Menu

Asset Tag: Evolution

July 7, 2019

Decay of sexual trait genes in an asexual parasitoid wasp.

Trait loss is a widespread phenomenon with pervasive consequences for a species’ evolutionary potential. The genetic changes underlying trait loss have only been clarified in a small number of cases. None of these studies can identify whether the loss of the trait under study was a result of neutral mutation accumulation or negative selection. This distinction is relatively clear-cut in the loss of sexual traits in asexual organisms. Male-specific sexual traits are not expressed and can only decay through neutral mutations, whereas female-specific traits are expressed and subject to negative selection. We present the genome of an asexual parasitoid wasp and compare it to that of a sexual lineage of the same species. We identify a short-list of 16 genes for which the asexual lineage carries deleterious SNP or indel variants, whereas the sexual lineage does not. Using tissue-specific expression data from other insects, we show that fifteen of these are expressed in male-specific reproductive tissues. Only one deleterious variant was found that is expressed in the female-specific spermathecae, a trait that is heavily degraded and thought to be under negative selection in L. clavipes. Although the phenotypic decay of male-specific sexual traits in asexuals is generally slow compared with the decay of female-specific sexual traits, we show that male-specific traits do indeed accumulate deleterious mutations as expected by theory. Our results provide an excellent starting point for detailed study of the genomics of neutral and selected trait decay.

Read more
July 7, 2019

Whole genome analysis of Yersinia ruckeri isolated over 27 years in Australia and New Zealand reveals geographical endemism over multiple lineages and recent evolution under host selection.

Yersinia ruckeri is a salmonid pathogen with widespread distribution in cool-temperate waters including Australia and New Zealand, two isolated environments with recently developed salmonid farming industries. Phylogenetic comparison of 58 isolates from Australia, New Zealand, USA, Chile, Finland and China based on non-recombinant core genome SNPs revealed multiple deep-branching lineages, with a most recent common ancestor estimated at 18?500 years BP (12?355-24?757 95% HPD) and evidence of Australasian endemism. Evolution within the Tasmanian Atlantic salmon serotype O1b lineage has been slow, with 63 SNPs describing the variance over 27 years. Isolates from the prevailing lineage are poorly/non-motile compared to a lineage pre-vaccination, introduced in 1997, which is highly motile but has not been isolated since from epizootics. A non-motile phenotype has arisen independently in Tasmania compared to Europe and USA through a frameshift in fliI, encoding the ATPase of the flagella cluster. We report for the first time lipopolysaccharide O-antigen serotype O2 isolates in Tasmania. This phenotype results from deletion of the O-antigen cluster and consequent loss of high-molecular-weight O-antigen. This phenomenon has occurred independently on three occasions on three continents (Australasia, North America and Asia) as O2 isolates from the USA, China and Tasmania share the O-antigen deletion but occupy distant lineages. Despite the European and North American origins of the Australasian salmonid stocks, the lineages of Y. ruckeri in Australia and New Zealand are distinct from those of the northern hemisphere, suggesting they are pre-existing ancient strains that have emerged and evolved with the introduction of susceptible hosts following European colonization.

Read more
July 7, 2019

Chromosome assembly of large and complex genomes using multiple references

Despite the rapid development of sequencing technologies, assembly of mammalian-scale genomes into complete chromosomes remains one of the most challenging problems in bioinformatics. To help address this difficulty, we developed Ragout, a reference-assisted assembly tool that now works for large and complex genomes. Taking one or more target assemblies (generated from an NGS assembler) and one or multiple related reference genomes, Ragout infers the evolutionary relationships between the genomes and builds the final assemblies using a genome rearrangement approach. Using Ragout, we transformed NGS assemblies of 15 different Mus musculus and one Mus spretus genomes into sets of complete chromosomes, leaving less than 5% of sequence unlocalized per set. Various benchmarks, including PCR testing and realigning of long PacBio reads, suggest only a small number of structural errors in the final assemblies, comparable with direct assembly approaches. Additionally, we applied Ragout to Mus caroli and Mus pahari genomes, which exhibit karyotype-scale variations compared to other genomes from the Muridae family. Chromosome color maps confirmed most large-scale rearrangements that Ragout detected.

