Inferring the history of interchromosomal gene transposition in Drosophila using n-dimensional parsimony

Retrogene Duplication and Expression Patterns Shaped by the Evolution of Sex Chromosomes in Malaria Mosquitoes

Genes that originate during evolution are an important source of novel biological functions. Retrogenes are functional copies of genes produced by retroduplication and as such are located in different genomic positions. To investigate retroposition patterns and retrogene expression, we computationally identified interchromosomal retroduplication events in nine portions of the phylogenetic history of malaria mosquitoes, making use of species that do or do not have classical sex chromosomes to test the roles of sex-linkage. We found 40 interchromosomal events and a significant excess of retroduplications from the X chromosome to autosomes among a set of young retrogenes. These young retroposition events occurred within the last 100 million years in lineages where all species possessed differentiated sex chromosomes. An analysis of available microarray and RNA-seq expression data for Anopheles gambiae showed that many of the young retrogenes evolved male-biased expression in the reproductive organs. Young autosomal retrogenes with increased meiotic or postmeiotic expression in the testes tend to be male biased. In contrast, older retrogenes, i.e., in lineages with undifferentiated sex chromosomes, do not show this particular chromosomal bias and are enriched for female-biased expression in reproductive organs. Our reverse-transcription PCR data indicates that most of the youngest retrogenes, which originated within the last 47.6 million years in the subgenus Cellia, evolved non-uniform expression patterns across body parts in the males and females of An. coluzzii. Finally, gene annotation revealed that mitochondrial function is a prominent feature of the young autosomal retrogenes. We conclude that mRNA-mediated gene duplication has produced a set of genes that contribute to mosquito reproductive functions and that different biases are revealed after the sex chromosomes evolve. Overall, these results suggest potential roles for the evolution of meiotic sex chromosome inactivation in males and of sexually antagonistic conflict related to mitochondrial energy function as the main selective pressures for X-to-autosome gene reduplication and testis-biased expression in these mosquito lineages.

Hybrid Sterility, Genetic Conflict and Complex Speciation: Lessons From the Drosophila simulans Clade Species

The three fruitfly species of the Drosophila simulans clade- D. simulans, D. mauritiana, and D. sechellia- have served as important models in speciation genetics for over 40 years. These species are reproductively isolated by geography, ecology, sexual signals, postmating-prezygotic interactions, and postzygotic genetic incompatibilities. All pairwise crosses between these species conform to Haldane's rule, producing fertile F1 hybrid females and sterile F1 hybrid males. The close phylogenetic proximity of the D. simulans clade species to the model organism, D. melanogaster, has empowered genetic analyses of their species differences, including reproductive incompatibilities. But perhaps no phenotype has been subject to more continuous and intensive genetic scrutiny than hybrid male sterility. Here we review the history, progress, and current state of our understanding of hybrid male sterility among the D. simulans clade species. Our aim is to integrate the available information from experimental and population genetics analyses bearing on the causes and consequences of hybrid male sterility. We highlight numerous conclusions that have emerged as well as issues that remain unresolved. We focus on the special role of sex chromosomes, the fine-scale genetic architecture of hybrid male sterility, and the history of gene flow between species. The biggest surprises to emerge from this work are that (i) genetic conflicts may be an important general force in the evolution of hybrid incompatibility, (ii) hybrid male sterility is polygenic with contributions of complex epistasis, and (iii) speciation, even among these geographically allopatric taxa, has involved the interplay of gene flow, negative selection, and positive selection. These three conclusions are marked departures from the classical views of speciation that emerged from the modern evolutionary synthesis.

Characteristics of the mitochondrial genome of Rana omeimontis and related species in Ranidae: Gene rearrangements and phylogenetic relationships

The Omei wood frog (Rana omeimontis), endemic to central China, belongs to the family Ranidae. In this study, we achieved detail knowledge about the mitogenome of the species. The length of the genome is 20,120 bp, including 13 protein-coding genes (PCGs), 22 tRNA genes, two rRNA genes, and a noncoding control region. Similar to other amphibians, we found that only nine genes (ND6 and eight tRNA genes) are encoded on the light strand (L) and other genes on the heavy strand (H). Totally, The base composition of the mitochondrial genome included 27.29% A, 28.85% T, 28.87% C, and 15.00% G, respectively. The control regions among the Rana species were found to exhibit rich genetic variability and A + T content. R. omeimontis was clustered together with R. chaochiaoensis in phylogenetic tree. Compared to R. amurensis and R. kunyuensi, it was more closely related to R. chaochiaoensis, and a new way of gene rearrangement (ND6-trnE-Cytb-D-loop-trnL2 (CUN)-ND5-D-loop) was also found in the mitogenome of R. amurensis and R. kunyuensi. Our results about the mitochondrial genome of R. omeimontis will contribute to the future studies on phylogenetic relationship and the taxonomic status of Rana and related Ranidae species.

