Recurrent deletion and gene presence/absence polymorphism: telomere dynamics dominate evolution at the tip of 3L in Drosophila melanogaster and D. simulans

Although Drosophila melanogaster has been the subject of intensive analysis of polymorphism and divergence, little is known about the distribution of variation at the most distal regions of chromosomes arms. Here we report a survey of genetic variation on the tip of 3L in D. melanogaster and D. simulans. Levels of single nucleotide polymorphism in the most distal euchromatic sequence are approximately one order of magnitude less than that typically observed in genomic regions of normal crossing over, consistent with what might be expected under models of linked selection in regions of low crossing over. However, despite this reduced level of nucleotide variation, we found abundant deletion polymorphism. These deletions create at least three gene presence/absence polymorphisms within D. melanogaster: the putative G-protein coupled receptor mthl-8 (which is the most distal known or predicted gene on 3L) and the unannotated mRNAs AY060886 and BT006009. Strikingly, D. simulans is also segregating deletions that cause mthl8 presence/absence polymorphism. Breakpoint sequencing and tests of correlations with segregating SNPs in D. melanogaster suggest that each deletion is unique. Cloned breakpoint sequences revealed the presence of Het-A elements just distal to unique, canonical euchromatic sequences. This pattern suggests a model in which repeated telomeric deficiencies cause deletions of euchromatic sequence followed by subsequent "healing" by retrotranposition of Het-A elements. These data reveal the dominance of telomeric dynamics on the evolution of closely linked sequences in Drosophila.     

The Drosophila melanogaster genome sequencing and annotation projects: a status report.

The sequence and genome annotations of Drosophila melanogaster were initially published in late 1999 and early 2000. Since then, the Berkeley Drosophila Genome Project (BDGP) and FlyBase have improved the quality of the sequence and reviewed the annotations by hand, respectively, to produce an account of the fruit fly genome that is of the highest quality. This review discusses the main features of this process, both from the point of view of the biology revealed in the end result and in the development of software that has been central to this genome sequencing and annotation project.     

A Physical Map of the X Chromosome of Drosophila Melanogaster: Cosmid Contigs and Sequence Tagged Sites

A physical map of the euchromatic X chromosome of Drosophila melanogaster has been constructed by assembling contiguous arrays of cosmids that were selected by screening a library with DNA isolated from microamplified chromosomal divisions. This map, consisting of 893 cosmids, covers ~64% of the euchromatic part of the chromosome. In addition, 568 sequence tagged sites (STS), in aggregate representing 120 kb of sequenced DNA, were derived from selected cosmids. Most of these STSs, spaced at an average distance of ~35 kb along the euchromatic region of the chromosome, represent DNA tags that can be used as entry points to the fruitfly genome. Furthermore, 42 genes have been placed on the physical map, either through the hybridization of specific probes to the cosmids or through the fact that they were represented among the STSs. These provide a link between the physical and the genetic maps of D. melanogaster. Nine novel genes have been tentatively identified in Drosophila on the basis of matches between STS sequences and sequences from other species.     

FlyBase: a Drosophila database. Flybase Consortium.

FlyBase (http://flybase.bio.indiana.edu/) is a comprehensive database of genetic and molecular data concerning Drosophila . FlyBase is maintained as a relational database (in Sybase) and is made available as html documents and flat files. The scope of FlyBase includes: genes, alleles (with phenotypes), aberrations, transposons, pointers to sequence data, gene products, maps, clones, stock lists, Drosophila workers and bibliographic references.     

The 1.688 repetitive DNA of Drosophila: concerted evolution at different genomic scales and association with genes

Concerted evolution leading to homogenization of tandemly repeated DNA arrays is widespread and important for genome evolution. We investigated the range and nature of the process at chromosomal and array levels using the 1.688 tandem repeats of Drosophila melanogaster where large arrays are present in the heterochromatin of chromosomes 2, 3, and X, and short arrays are found in the euchromatin of the same chromosomes. Analysis of 326 euchromatic and heterochromatic repeats from 52 arrays showed that the homogenization of 1.688 repeats occurred differentially for distinct genomic regions, from euchromatin to heterochromatin and from local arrays to chromosomes. We further found that most euchromatic arrays are either close to, or are within introns of, genes. The short size of euchromatic arrays (one to five repeats) could be selectively constrained by their role as gene regulators, a situation similar to the so-called "tuning knobs."     