Read more
July 7, 2019

Spontaneous chloroplast mutants mostly occur by replication slippage and show a biased pattern in the plastome of Oenothera.

Spontaneous plastome mutants have been used as a research tool since the beginning of genetics. However, technical restrictions have severely limited their contributions to research in physiology and molecular biology. Here, we used full plastome sequencing to systematically characterize a collection of 51 spontaneous chloroplast mutants in Oenothera (evening primrose). Most mutants carry only a single mutation. Unexpectedly, the vast majority of mutations do not represent single nucleotide polymorphisms but are insertions/deletions originating from DNA replication slippage events. Only very few mutations appear to be caused by imprecise double-strand break repair, nucleotide misincorporation during replication, or incorrect nucleotide excision repair following oxidative damage. U-turn inversions were not detected. Replication slippage is induced at repetitive sequences that can be very small and tend to have high A/T content. Interestingly, the mutations are not distributed randomly in the genome. The underrepresentation of mutations caused by faulty double-strand break repair might explain the high structural conservation of seed plant plastomes throughout evolution. In addition to providing a fully characterized mutant collection for future research on plastid genetics, gene expression, and photosynthesis, our work identified the spectrum of spontaneous mutations in plastids and reveals that this spectrum is very different from that in the nucleus.© 2016 American Society of Plant Biologists. All rights reserved.

Read more
July 7, 2019

Susan Celniker: Foundational resources to study a dynamic genome.

The Genetics Society of America’s George W. Beadle Award honors individuals who have made outstanding contributions to the community of genetics researchers and who exemplify the qualities of its namesake. The 2016 recipient, Susan E. Celniker, played a key role in the sequencing, annotation, and characterization of the Drosophila genome. She participated in early sequencing efforts at the Lawrence Berkeley National Laboratory and led the modENCODE Fly Transcriptome Consortium. Her efforts were critical to ensuring that the Drosophila genome was well-annotated, making it one of the best curated animal genomes available. As the Principal Investigator for the BDGP, Celniker has enabled the study of proteomes by creating a collection of over 13,000 clones that match annotated genes for protein expression in cells or transgenic flies, and she has established the most comprehensive spatial gene expression atlas in any organism, with in situ imaging of more than 80% of the Drosophila protein-coding transcriptome through embryogenesis. In addition to providing the research community with these invaluable resources and reagents, she continues to develop new tools and datasets for genetics researchers to explore the spatial and temporal control of gene expression.

Read more
July 7, 2019

Lepidoptera genomes: current knowledge, gaps and future directions.

Butterflies and moths (Lepidoptera) are one of the most ecologically diverse and speciose insect orders. With recent advances in genomics, new Lepidoptera genomes are regularly being sequenced, and many of them are playing principal roles in genomics studies, particularly in the fields of phylo-genomics and functional genomics. Thus far, assembled genomes are only available for <10 of the 43 Lepidoptera superfamilies. Nearly all are model species, found in the speciose clade Ditrysia. Community support for Lepidoptera genomics is growing with successful management and dissemination of data and analytical tools in centralized databases. With genomic studies quickly becoming integrated with ecological and evolutionary research, the Lepidoptera community will unquestionably benefit from new high-quality reference genomes that are more evenly distributed throughout the order. Copyright © 2018 Elsevier Inc. All rights reserved.

Read more
July 7, 2019

Inferring synteny between genome assemblies: a systematic evaluation.