Heterochromatin-Enriched Assemblies Reveal the Sequence and Organization of the Drosophila melanogaster Y Chromosome

Heterochromatic regions of the genome are repeat-rich and poor in protein coding genes, and are therefore underrepresented in even the best genome assemblies. One of the most difficult regions of the genome to assemble are sex-limited chromosomes. The Drosophila melanogaster Y chromosome is entirely heterochromatic, yet has wide-ranging effects on male fertility, fitness, and genome-wide gene expression. The genetic basis of this phenotypic variation is difficult to study, in part because we do not know the detailed organization of the Y chromosome. To study Y chromosome organization in D. melanogaster, we develop an assembly strategy involving the in silico enrichment of heterochromatic long single-molecule reads and use these reads to create targeted de novo assemblies of heterochromatic sequences. We assigned contigs to the Y chromosome using Illumina reads to identify male-specific sequences. Our pipeline extends the D. melanogaster reference genome by 11.9 Mb, closes 43.8% of the gaps, and improves overall contiguity. The addition of 10.6 MB of Y-linked sequence permitted us to study the organization of repeats and genes along the Y chromosome. We detected a high rate of duplication to the pericentric regions of the Y chromosome from other regions in the genome. Most of these duplicated genes exist in multiple copies. We detail the evolutionary history of one sex-linked gene family, crystal-Stellate While the Y chromosome does not undergo crossing over, we observed high gene conversion rates within and between members of the crystal-Stellate gene family, Su(Ste), and PCKR, compared to genome-wide estimates. Our results suggest that gene conversion and gene duplication play an important role in the evolution of Y-linked genes.

Comparison of Oryza sativa and Oryza brachyantha Genomes Reveals Selection-Driven Gene Escape from the Centromeric Regions

Centromeres are dynamic chromosomal regions, and the genetic and epigenetic environment of the centromere is often regarded as oppressive to protein-coding genes. Here, we used comparative genomic and phylogenomic approaches to study the evolution of centromeres and centromere-linked genes in the genus Oryza We report a 12.4-Mb high-quality BAC-based pericentromeric assembly for Oryza brachyantha, which diverged from cultivated rice (Oryza sativa) ∼15 million years ago. The synteny analyses reveal seven medium (>50 kb) pericentric inversions in O. sativa and 10 in O. brachyantha Of these inversions, three resulted in centromere movement (Chr1, Chr7, and Chr9). Additionally, we identified a potential centromere-repositioning event, in which the ancestral centromere on chromosome 12 in O. brachyantha jumped ∼400 kb away, possibly mediated by a duplicated transposition event (>28 kb). More strikingly, we observed an excess of syntenic gene loss at and near the centromeric regions (P < 2.2 × 10^-16). Most (33/47) of the missing genes moved to other genomic regions; therefore such excess could be explained by the selective loss of the copy in or near centromeric regions after gene duplication. The pattern of gene loss immediately adjacent to centromeric regions suggests centromere chromatin dynamics (e.g., spreading or microrepositioning) may drive such gene loss.

Genes Relocated Between Drosophila Chromosome Arms Evolve Under Relaxed Selective Constraints Relative to Non-Relocated Genes

Gene duplication creates a second copy of a gene either in tandem to the ancestral locus or dispersed to another chromosomal location. When the ancestral copy of a dispersed duplicate is lost from the genome, it creates the appearance that the gene was "relocated" from the ancestral locus to the derived location. Gene relocations may be as common as canonical dispersed duplications in which both the ancestral and derived copies are retained. Relocated genes appear to be under more selective constraints than the derived copies of canonical duplications, and they are possibly as conserved as single-copy non-relocated genes. To test this hypothesis, we combined comparative genomics, population genetics, gene expression, and functional analyses to assess the selection pressures acting on relocated, duplicated, and non-relocated single-copy genes in Drosophila genomes. We find that relocated genes evolve faster than single-copy non-relocated genes, and there is no evidence that this faster evolution is driven by positive selection. In addition, relocated genes are less essential for viability and male fertility than single-copy non-relocated genes, suggesting that relocated genes evolve fast because of relaxed selective constraints. However, relocated genes evolve slower than the derived copies of canonical dispersed duplicated genes. We therefore conclude that relocated genes are under more selective constraints than canonical duplicates, but are not as conserved as single-copy non-relocated genes.

