 17% of sites in D. yakuba modified with deletions. These secondary deletions are consistent with the action of the large loop mismatch repair system acting to remove polymorphic tandem duplication, resulting in rapid dynamics of gain and loss in duplicated alleles and a richer substrate of genetic novelty than has been previously reported. Most duplications are present in only single strains, suggesting deleterious impacts are common. D. simulans shows larger numbers of whole gene duplications in comparison to larger proportions of gene fragments in D. yakuba. D. simulans displays an excess of high frequency variants on the X chromosome, consistent with adaptive evolution through duplications on the D. simulans X or demographic forces driving duplicates to high frequency. We identify 78 chimeric genes in D. yakuba and 38 chimeric genes in D. simulans, as well as 143 cases of recruited non-coding sequence in D. yakuba and 96 in D. simulans, in agreement with rates of chimeric gene origination in D. melanogaster. Together, these results suggest that tandem duplications often result in complex variation beyond whole gene duplications that offers a rich substrate of standing variation that is likely to contribute both to detrimental phenotypes and disease, as well as to adaptive evolutionary change.Comment: Revised Version- Accepted at Molecular Biology and Evolutio

Transcriptome Complexities Across Eukaryotes

Genomic complexity is a growing field of evolution, with case studies for comparative evolutionary analyses in model and emerging non-model systems. Understanding complexity and the functional components of the genome is an untapped wealth of knowledge ripe for exploration. With the "remarkable lack of correspondence" between genome size and complexity, there needs to be a way to quantify complexity across organisms. In this study we use a set of complexity metrics that allow for evaluation of changes in complexity using TranD. We ascertain if complexity is increasing or decreasing across transcriptomes and at what structural level, as complexity is varied. We define three metrics -- TpG, EpT, and EpG in this study to quantify the complexity of the transcriptome that encapsulate the dynamics of alternative splicing. Here we compare complexity metrics across 1) whole genome annotations, 2) a filtered subset of orthologs, and 3) novel genes to elucidate the impacts of ortholog and novel genes in transcriptome analysis. We also derive a metric from Hong et al., 2006, Effective Exon Number (EEN), to compare the distribution of exon sizes within transcripts against random expectations of uniform exon placement. EEN accounts for differences in exon size, which is important because novel genes differences in complexity for orthologs and whole transcriptome analyses are biased towards low complexity genes with few exons and few alternative transcripts. With our metric analyses, we are able to implement changes in complexity across diverse lineages with greater precision and accuracy than previous cross-species comparisons under ortholog conditioning. These analyses represent a step forward toward whole transcriptome analysis in the emerging field of non-model evolutionary genomics, with key insights for evolutionary inference of complexity changes on deep timescales across the tree of life. We suggest a means to quantify biases generated in ortholog calling and correct complexity analysis for lineage-specific effects. With these metrics, we directly assay the quantitative properties of newly formed lineage-specific genes as they lower complexity in transcriptomes.Comment: 33 pages main text; 6 main figures; 25 pages of supplement; 1 supplementary table; 24 Supp Figures; 58 pages tota

Gene content and distribution in the nuclear genome of Fragaria vesca

Thirty fosmids were randomly selected from a library of Fragaria vesca subsp. americana (cv. Pawtuckaway) DNA. These fosmid clones were individually sheared, and ∼4- to 5-kb fragments were subcloned. Subclones on a single 384-well plate were sequenced bidirectionally for each fosmid. Assembly of these data yielded 12 fosmid inserts completely sequenced, 14 inserts as 2 to 3 contiguous sequences (contigs), and 4 inserts with 5 to 9 contigs. In most cases, a single unambiguous contig order and orientation was determined, so no further finishing was required to identify genes and their relative arrangement. One hundred fifty-eight genes were identified in the ∼1.0 Mb of nuclear genomic DNA that was assembled. Because these fosmids were randomly chosen, this allowed prediction of the genetic content of the entire ∼200 Mb F. vesca genome as about 30,500 protein-encoding genes, plus >4700 truncated gene fragments. The genes are mostly arranged in gene-rich regions, to a variable degree intermixed with transposable elements (TEs). The most abundant TEs in F. vesca were found to be long terminal repeat (LTR) retrotransposons, and these comprised about 13% of the DNA analyzed. Over 30 new repeat families were discovered, mostly TEs, and the total TE content of F. vesca is predicted to be at least 16%.EEA BalcarceFil: Pontaroli, Ana Clara. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce; Argentina. University of Georgia. Department of Genetics; Estados UnidosFil: Rogers, Rebekah L. Harvard University. Department of Organismic and Evolutionary Biology; Estados Unidos. University of Georgia. Department of Genetics; Estados UnidosFil: Qian, Zhang. University of New Hampshire. Department of Biological Sciences; Estados UnidosFil: Shields, Melanie E. University of New Hampshire. Department of Biological Sciences; Estados UnidosFil: Davis, Thomas M. University of New Hampshire. Department of Biological Sciences; Estados UnidosFil: Folta, Kevin M. University of Florida. Horticultural Sciences Department; Estados UnidosFil: SanMiguel, Phillip. Purdue University. Department of Horticulture and Landscape Architecture; Estados UnidosFil: Bennetzen, Jeffrey L. University of Georgia. Department of Genetics; Estados Unido