Genome assemblies across all domains of life are being produced routinely. Initial analysis of a new genome usually includes annotation and comparative genomics. Synteny provides a framework in which conservation of homologous genes and gene order is identified between genomes of different species. The availability of human and mouse genomes paved the way for algorithm development in large-scale synteny mapping, which eventually became an integral part of comparative genomics. Synteny analysis is regularly performed on assembled sequences that are fragmented, neglecting the fact that most methods were developed using complete genomes. It is unknown to what extent draft assemblies lead to errors in such analysis.We fragmented genome assemblies of model nematodes to various extents and conducted synteny identification and downstream analysis. We first show that synteny between species can be underestimated up to 40% and find disagreements between popular tools that infer synteny blocks. This inconsistency and further demonstration of erroneous gene ontology enrichment tests raise questions about the robustness of previous synteny analysis when gold standard genome sequences remain limited. In addition, assembly scaffolding using a reference guided approach with a closely related species may result in chimeric scaffolds with inflated assembly metrics if a true evolutionary relationship was overlooked. Annotation quality, however, has minimal effect on synteny if the assembled genome is highly contiguous.Our results show that a minimum N50 of 1 Mb is required for robust downstream synteny analysis, which emphasizes the importance of gold standard genomes to the science community, and should be achieved given the current progress in sequencing technology.

Read more
July 7, 2019

Supergene evolution triggered by the introgression of a chromosomal inversion.

Supergenes are groups of tightly linked loci whose variation is inherited as a single Mendelian locus and are a common genetic architecture for complex traits under balancing selection [1-8]. Supergene alleles are long-range haplotypes with numerous mutations underlying distinct adaptive strategies, often maintained in linkage disequilibrium through the suppression of recombination by chromosomal rearrangements [1, 5, 7-9]. However, the mechanism governing the formation of supergenes is not well understood and poses the paradox of establishing divergent functional haplotypes in the face of recombination. Here, we show that the formation of the supergene alleles encoding mimicry polymorphism in the butterfly Heliconius numata is associated with the introgression of a divergent, inverted chromosomal segment. Haplotype divergence and linkage disequilibrium indicate that supergene alleles, each allowing precise wing-pattern resemblance to distinct butterfly models, originate from over a million years of independent chromosomal evolution in separate lineages. These “superalleles” have evolved from a chromosomal inversion captured by introgression and maintained in balanced polymorphism, triggering supergene inheritance. This mode of evolution involving the introgression of a chromosomal rearrangement is likely to be a common feature of complex structural polymorphisms associated with the coexistence of distinct adaptive syndromes. This shows that the reticulation of genealogies may have a powerful influence on the evolution of genetic architectures in nature. Copyright © 2018 Elsevier Ltd. All rights reserved.

Read more
July 7, 2019

The sequenced angiosperm genomes and genome databases.

Angiosperms, the flowering plants, provide the essential resources for human life, such as food, energy, oxygen, and materials. They also promoted the evolution of human, animals, and the planet earth. Despite the numerous advances in genome reports or sequencing technologies, no review covers all the released angiosperm genomes and the genome databases for data sharing. Based on the rapid advances and innovations in the database reconstruction in the last few years, here we provide a comprehensive review for three major types of angiosperm genome databases, including databases for a single species, for a specific angiosperm clade, and for multiple angiosperm species. The scope, tools, and data of each type of databases and their features are concisely discussed. The genome databases for a single species or a clade of species are especially popular for specific group of researchers, while a timely-updated comprehensive database is more powerful for address of major scientific mysteries at the genome scale. Considering the low coverage of flowering plants in any available database, we propose construction of a comprehensive database to facilitate large-scale comparative studies of angiosperm genomes and to promote the collaborative studies of important questions in plant biology.

Read more
July 7, 2019

Identification of repetitive DNA sequences in the Chrysanthemum boreale genome

We previously revealed that the Chrysanthemum boreale genome is highly repetitive; however, the types and nucleotide sequences of repetitive DNA in this diploid wild chrysanthemum are not known. Here, we characterized repetitive DNA sequences in the C. boreale genome by analysing genomic sequences obtained by Illumina sequencing and confirmed their repetitive nature by conducting fluorescence in situ hybridization (FISH) analyses. Annotation of the obtained DNA sequences revealed that microsatellite-containing genomic sequences exhibited similarity with genomic sequences in Chrysanthemum morifolium, indicating sequence conservation of repetitive DNA sequences between the two Chrysanthemum species. Two superfamilies of repetitive DNA, Copia and Gypsy, belonging to the long-terminal repeat (LTR) class of retrotransposons, are abundant in the C. boreale genome. We propose that Copia and Gypsy retroelements contribute to the current genome architecture of C. boreale. Whole genome sequencing, which is currently in progress, will reveal the extent to which these repetitive DNA sequences contribute.

