Viktoria Nizhynska

1,135 Genomes Reveal the Global Pattern of Polymorphism in Arabidopsis thaliana

Summary Arabidopsis thaliana serves as a model organism for the study of fundamental physiological, cellular, and molecular processes. It has also greatly advanced our understanding of intraspecific genome variation. We present a detailed map of variation in 1,135 high-quality re-sequenced natural inbred lines representing the native Eurasian and North African range and recently colonized North America. We identify relict populations that continue to inhabit ancestral habitats, primarily in the Iberian Peninsula. They have mixed with a lineage that has spread to northern latitudes from an unknown glacial refugium and is now found in a much broader spectrum of habitats. Insights into the history of the species and the fine-scale distribution of genetic diversity provide the basis for full exploitation of A. thaliana natural variation through integration of genomes and epigenomes with molecular and non-molecular phenotypes.

Massive genomic variation and strong selection in Arabidopsis thaliana lines from Sweden

Despite advances in sequencing, the goal of obtaining a comprehensive view of genetic variation in populations is still far from reached. We sequenced 180 lines of A. thaliana from Sweden to obtain as complete a picture as possible of variation in a single region. Whereas simple polymorphisms in the unique portion of the genome are readily identified, other polymorphisms are not. The massive variation in genome size identified by flow cytometry seems largely to be due to 45S rDNA copy number variation, with lines from northern Sweden having particularly large numbers of copies. Strong selection is evident in the form of long-range linkage disequilibrium (LD), as well as in LD between nearby compensatory mutations. Many footprints of selective sweeps were found in lines from northern Sweden, and a massive global sweep was shown to have involved a 700-kb transposition.

Sequencing of the genus Arabidopsis identifies a complex history of nonbifurcating speciation and abundant trans-specific polymorphism

The notion of species as reproductively isolated units related through a bifurcating tree implies that gene trees should generally agree with the species tree and that sister taxa should not share polymorphisms unless they diverged recently and should be equally closely related to outgroups. It is now possible to evaluate this model systematically. We sequenced multiple individuals from 27 described taxa representing the entire Arabidopsis genus. Cluster analysis identified seven groups, corresponding to described species that capture the structure of the genus. However, at the level of gene trees, only the separation of Arabidopsis thaliana from the remaining species was universally supported, and, overall, the amount of shared polymorphism demonstrated that reproductive isolation was considerably more recent than the estimated divergence times. We uncovered multiple cases of past gene flow that contradict a bifurcating species tree. Finally, we showed that the pattern of divergence differs between gene ontologies, suggesting a role for selection.

GMI1, a structural-maintenance-of-chromosomes-hinge domain-containing protein, is involved in somatic homologous recombination in Arabidopsis

DNA double-strand breaks (DSBs) pose one of the most severe threats to genome integrity, potentially leading to cell death. After detection of a DSB, the DNA damage and repair response is initiated and the DSB is repaired by non-homologous end joining and/or homologous recombination. Many components of these processes are still unknown in Arabidopsis thaliana. In this work, we characterized γ-irradiation and mitomycin C induced 1 (GMI1), a member of the SMC-hinge domain-containing protein family. RT-PCR analysis and promoter-GUS fusion studies showed that γ-irradiation, the radio-mimetic drug bleocin, and the DNA cross-linking agent mitomycin C strongly enhance GMI1 expression particularly in meristematic tissues. The induction of GMI1 by γ-irradiation depends on the signalling kinase Ataxia telangiectasia-mutated (ATM) but not on ATM and Rad3-related (ATR). Epistasis analysis of single and double mutants demonstrated that ATM acts upstream of GMI1 while the atr gmi1-2 double mutant was more sensitive than the respective single mutants. Comet assay revealed a reduced rate of DNA double-strand break repair in gmi1 mutants during the early recovery phase after exposure to bleocin. Moreover, the rate of homologous recombination of a reporter construct was strongly reduced in gmi1 mutant plants upon exposure to bleocin or mitomycin C. GMI1 is the first member of its protein family known to be involved in DNA repair.

