Boudewijn ten Hallers

The genome of the green anole lizard and a comparative analysis with birds and mammals

The evolution of the amniotic egg was one of the great evolutionary innovations in the history of life, freeing vertebrates from an obligatory connection to water and thus permitting the conquest of terrestrial environments. Among amniotes, genome sequences are available for mammals and birds, but not for non-avian reptiles. Here we report the genome sequence of the North American green anole lizard, Anolis carolinensis. We find that A. carolinensis microchromosomes are highly syntenic with chicken microchromosomes, yet do not exhibit the high GC and low repeat content that are characteristic of avian microchromosomes. Also, A. carolinensis mobile elements are very young and diverse—more so than in any other sequenced amniote genome. The GC content of this lizard genome is also unusual in its homogeneity, unlike the regionally variable GC content found in mammals and birds. We describe and assign sequence to the previously unknown A. carolinensis X chromosome. Comparative gene analysis shows that amniote egg proteins have evolved significantly more rapidly than other proteins. An anole phylogeny resolves basal branches to illuminate the history of their repeated adaptive radiations.

Gibbon genome and the fast karyotype evolution of small apes.

Gibbons are small arboreal apes that display an accelerated rate of evolutionary chromosomal rearrangement and occupy a key node in the primate phylogeny between Old World monkeys and great apes. Here we present the assembly and analysis of a northern white-cheeked gibbon (Nomascus leucogenys) genome. We describe the propensity for a gibbon-specific retrotransposon (LAVA) to insert into chromosome segregation genes and alter transcription by providing a premature termination site, suggesting a possible molecular mechanism for the genome plasticity of the gibbon lineage. We further show that the gibbon genera (Nomascus, Hylobates, Hoolock and Symphalangus) experienced a near-instantaneous radiation ∼5 million years ago, coincident with major geographical changes in southeast Asia that caused cycles of habitat compression and expansion. Finally, we identify signatures of positive selection in genes important for forelimb development (TBX5) and connective tissues (COL1A1) that may have been involved in the adaptation of gibbons to their arboreal habitat.

Single-molecule sequencing of the desiccation-tolerant grass Oropetium thomaeum

Plant genomes, and eukaryotic genomes in general, are typically repetitive, polyploid and heterozygous, which complicates genome assembly. The short read lengths of early Sanger and current next-generation sequencing platforms hinder assembly through complex repeat regions, and many draft and reference genomes are fragmented, lacking skewed GC and repetitive intergenic sequences, which are gaining importance due to projects like the Encyclopedia of DNA Elements (ENCODE). Here we report the whole-genome sequencing and assembly of the desiccation-tolerant grass Oropetium thomaeum. Using only single-molecule real-time sequencing, which generates long (>16 kilobases) reads with random errors, we assembled 99% (244 megabases) of the Oropetium genome into 625 contigs with an N50 length of 2.4 megabases. Oropetium is an example of a ‘near-complete’ draft genome which includes gapless coverage over gene space as well as intergenic sequences such as centromeres, telomeres, transposable elements and rRNA clusters that are typically unassembled in draft genomes. Oropetium has 28,466 protein-coding genes and 43% repeat sequences, yet with 30% more compact euchromatic regions it is the smallest known grass genome. The Oropetium genome demonstrates the utility of single-molecule real-time sequencing for assembling high-quality plant and other eukaryotic genomes, and serves as a valuable resource for the plant comparative genomics community.

A high-resolution map of synteny disruptions in gibbon and human genomes.

Gibbons are part of the same superfamily (Hominoidea) as humans and great apes, but their karyotype has diverged faster from the common hominoid ancestor. At least 24 major chromosome rearrangements are required to convert the presumed ancestral karyotype of gibbons into that of the hominoid ancestor. Up to 28 additional rearrangements distinguish the various living species from the common gibbon ancestor. Using the northern white-cheeked gibbon (2n = 52) (Nomascus leucogenys leucogenys) as a model, we created a high-resolution map of the homologous regions between the gibbon and human. The positions of 100 synteny breakpoints relative to the assembled human genome were determined at a resolution of about 200 kb. Interestingly, 46% of the gibbon–human synteny breakpoints occur in regions that correspond to segmental duplications in the human lineage, indicating a common source of plasticity leading to a different outcome in the two species. Additionally, the full sequences of 11 gibbon BACs spanning evolutionary breakpoints reveal either segmental duplications or interspersed repeats at the exact breakpoint locations. No specific sequence element appears to be common among independent rearrangements. We speculate that the extraordinarily high level of rearrangements seen in gibbons may be due to factors that increase the incidence of chromosome breakage or fixation of the derivative chromosomes in a homozygous state.