Read more
July 7, 2019

Tracing the de novo origin of protein-coding genes in yeast.

De novo genes are very important for evolutionary innovation. However, how these genes originate and spread remains largely unknown. To better understand this, we rigorously searched for de novo genes in Saccharomyces cerevisiae S288C and examined their spread and fixation in the population. Here, we identified 84 de novo genes in S. cerevisiae S288C since the divergence with their sister groups. Transcriptome and ribosome profiling data revealed at least 8 (10%) and 28 (33%) de novo genes being expressed and translated only under specific conditions, respectively. DNA microarray data, based on 2-fold change, showed that 87% of the de novo genes are regulated during various biological processes, such as nutrient utilization and sporulation. Our comparative and evolutionary analyses further revealed that some factors, including single nucleotide polymorphism (SNP)/indel mutation, high GC content, and DNA shuffling, contribute to the birth of de novo genes, while domestication and natural selection drive the spread and fixation of these genes. Finally, we also provide evidence suggesting the possible parallel origin of a de novo gene between S. cerevisiae and Saccharomyces paradoxus Together, our study provides several new insights into the origin and spread of de novo genes.IMPORTANCE Emergence of de novo genes has occurred in many lineages during evolution, but the birth, spread, and function of these genes remain unresolved. Here we have searched for de novo genes from Saccharomyces cerevisiae S288C using rigorous methods, which reduced the effects of bad annotation and genomic gaps on the identification of de novo genes. Through this analysis, we have found 84 new genes originating de novo from previously noncoding regions, 87% of which are very likely involved in various biological processes. We noticed that 10% and 33% of de novo genes were only expressed and translated under specific conditions, therefore, verification of de novo genes through transcriptome and ribosome profiling, especially from limited expression data, may underestimate the number of bona fide new genes. We further show that SNP/indel mutation, high GC content, and DNA shuffling could be involved in the birth of de novo genes, while domestication and natural selection drive the spread and fixation of these genes. Finally, we provide evidence suggesting the possible parallel origin of a new gene. Copyright © 2018 Wu and Knudson.

Read more
July 7, 2019

An improved approach for reconstructing consensus repeats from short sequence reads

Repeat elements are important components of most eukaryotic genomes. Most existing tools for repeat analysis rely either on high quality reference genomes or existing repeat libraries. Thus, it is still challenging to do repeat analysis for species with highly repetitive or complex genomes which often do not have good reference genomes or annotated repeat libraries. Recently we developed a computational method called REPdenovo that constructs consensus repeat sequences directly from short sequence reads, which outperforms an existing tool called RepARK. One major issue with REPdenovo is that it doesn’t perform well for repeats with relatively high divergence rates or low copy numbers. In this paper, we present an improved approach for constructing consensus repeats directly from short reads. Comparing with the original REPdenovo, the improved approach uses more repeat-related k-mers and improves repeat assembly quality using a consensus-based k-mer processing method.

Read more
July 7, 2019

Loss of RXFP2 and INSL3 genes in Afrotheria shows that testicular descent is the ancestral condition in placental mammals.

Descent of testes from a position near the kidneys into the lower abdomen or into the scrotum is an important developmental process that occurs in all placental mammals, with the exception of five afrotherian lineages. Since soft-tissue structures like testes are not preserved in the fossil record and since key parts of the placental mammal phylogeny remain controversial, it has been debated whether testicular descent is the ancestral or derived condition in placental mammals. To resolve this debate, we used genomic data of 71 mammalian species and analyzed the evolution of two key genes (relaxin/insulin-like family peptide receptor 2 [RXFP2] and insulin-like 3 [INSL3]) that induce the development of the gubernaculum, the ligament that is crucial for testicular descent. We show that both RXFP2 and INSL3 are lost or nonfunctional exclusively in four afrotherians (tenrec, cape elephant shrew, cape golden mole, and manatee) that completely lack testicular descent. The presence of remnants of once functional orthologs of both genes in these afrotherian species shows that these gene losses happened after the split from the placental mammal ancestor. These “molecular vestiges” provide strong evidence that testicular descent is the ancestral condition, irrespective of persisting phylogenetic discrepancies. Furthermore, the absence of shared gene-inactivating mutations and our estimates that the loss of RXFP2 happened at different time points strongly suggest that testicular descent was lost independently in Afrotheria. Our results provide a molecular mechanism that explains the loss of testicular descent in afrotherians and, more generally, highlight how molecular vestiges can provide insights into the evolution of soft-tissue characters.