TE-Locate: A Tool to Locate and Group Transposable Element Occurrences Using Paired-End Next-Generation Sequencing Data.

Transposable elements (TEs) are common mobile DNA elements present in nearly all genomes. Since the movement of TEs within a genome can sometimes have phenotypic consequences, an accurate report of TE actions is desirable. To this end, we developed TE-Locate, a computational tool that uses paired-end reads to identify the novel locations of known TEs. TE-Locate can utilize either a database of TE sequences, or annotated TEs within the reference sequence of interest. This makes TE-Locate useful in the search for any mobile sequence, including retrotransposed gene copies. One major concern is to act on the correct hierarchy level, thereby avoiding an incorrect calling of a single insertion as multiple events of TEs with high sequence similarity. We used the (super)family level, but TE-Locate can also use any other level, right down to the individual transposable element. As an example of analysis with TE-Locate, we used the Swedish population in the 1,001 Arabidopsis genomes project, and presented the biological insights gained from the novel TEs, inducing the association between different TE superfamilies. The program is freely available, and the URL is provided in the end of the paper.

PoolHap: Inferring Haplotype Frequencies from Pooled Samples by Next Generation Sequencing

With the advance of next-generation sequencing (NGS) technologies, increasingly ambitious applications are becoming feasible. A particularly powerful one is the sequencing of polymorphic, pooled samples. The pool can be naturally occurring, as in the case of multiple pathogen strains in a blood sample, multiple types of cells in a cancerous tissue sample, or multiple isoforms of mRNA in a cell. In these cases, its difficult or impossible to partition the subtypes experimentally before sequencing, and those subtype frequencies must hence be inferred. In addition, investigators may occasionally want to artificially pool the sample of a large number of individuals for reasons of cost-efficiency, e.g., when carrying out genetic mapping using bulked segregant analysis. Here we describe PoolHap, a computational tool for inferring haplotype frequencies from pooled samples when haplotypes are known. The key insight into why PoolHap works is that the large number of SNPs that come with genome-wide coverage can compensate for the uneven coverage across the genome. The performance of PoolHap is illustrated and discussed using simulated and real data. We show that PoolHap is able to accurately estimate the proportions of haplotypes with less than 2% error for 34-strain mixtures with 2X total coverage Arabidopsis thaliana whole genome polymorphism data. This method should facilitate greater biological insight into heterogeneous samples that are difficult or impossible to isolate experimentally. Software and users manual are freely available at http://arabidopsis.gmi.oeaw.ac.at/quan/poolhap/.

Germline replications and somatic mutation accumulation are independent of vegetative life span in Arabidopsis.

Significance In contrast to animals, plants lack a segregated germline. Instead, all plant tissue is derived from small clusters of meristematic cells that throughout development give rise to vegetative tissues and only late in development to reproductive organs. Hence, mutations acquired during vegetative growth can be passed on to offspring; it is generally believed that older plants will acquire more such mutations over their lifetimes due to replication during cell division. Here we measure the number of DNA replications occurring in the cell lineage leading to gametes and demonstrate that the number of replications is independent of life span, suggesting that older plants may not be passing on more mutations to their offspring relative to younger plants. In plants, gametogenesis occurs late in development, and somatic mutations can therefore be transmitted to the next generation. Longer periods of growth are believed to result in an increase in the number of cell divisions before gametogenesis, with a concomitant increase in mutations arising due to replication errors. However, there is little experimental evidence addressing how many cell divisions occur before gametogenesis. Here, we measured loss of telomeric DNA and accumulation of replication errors in Arabidopsis with short and long life spans to determine the number of replications in lineages leading to gametes. Surprisingly, the number of cell divisions within the gamete lineage is nearly independent of both life span and vegetative growth. One consequence of the relatively stable number of replications per generation is that older plants may not pass along more somatically acquired mutations to their offspring. We confirmed this hypothesis by genomic sequencing of progeny from young and old plants. This independence can be achieved by hierarchical arrangement of cell divisions in plant meristems where vegetative growth is primarily accomplished by expansion of cells in rapidly dividing meristematic zones, which are only rarely refreshed by occasional divisions of more quiescent cells. We support this model by 5-ethynyl-2′-deoxyuridine retention experiments in shoot and root apical meristems. These results suggest that stem-cell organization has independently evolved in plants and animals to minimize mutations by limiting DNA replication.