The Genomic Impact of Gene Retrocopies: What Have We Learned from Comparative Genomics, Population Genomics, and Transcriptomic Analyses?

Gene duplication is a major driver of organismal evolution. Gene retroposition is a mechanism of gene duplication whereby a gene's transcript is used as a template to generate retroposed gene copies, or retrocopies. Intriguingly, the formation of retrocopies depends upon the enzymatic machinery encoded by retrotransposable elements, genomic parasites occurring in the majority of eukaryotes. Most retrocopies are depleted of the regulatory regions found upstream of their parental genes; therefore, they were initially considered transcriptionally incompetent gene copies, or retropseudogenes. However, examples of functional retrocopies, or retrogenes, have accumulated since the 1980s. Here, we review what we have learned about retrocopies in animals, plants and other eukaryotic organisms, with a particular emphasis on comparative and population genomic analyses complemented with transcriptomic datasets. In addition, these data have provided information about the dynamics of the different "life cycle" stages of retrocopies (i.e., polymorphic retrocopy number variants, fixed retropseudogenes and retrogenes) and have provided key insights into the retroduplication mechanisms, the patterns and evolutionary forces at work during the fixation process and the biological function of retrogenes. Functional genomic and transcriptomic data have also revealed that many retropseudogenes are transcriptionally active and a biological role has been experimentally determined for many. Finally, we have learned that not only non-long terminal repeat retroelements but also long terminal repeat retroelements play a role in the emergence of retrocopies across eukaryotes. This body of work has shown that mRNA-mediated duplication represents a widespread phenomenon that produces an array of new genes that contribute to organismal diversity and adaptation.

Spermatogenesis Drives Rapid Gene Creation and Masculinization of the X Chromosome in Stalk-Eyed Flies (Diopsidae)

Throughout their evolutionary history, genomes acquire new genetic material that facilitates phenotypic innovation and diversification. Developmental processes associated with reproduction are particularly likely to involve novel genes. Abundant gene creation impacts the evolution of chromosomal gene content and general regulatory mechanisms such as dosage compensation. Numerous studies in model organisms have found complex and, at times contradictory, relationships among these genomic attributes highlighting the need to examine these patterns in other systems characterized by abundant sexual selection. Therefore, we examined the association among novel gene creation, tissue-specific gene expression, and chromosomal gene content within stalk-eyed flies. Flies in this family are characterized by strong sexual selection and the presence of a newly evolved X chromosome. We generated RNA-seq transcriptome data from the testes for three species within the family and from seven additional tissues in the highly dimorphic species, Teleopsis dalmanni. Analysis of dipteran gene orthology reveals dramatic testes-specific gene creation in stalk-eyed flies, involving numerous gene families that are highly conserved in other insect groups. Identification of X-linked genes for the three species indicates that the X chromosome arose prior to the diversification of the family. The most striking feature of this X chromosome is that it is highly masculinized, containing nearly twice as many testes-specific genes as expected based on its size. All the major processes that may drive differential sex chromosome gene content—creation of genes with male-specific expression, development of male-specific expression from pre-existing genes, and movement of genes with male-specific expression—are elevated on the X chromosome of T. dalmanni. This masculinization occurs despite evidence that testes expressed genes do not achieve the same levels of gene expression on the X chromosome as they do on the autosomes.