A Small, Variable, and Irregular Killer Cell Ig-Like Receptor Locus Accompanies the Absence of MHC-C and MHC-G in Gibbons

The killer cell Ig-like receptors (KIRs) of NK cells recognize MHC class I ligands and function in placental reproduction and immune defense against pathogens. During the evolution of monkeys, great apes, and humans, an ancestral KIR3DL gene expanded to become a diverse and rapidly evolving gene family of four KIR lineages. Characterizing the KIR locus are three framework regions, defining two intervals of variable gene content. By analysis of four KIR haplotypes from two species of gibbon, we find that the smaller apes do not conform to these rules. Although diverse and irregular in structure, the gibbon haplotypes are unusually small, containing only two to five functional genes. Comparison with the predicted ancestral hominoid KIR haplotype indicates that modern gibbon KIR haplotypes were formed by a series of deletion events, which created new hybrid genes as well as eliminating ancestral genes. Of the three framework regions, only KIR3DL3 (lineage V), defining the 5′ end of the KIR locus, is present and intact on all gibbon KIR haplotypes. KIR2DL4 (lineage I) defining the central framework region has been a major target for elimination or inactivation, correlating with the absence of its putative ligand, MHC-G, in gibbons. Similarly, the MHC-C–driven expansion of lineage III KIR genes in great apes has not occurred in gibbons because they lack MHC-C. Our results indicate that the selective forces shaping the size and organization of the gibbon KIR locus differed from those acting upon the KIR of other hominoid species.

Evolution of a bitter taste receptor gene cluster in a New World sparrow.

Bitter taste perception likely evolved as a protective mechanism against the ingestion of harmful compounds in food. The evolution of the taste receptor type 2 (TAS2R) gene family, which encodes the chemoreceptors that are directly responsible for the detection of bitter compounds, has therefore been of considerable interest. Though TAS2R repertoires have been characterized for a number of species, to date the complement of TAS2Rs from just one bird, the chicken, which had a notably small number of TAS2Rs, has been established. Here, we used targeted mapping and genomic sequencing in the white-throated sparrow (Zonotrichia albicollis) and sample sequencing in other closely related birds to reconstruct the history of a TAS2R gene cluster physically linked to the break points of an evolutionary chromosomal rearrangement. In the white-throated sparrow, this TAS2R cluster encodes up to 18 functional bitter taste receptors and likely underwent a large expansion that predates and/or coincides with the radiation of the Emberizinae subfamily into the New World. In addition to signatures of gene birth-and-death evolution within this cluster, estimates of Ka/Ks for the songbird TAS2Rs were similar to those previously observed in mammals, including humans. Finally, comparison of the complete genomic sequence of the cluster from two common haplotypes in the white-throated sparrow revealed a number of nonsynonymous variants and differences in functional gene content within this species. These results suggest that interspecies and intraspecies genetic variability does exist in avian TAS2Rs and that these differences could contribute to variation in bitter taste perception in birds.

Construction of an American mink Bacterial Artificial Chromosome (BAC) library and sequencing candidate genes important for the fur industry

BackgroundBacterial artificial chromosome (BAC) libraries continue to be invaluable tools for the genomic analysis of complex organisms. Complemented by the newly and fast growing deep sequencing technologies, they provide an excellent source of information in genomics projects.ResultsHere, we report the construction and characterization of the CHORI-231 BAC library constructed from a Danish-farmed, male American mink (Neovison vison). The library contains approximately 165,888 clones with an average insert size of 170 kb, representing approximately 10-fold coverage. High-density filters, each consisting of 18,432 clones spotted in duplicate, have been produced for hybridization screening and are publicly available. Overgo probes derived from expressed sequence tags (ESTs), representing 21 candidate genes for traits important for the mink industry, were used to screen the BAC library. These included candidate genes for coat coloring, hair growth and length, coarseness, and some receptors potentially involved in viral diseases in mink. The extensive screening yielded positive results for 19 of these genes. Thirty-five clones corresponding to 19 genes were sequenced using 454 Roche, and large contigs (184 kb in average) were assembled. Knowing the complete sequences of these candidate genes will enable confirmation of the association with a phenotype and the finding of causative mutations for the targeted phenotypes.Additionally, 1577 BAC clones were end sequenced; 2505 BAC end sequences (80% of BACs) were obtained. An excess of 2 Mb has been analyzed, thus giving a snapshot of the mink genome.ConclusionsThe availability of the CHORI-321 American mink BAC library will aid in identification of genes and genomic regions of interest. We have demonstrated how the library can be used to identify specific genes of interest, develop genetic markers, and for BAC end sequencing and deep sequencing of selected clones. To our knowledge, this is the first report of 454 sequencing of selected BAC clones in mammals and re-assures the suitability of this technique for obtaining the sequence information of genes of interest in small genomics projects. The BAC end sequences described in this paper have been deposited in the GenBank data library [HN339419-HN341884, HN604664-HN604702]. The 454 produced contigs derived from selected clones are deposited with reference numbers [GenBank: JF288166-JF288183 & JF310744].