A Chado case study: an ontology-based modular schema for representing genome-associated biological information

MOTIVATION: A few years ago, FlyBase undertook to design a new database schema to store Drosophila data. It would fully integrate genomic sequence and annotation data with bibliographic, genetic, phenotypic and molecular data from the literature representing a distillation of the first 100 years of research on this major animal model system. In developing this new integrated schema, FlyBase also made a commitment to ensure that its design was generic, extensible and available as open source, so that it could be employed as the core schema of any model organism data repository, thereby avoiding redundant software development and potentially increasing interoperability. Our question was whether we could create a relational database schema that would be successfully reused. RESULTS: Chado is a relational database schema now being used to manage biological knowledge for a wide variety of organisms, from human to pathogens, especially the classes of information that directly or indirectly can be associated with genome sequences or the primary RNA and protein products encoded by a genome. Biological databases that conform to this schema can interoperate with one another, and with application software from the Generic Model Organism Database (GMOD) toolkit. Chado is distinctive because its design is driven by ontologies. The use of ontologies (or controlled vocabularies) is ubiquitous across the schema, as they are used as a means of typing entities. The Chado schema is partitioned into integrated subschemas (modules), each encapsulating a different biological domain, and each described using representations in appropriate ontologies. To illustrate this methodology, we describe here the Chado modules used for describing genomic sequences. AVAILABILITY: GMOD is a collaboration of several model organism database groups, including FlyBase, to develop a set of open-source software for managing model organism data. The Chado schema is freely distributed under the terms of the Artistic License (http://www.opensource.org/licenses/artistic-license.php) from GMOD (www.gmod.org).     

Mapping the Human Reference Genome’s Missing Sequence by Three-Way Admixture in Latino Genomes

A principal obstacle to completing maps and analyses of the human genome involves the genome’s “inaccessible” regions: sequences (often euchromatic and containing genes) that are isolated from the rest of the euchromatic genome by heterochromatin and other repeat-rich sequence. We describe a way to localize these sequences by using ancestry linkage disequilibrium in populations that derive ancestry from at least three continents, as is the case for Latinos. We used this approach to map the genomic locations of almost 20 megabases of sequence unlocalized or missing from the current human genome reference (NCBI Genome GRCh37)—a substantial fraction of the human genome’s remaining unmapped sequence. We show that the genomic locations of most sequences that originated from fosmids and larger clones can be admixture mapped in this way, by using publicly available whole-genome sequence data. Genome assembly efforts and future builds of the human genome reference will be strongly informed by this localization of genes and other euchromatic sequences that are embedded within highly repetitive pericentromeric regions.     

FlyBase: a Drosophila database. The FlyBase consortium.

FlyBase is a database of genetic and molecular data concerning Drosophila. FlyBase is maintained as a relational database (in Sybase) and is made available as html documents and flat files. The scope of FlyBase includes: genes, alleles (and phenotypes), aberrations, transposons, pointers to sequence data, clones, stock lists, Drosophila workers and bibliographic references. The Encyclopedia of Drosophila is a joint effort between FlyBase and the Berkeley Drosophila Genome Project which integrates FlyBase data with those from the BDGP.     

FlyBase--the Drosophila database. The FlyBase Consortium.

FlyBase is a database of genetic and molecular data concerning Drosophila. FlyBase is maintained as a relational database (in Sybase) and is available from the ftp.bio.indiana.edu Gopher server. The scope of FlyBase includes: genes, alleles, aberrations, pointers to sequence data, stock lists, Drosophila workers and bibliographic references.     

Circumventing heterozygosity: sequencing the amplified genome of a single haploid Drosophila melanogaster embryo

Heterozygosity is a major challenge to efficient, high-quality genomic assembly and to the full genomic survey of polymorphism and divergence. In Drosophila melanogaster lines derived from equatorial populations are particularly resistant to inbreeding, thus imposing a major barrier to the determination and analyses of genomic variation in natural populations of this model organism. Here we present a simple genome sequencing protocol based on the whole-genome amplification of the gynogenetically derived haploid genome of a progeny of females mated to males homozygous for the recessive male sterile mutation, ms(3)K81. A single "lane" of paired-end sequences (2 × 76 bp) provides a good syntenic assembly with >95% high-quality coverage (more than five reads). The amplification of the genomic DNA moderately inflates the variation in coverage across the euchromatic portion of the genome. It also increases the frequency of chimeric clones. But the low frequency and random genomic distribution of the chimeric clones limits their impact on the final assemblies. This method provides a solid path forward for population genomic sequencing and offers applications to many other systems in which small amounts of genomic DNA have unique experimental relevance.     