Genomics of ecological adaptation in cactophilic Drosophila

Cactophilic Drosophila species provide a valuable model to study gene–environment interactions and ecological adaptation. Drosophila buzzatii and Drosophila mojavensis are two cactophilic species that belong to the repleta group, but have very different geographical distributions and primary host plants. To investigate the genomic basis of ecological adaptation, we sequenced the genome and developmental transcriptome of D. buzzatii and compared its gene content with that of D. mojavensis and two other noncactophilic Drosophila species in the same subgenus. The newly sequenced D. buzzatii genome (161.5 Mb) comprises 826 scaffolds (>3 kb) and contains 13,657 annotated protein-coding genes. Using RNA sequencing data of five life-stages we found expression of 15,026 genes, 80% protein-coding genes, and 20% noncoding RNA genes. In total, we detected 1,294 genes putatively under positive selection. Interestingly, among genes under positive selection in the D. mojavensis lineage, there is an excess of genes involved in metabolism of heterocyclic compounds that are abundant in Stenocereus cacti and toxic to nonresident Drosophila species. We found 117 orphan genes in the shared D. buzzatii–D. mojavensis lineage. In addition, gene duplication analysis identified lineage-specific expanded families with functional annotations associated with proteolysis, zinc ion binding, chitin binding, sensory perception, ethanol tolerance, immunity, physiology, and reproduction. In summary, we identified genetic signatures of adaptation in the shared D. buzzatii–D. mojavensis lineage, and in the two separate D. buzzatii and D. mojavensis lineages. Many of the novel lineage-specific genomic features are promising candidates for explaining the adaptation of these species to their distinct ecological niches.

Mosquito genomics. Highly evolvable malaria vectors: the genomes of 16 Anopheles mosquitoes

Variation in vectorial capacity for human malaria among Anopheles mosquito species is determined by many factors, including behavior, immunity, and life history. To investigate the genomic basis of vectorial capacity and explore new avenues for vector control, we sequenced the genomes of 16 anopheline mosquito species from diverse locations spanning ~100 million years of evolution. Comparative analyses show faster rates of gene gain and loss, elevated gene shuffling on the X chromosome, and more intron losses, relative to Drosophila. Some determinants of vectorial capacity, such as chemosensory genes, do not show elevated turnover but instead diversify through protein-sequence changes. This dynamism of anopheline genes and genomes may contribute to their flexible capacity to take advantage of new ecological niches, including adapting to humans as primary hosts.

Relocation facilitates the acquisition of short cis-regulatory regions that drive the expression of retrogenes during spermatogenesis in Drosophila

Retrogenes are functional processed copies of genes that originate via the retrotranscription of an mRNA intermediate and often exhibit testis-specific expression. Although this expression pattern appears to be favored by selection, the origin of such expression bias remains unexplained. Here, we study the regulation of two young testis-specific Drosophila retrogenes, Dntf-2r and Pros28.1A, using genetic transformation and the enhanced green fluorescent protein reporter gene in Drosophila melanogaster. We show that two different short (<24 bp) regions upstream of the transcription start sites (TSSs) act as testis-specific regulatory motifs in these genes. The Dntf-2r regulatory region is similar to the known β2 tubulin 14-bp testis motif (β2-tubulin gene upstream element 1 [β2-UE1]). Comparative sequence analyses reveal that this motif was already present before the Dntf-2r insertion and was likely driving the transcription of a noncoding RNA. We also show that the β2-UE1 occurs in the regulatory regions of other testis-specific retrogenes, and is functional in either orientation. In contrast, the Pros28.1A testes regulatory region in D. melanogaster appears to be novel. Only Pros28.1B, an older paralog of the Pros28.1 gene family, seems to carry a similar regulatory sequence. It is unclear how the Pros28.1A regulatory region was acquired in D. melanogaster, but it might have evolved de novo from within a region that may have been preprimed for testes expression. We conclude that relocation is critical for the evolutionary origin of male germline-specific cis-regulatory regions of retrogenes because expression depends on either the site of the retrogene insertion or the sequence changes close to the TSS thereafter. As a consequence we infer that positive selection will play a role in the evolution of these regulatory regions and can often act from the moment of the retrocopy insertion.

The genomic distribution of sex-biased genes in drosophila serrata: X chromosome demasculinization, feminization, and hyperexpression in both sexes

The chromosomal distribution of genes with sex-biased expression is often nonrandom, and in species with XY sex chromosome systems, it is common to observe a deficit of X-linked male-biased genes and an excess of X-linked female-biased genes. One explanation for this pattern is that sex-specific selection has shaped the gene content of the X. Alternatively, the deficit of male-biased and excess of female-biased genes could be an artifact of differences between the sexes in the global expression level of their X chromosome(s), perhaps brought about by a lack of dosage compensation in males and hyperexpression in females. In the montium fruit fly, Drosophila serrata, both these explanations can account for a deficit of male-biased and excess of female-biased X-linked genes. Using genome-wide expression data from multiple male and female tissues (n = 176 hybridizations), we found that testis- and accessory gland-specific genes are underrepresented whereas female ovary-specific genes are overrepresented on the X chromosome, suggesting that X-linkage is disfavored for male function genes but favored for female function genes. However, genes with such sex-specific functions did not fully account for the deficit of male-biased and excess of female-biased X-linked genes. We did, however, observe sex differences in the global expression level of the X chromosome and autosomes. Surprisingly, and in contrast to other species where a lack of dosage compensation in males is responsible, we found that hyperexpression of X-linked genes in both sexes leads to this imbalance in D. serrata. Our results highlight how common genomic distributions of sex-biased genes, even among closely related species, may arise via quite different evolutionary processes.