Long-Range Massively Parallel Mate Pair Sequencing Detects Distinct Mutations and Similar Patterns of Structural Mutability in Two Breast Cancer Cell Lines

Cancer genomes frequently undergo genomic instability resulting in accumulation of chromosomal rearrangement. To date, one of the main challenges has been to confidently and accurately identify these rearrangements by using short-read massively parallel sequencing. We were able to improve cancer rearrangement detection by combining two distinct massively parallel sequencing strategies: fosmid-sized (36 kb on average) and standard 5 kb mate pair libraries. We applied this combined strategy to map rearrangements in two breast cancer cell lines, MCF7 and HCC1954. We detected and validated a total of 91 somatic rearrangements in MCF7 and 25 in HCC1954, including genomic alterations corresponding to previously reported transcript aberrations in these two cell lines. Each of the genomes contains two types of breakpoints: clustered and dispersed. In both cell lines, the dispersed breakpoints show enrichment for low copy repeats, while the clustered breakpoints associate with high copy number amplifications. Comparing the two genomes, we observed highly similar structural mutational spectra affecting different sets of genes, pointing to similar histories of genomic instability against the background of very different gene network perturbations.

Development of genomic resources for the prairie vole ( Microtus ochrogaster ): construction of a BAC library and vole-mouse comparative cytogenetic map

BackgroundThe prairie vole (Microtus ochrogaster) is a premier animal model for understanding the genetic and neurological basis of social behaviors. Unlike other biomedical models, prairie voles display a rich repertoire of social behaviors including the formation of long-term pair bonds and biparental care. However, due to a lack of genomic resources for this species, studies have been limited to a handful of candidate genes. To provide a substrate for future development of genomic resources for this unique model organism, we report the construction and characterization of a bacterial artificial chromosome (BAC) library from a single male prairie vole and a prairie vole-mouse (Mus musculus) comparative cytogenetic map.ResultsWe constructed a prairie vole BAC library (CHORI-232) consisting of 194,267 recombinant clones with an average insert size of 139 kb. Hybridization-based screening of the gridded library at 19 loci established that the library has an average depth of coverage of ~10×. To obtain a small-scale sampling of the prairie vole genome, we generated 3884 BAC end-sequences totaling ~2.8 Mb. One-third of these BAC-end sequences could be mapped to unique locations in the mouse genome, thereby anchoring 1003 prairie vole BAC clones to an orthologous position in the mouse genome. Fluorescence in situ hybridization (FISH) mapping of 62 prairie vole clones with BAC-end sequences mapping to orthologous positions in the mouse genome was used to develop a first-generation genome-wide prairie vole-mouse comparative cytogenetic map. While conserved synteny was observed between this pair of rodent genomes, rearrangements between the prairie vole and mouse genomes were detected, including a minimum of five inversions and 16 inter-chromosomal rearrangements.ConclusionsThe construction of the prairie vole BAC library and the vole-mouse comparative cytogenetic map represent the first genome-wide modern genomic resources developed for this species. The BAC library will support future genomic, genetic and molecular characterization of this genome and species, and the isolation of clones of high interest to the vole research community will allow for immediate characterization of the regulatory and coding sequences of genes known to play important roles in social behaviors. In addition, these resources provide an excellent platform for future higher resolution cytogenetic mapping and full genome sequencing.

Comprehensive Definition of Genome Features in Spirodela polyrhiza by High-Depth Physical Mapping and Short-Read DNA Sequencing Strategies

&NA; Spirodela polyrhiza is a fast‐growing aquatic monocot with highly reduced morphology, genome size and number of protein‐coding genes. Considering these biological features of Spirodela and its basal position in the monocot lineage, understanding its genome architecture could shed light on plant adaptation and genome evolution. Like many draft genomes, however, the 158‐Mb Spirodela genome sequence has not been resolved to chromosomes, and important genome characteristics have not been defined. Here we deployed rapid genome‐wide physical maps combined with high‐coverage short‐read sequencing to resolve the 20 chromosomes of Spirodela and to empirically delineate its genome features. Our data revealed a dramatic reduction in the number of the rDNA repeat units in Spirodela to fewer than 100, which is even fewer than that reported for yeast. Consistent with its unique phylogenetic position, small RNA sequencing revealed 29 Spirodela‐specific microRNA, with only two being shared with Elaeis guineensis (oil palm) and Musa balbisiana (banana). Combining DNA methylation data and small RNA sequencing enabled the accurate prediction of 20.5% long terminal repeats (LTRs) that doubled the previous estimate, and revealed a high Solo:Intact LTR ratio of 8.2. Interestingly, we found that Spirodela has the lowest global DNA methylation levels (9%) of any plant species tested. Taken together our results reveal a genome that has undergone reduction, likely through eliminating non‐essential protein coding genes, rDNA and LTRs. In addition to delineating the genome features of this unique plant, the methodologies described and large‐scale genome resources from this work will enable future evolutionary and functional studies of this basal monocot family. Significance statement Spirodela polyrhiza is a fast‐growing aquatic and basal monocot with a small genome, so understanding its genome architecture should inform plant adaptation strategies and genome evolution. However, long repetitive regions are formidable challenges for the proper assembly, annotation and closure of genome projects. Here we overcame this bottleneck by combining high‐throughput sequencing and genome mapping technologies to generate a validated sequence map of the 20 Spirodela polyrhiza chromosomes. Genome‐wide profiling data revealed a genome in a cycle of reduction with very low global DNA methylation and a reduced ribosomal DNA as well as gene content.