On the abundance and distribution of transposable elements in the genome of Drosophila melanogaster

The abundance and distribution of transposable elements (TEs) in a representative part of the euchromatic genome of Drosophila melanogaster were studied by analyzing the sizes and locations of TEs of all known families in the genomic sequences of chromosomes 2R, X, and 4. TEs contribute to up to 2% of the sequenced DNA, which corresponds roughly to the euchromatin of these chromosomes. This estimate is lower than that previously available from in situ data and suggests that TEs accumulate in the heterochromatin more intensively than was previously thought. We have also found that TEs are not distributed at random in the chromosomes and that their abundance is more strongly associated with local recombination rates, rather than with gene density. The results are compatible with the ectopic exchange model, which proposes that selection against deleterious effects of chromosomal rearrangements is a major force opposing element spread in the genome of this species. Selection against insertional mutations also influences the observed patterns, such as an absence of insertions in coding regions. The results of the analyses are discussed in the light of recent findings on the distribution of TEs in other species.     

Apollo: a sequence annotation editor

The well-established inaccuracy of purely computational methods for annotating genome sequences necessitates an interactive tool to allow biological experts to refine these approximations by viewing and independently evaluating the data supporting each annotation. Apollo was developed to meet this need, enabling curators to inspect genome annotations closely and edit them. FlyBase biologists successfully used Apollo to annotate the Drosophila melanogaster genome and it is increasingly being used as a starting point for the development of customized annotation editing tools for other genome projects.     

FlyBase: the Drosophila database. The Flybase Consortium.

FlyBase is a database of genetic and molecular data concerning Drosophila. FlyBase is maintained as a relational database (in Sybase). The scope of FlyBase includes: genes, alleles (and phenotypes), aberrations, pointers to sequence data, clones, stock lists, Drosophila workers and bibliographic references. FlyBase is also available on CD-ROM for Macintosh systems (Encyclopaedia of Drosophila).     

HB mobile element in the Drosophila melanogaster genome: structural and functional analyses

The structure was analyzed for 60 annotated copies of the mobile genetic element (MGE) HB from the Drosophila melanogaster genome. The genomic distribution of HB copies was studied, and preferential insertion sites (hot spots) were identified, which presumably amount to several kilobases. Structural analysis of the open reading frame (ORF) and terminal repeats of HB was performed. All 26 HB copies retaining the ORF sequence have a stop codon in the same position. Consequently, the HB ORF proved indeed to code for an enzyme of 148 amino acid residues, relatively small for Tc1-family transposases. The ORF consensus sequence was established. HB{}1185 was identified as the only HB copy potentially coding for a functional protein. All 37 repeat-containing HB copies were analyzed. Of these, only four had functional terminal sequences, lacking, however, a functional transposase gene. A new 7762-bp copy of MGE roo was found in the D. melanogaster genome; the copy was earlier unavailable from databases and represents an insert in the HB{}1605 sequence.     

New Assembly,Reannotation and Analysis of the Entamoeba histolytica Genome Reveal New Genomic Features and Protein Content Information

Background

In order to maintain genome information accurately and relevantly, original genome annotations need to be updated and evaluated regularly. Manual reannotation of genomes is important as it can significantly reduce the propagation of errors and consequently diminishes the time spent on mistaken research. For this reason, after five years from the initial submission of the Entamoeba histolytica draft genome publication, we have re-examined the original 23 Mb assembly and the annotation of the predicted genes.

Principal Findings

The evaluation of the genomic sequence led to the identification of more than one hundred artifactual tandem duplications that were eliminated by re-assembling the genome. The reannotation was done using a combination of manual and automated genome analysis. The new 20 Mb assembly contains 1,496 scaffolds and 8,201 predicted genes, of which 60% are identical to the initial annotation and the remaining 40% underwent structural changes. Functional classification of 60% of the genes was modified based on recent sequence comparisons and new experimental data. We have assigned putative function to 3,788 proteins (46% of the predicted proteome) based on the annotation of predicted gene families, and have identified 58 protein families of five or more members that share no homology with known proteins and thus could be entamoeba specific. Genome analysis also revealed new features such as the presence of segmental duplications of up to 16 kb flanked by inverted repeats, and the tight association of some gene families with transposable elements.

Significance

This new genome annotation and analysis represents a more refined and accurate blueprint of the pathogen genome, and provides an upgraded tool as reference for the study of many important aspects of E. histolytica biology, such as genome evolution and pathogenesis.