The duplication-loss small phylogeny problem: from cherries to trees

The reconstruction of the history of evolutionary genome-wide events among a set of related organisms is of great biological interest since it can help to reveal the genomic basis of phenotypes. The sequencing of whole genomes faciliates the study of gene families that vary in size through duplication and loss events, like transfer RNA. However, a high sequence similarity often does not allow one to distinguish between orthologs and paralogs. Previous methods have addressed this difficulty by taking into account flanking regions of members of a family independently. We go one step further by inferring the order of genes of (a set of) families for ancestral genomes by considering the order of these genes on sequenced genomes. We present a novel branch-and-cut algorithm to solve the two species small phylogeny problem in the evolutionary model of duplications and losses. On average, our implementation, DupLoCut, improves the running time of a recently proposed method in the experiments on six Vibrionaceae lineages by a factor of ∼200. Besides the mere improvement in running time, the efficiency of our approach allows us to extend our model from cherries of a species tree, that is, subtrees with two leaves, to the median of three species setting. Being able to determine the median of three species is of key importance to one of the most common approaches to ancestral reconstruction, and our experiments show that its repeated computation considerably reduces the number of duplications and losses along the tree both on simulated instances comprising 128 leaves and a set of Bacillus genomes. Furthermore, in our simulations we show that a reduction in cost goes hand in hand with an improvement of the predicted ancestral genomes. Finally, we prove that the small phylogeny problem in the duplication-loss model is NP-complete already for two species.

Asymmetric functional divergence of young, dispersed gene duplicates in Arabidopsis thaliana

One prediction of the classic Ohno model of gene duplication predicts that new genes form from the asymmetric functional divergence of a newly arisen, redundant duplicate locus. In order to understand the mechanisms which give rise to functional divergence of newly formed dispersed duplicates, we assessed the expression and molecular evolutionary divergence of a suite of 19 highly similar dispersed duplicates in Arabidopsis thaliana. These duplicates have a K sil equal to or less than 5 % and are specific to the A. thaliana lineage; thus, they predictably represent some of the youngest duplicates in the A. thaliana genome. We found that the majority of young duplicate loci exhibit asymmetric expression patterns, with the daughter locus exhibiting reduced expression across all tissues analyzed relative to the progenitor locus or simply not expressed. Furthermore, daughter loci, on the whole, have significantly more nonsynonymous substitutions than the progenitor loci. We also identified four pairs of loci which exhibit significant (P < 0.05) evolutionary rate asymmetry, three of which exhibit elevated dN/dS in the duplicate copy. We suggest, based on these data, that functional diversification initially takes the form of asymmetric regulatory divergence that can be a direct consequence of the mode of duplication. The reduced and/or absence of expression in the daughter copy relaxes functional constraint on its protein coding sequence leading to the asymmetric accumulation of nonsynonymous mutations. Thus, our data both affirm Ohno's prediction while explaining the mechanism by which functional divergence initially occurs following duplication for dispersed gene duplicates.

Evidence of a paucity of genes that interact with the mitochondrion on the X in mammals

Mitochondria are essential organelles whose replication, development, and physiology are dependent upon coordinated gene interactions with both the mitochondrial and the nuclear genomes. The evolution of coadapted (CA) nuclear-mitochondrial gene combinations would be facilitated if such nuclear genes were located on the X-chromosome instead of on the autosomes because of the increased probability of cotransmission. Here, we test the prediction of the CA hypothesis by investigating the chromosomal distribution of nuclear genes that interact with mitochondria. Using the online genome database BIOMART, we compared the density of genes that have a mitochondrion cellular component annotation across chromosomes in 16 vertebrates. We find a strong and highly significant genomic pattern against the CA hypothesis: nuclear genes interacting with the mitochondrion are significantly underrepresented on the X-chromosome in mammals but not in birds. We interpret our findings in terms of sexual conflict as a mechanism that may generate the observed pattern. Our finding extends single-gene theory for the evolution of sexually antagonistic genes to nuclear-mitochondrial gene combinations.