Read more
July 7, 2019

Genomes and transcriptomes of duckweeds.

Duckweeds (Lemnaceae family) are the smallest flowering plants that adapt to the aquatic environment. They are regarded as the promising sustainable feedstock with the characteristics of high starch storage, fast propagation, and global distribution. The duckweed genome size varies 13-fold ranging from 150 Mb in Spirodela polyrhiza to 1,881 Mb in Wolffia arrhiza. With the development of sequencing technology and bioinformatics, five duckweed genomes from Spirodela and Lemna genera are sequenced and assembled. The genome annotations discover that they share similar protein orthologs, whereas the repeat contents could mainly explain the genome size difference. The gene families responsible for cell growth and expansion, lignin biosynthesis, and flowering are greatly contracted. However, the gene family of glutamate synthase has experienced expansion, indicating their significance in ammonia assimilation and nitrogen transport. The transcriptome is comprehensively sequenced for the genera of Spirodela, Landoltia, and Lemna, including various treatments such as abscisic acid, radiation, heavy metal, and starvation. The analysis of the underlying molecular mechanism and the regulatory network would accelerate their applications in the fields of bioenergy and phytoremediation. The comparative genomics has shown that duckweed genomes contain relatively low gene numbers and more contracted gene families, which may be in parallel with their highly reduced morphology with a simple leaf and primary roots. Still, we are waiting for the advancement of the long read sequencing technology to resolve the complex genomes and transcriptomes for unsequenced Wolffiella and Wolffia due to the large genome sizes and the similarity in their polyploidy.

Read more
July 7, 2019

The recombination landscape of Drosophila virilis is robust to transposon activation in hybrid dysgenesis

DNA damage in the germline is a double-edged sword. Induced double-strand breaks establish the foundation for meiotic recombination and proper chromosome segregation but can also pose a significant challenge for genome stability. Within the germline, transposable elements are powerful agents of double-strand break formation. How different types of DNA damage are resolved within the germline is poorly understood. For example, little is known about the relationship between the frequency of double-stranded breaks, both endogenous and exogenous, and the decision to repair DNA through one of the many pathways, including crossing over and gene conversion. Here we use the Drosophila virilis hybrid dysgenesis model to determine how recombination landscapes change under transposable element activation. In this system, a cross between two strains of D. virilis with divergent transposable element profiles results in the hybrid dysgenesis phenotype, which includes the germline activation of diverse transposable elements, reduced fertility, and male recombination. However, only one direction of the cross results in hybrid dysgenesis. This allows the study of recombination in genetically identical F1 females; those with baseline levels of programmed DNA damage and those with an increased level of DNA damage resulting from transposable element proliferation. Using multiplexed shotgun genotyping to map crossover events, we compared the recombination landscapes of hybrid dysgenic and non-hybrid dysgenic individuals. The frequency and distribution of meiotic recombination appears to be robust during hybrid dysgenesis. However, hybrid dysgenesis is also associated with occasional clusters of recombination derived from single dysgenic F1 mothers. The clusters of recombination are hypothesized to be the result of mitotic crossovers during early germline development. Overall, these results show that meiotic recombination in D. virilis is robust to the damage caused by transposable elements during early development.

Read more

Subscribe for blog updates:

Talk with an expert

If you have a question, need to check the status of an order, or are interested in purchasing an instrument, we're here to help.