Multiple alleles at a single locus control seed dormancy in Swedish Arabidopsis

Seed dormancy is a complex life history trait that determines the timing of germination and is crucial for local adaptation. Genetic studies of dormancy are challenging, because the trait is highly plastic and strongly influenced by the maternal environment. Using a combination of statistical and experimental approaches, we show that multiple alleles at the previously identified dormancy locus DELAY OF GERMINATION1 jointly explain as much as 57% of the variation observed in Swedish Arabidopsis thaliana, but give rise to spurious associations that seriously mislead genome-wide association studies unless modeled correctly. Field experiments confirm that the major alleles affect germination as well as survival under natural conditions, and demonstrate that locally adaptive traits can sometimes be dissected genetically. DOI: http://dx.doi.org/10.7554/eLife.22502.001

Unstable Inheritance of 45S rRNA Genes in Arabidopsis thaliana

The considerable genome size variation in Arabidopsis thaliana has been shown largely to be due to copy number variation (CNV) in 45S ribosomal RNA (rRNA) genes. Surprisingly, attempts to map this variation by means of genome-wide association studies (GWAS) failed to identify either of the two likely sources, namely the nucleolus organizer regions (NORs). Instead, GWAS implicated a trans-acting locus, as if rRNA gene CNV was a phenotype rather than a genotype. To explain these results, we investigated the inheritance and stability of rRNA gene copy number using the variety of genetic resources available in A. thaliana — F2 crosses, recombinant inbred lines, the multiparent advanced-generation inter-cross population, and mutation accumulation lines. Our results clearly show that rRNA gene CNV can be mapped to the NORs themselves, with both loci contributing equally to the variation. However, NOR size is unstably inherited, and dramatic copy number changes are visible already within tens of generations, which explains why it is not possible to map the NORs using GWAS. We did not find any evidence of trans-acting loci in crosses, which is also expected since changes due to such loci would take very many generations to manifest themselves. rRNA gene copy number is thus an interesting example of “missing heritability”—a trait that is heritable in pedigrees, but not in the general population.

Genome sequencing reveals the origin of the allotetraploid Arabidopsis suecica

Abstract Polyploidy is an example of instantaneous speciation when it involves the formation of a new cytotype that is incompatible with the parental species. Because new polyploid individuals are likely to be rare, establishment of a new species is unlikely unless polyploids are able to reproduce through self-fertilization (selfing), or asexually. Conversely, selfing (or asexuality) makes it possible for polyploid species to originate from a single individual—a bona fide speciation event. The extent to which this happens is not known. Here, we consider the origin of Arabidopsis suecica, a selfing allopolyploid between Arabidopsis thaliana and Arabidopsis arenosa, which has hitherto been considered to be an example of a unique origin. Based on whole-genome re-sequencing of 15 natural A. suecica accessions, we identify ubiquitous shared polymorphism with the parental species, and hence conclusively reject a unique origin in favor of multiple founding individuals. We further estimate that the species originated after the last glacial maximum in Eastern Europe or central Eurasia (rather than Sweden, as the name might suggest). Finally, annotation of the self-incompatibility loci in A. suecica revealed that both loci carry non-functional alleles. The locus inherited from the selfing A. thaliana is fixed for an ancestral non-functional allele, whereas the locus inherited from the outcrossing A. arenosa is fixed for a novel loss-of-function allele. Furthermore, the allele inherited from A. thaliana is predicted to transcriptionally silence the allele inherited from A. arenosa, suggesting that loss of self-incompatibility may have been instantaneous.