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The doubling of genomes does not cause increased plant speciation unless the progenitor lineages are highly fit. Polyploidy, the doubling of genomic content, is a widespread feature, especially among plants, yet its macroevolutionary impacts are contentious. Traditionally, polyploidy has been considered an evolutionary dead end, whereas recent genomic studies suggest that polyploidy has been a key driver of macroevolutionary success. We examined the consequences of polyploidy on the time scale of genera across a diverse set of vascular plants, encompassing hundreds of inferred polyploidization events. Likelihood-based analyses indicate that polyploids generally exhibit lower speciation rates and higher extinction rates than diploids, providing the first quantitative corroboration of the dead-end hypothesis. The increased speciation rates of diploids can, in part, be ascribed to their capacity to speciate via polyploidy. Only particularly fit lineages of polyploids may persist to enjoy longer-term evolutionary success.

Bin Han and colleagues performed low-coverage sequencing of 517 rice landraces and constructed a high-density haplotype map of the rice genome. They have used this resource to carry out genome-wide association studies for 14 agronomic traits and identify 80 loci with strong association signals. Uncovering the genetic basis of agronomic traits in crop landraces that have adapted to various agro-climatic conditions is important to world food security. Here we have identified ∼3.6 million SNPs by sequencing 517 rice landraces and constructed a high-density haplotype map of the rice genome using a novel data-imputation method. We performed genome-wide association studies (GWAS) for 14 agronomic traits in the population of Oryza sativa indica subspecies. The loci identified through GWAS explained ∼36% of the phenotypic variance, on average. The peak signals at six loci were tied closely to previously identified genes. This study provides a fundamental resource for rice genetics research and breeding, and demonstrates that an approach integrating second-generation genome sequencing and GWAS can be used as a powerful complementary strategy to classical biparental cross-mapping for dissecting complex traits in rice.

Riccardo Velasco and colleagues report the genome sequence of the 'Golden Delicious' domesticated apple. These data shed new insight into the genomic events that preceded the origin of this crop. We report a high-quality draft genome sequence of the domesticated apple ( Malus × domestica ). We show that a relatively recent (>50 million years ago) genome-wide duplication (GWD) has resulted in the transition from nine ancestral chromosomes to 17 chromosomes in the Pyreae. Traces of older GWDs partly support the monophyly of the ancestral paleohexaploidy of eudicots. Phylogenetic reconstruction of Pyreae and the genus Malus , relative to major Rosaceae taxa, identified the progenitor of the cultivated apple as M. sieversii . Expansion of gene families reported to be involved in fruit development may explain formation of the pome, a Pyreae-specific false fruit that develops by proliferation of the basal part of the sepals, the receptacle. In apple, a subclade of MADS-box genes, normally involved in flower and fruit development, is expanded to include 15 members, as are other gene families involved in Rosaceae-specific metabolism, such as transport and assimilation of sorbitol.

Soybean (Glycine max) is one of the most important crop plants for seed protein and oil content, and for its capacity to fix atmospheric nitrogen through symbioses with soil-borne microorganisms. We sequenced the 1.1-gigabase genome by a whole-genome shotgun approach and integrated it with physical and high-density genetic maps to create a chromosome-scale draft sequence assembly. We predict 46,430 protein-coding genes, 70% more than Arabidopsis and similar to the poplar genome which, like soybean, is an ancient polyploid (palaeopolyploid). About 78% of the predicted genes occur in chromosome ends, which comprise less than one-half of the genome but account for nearly all of the genetic recombination. Genome duplications occurred at approximately 59 and 13 million years ago, resulting in a highly duplicated genome with nearly 75% of the genes present in multiple copies. The two duplication events were followed by gene diversification and loss, and numerous chromosome rearrangements. An accurate soybean genome sequence will facilitate the identification of the genetic basis of many soybean traits, and accelerate the creation of improved soybean varieties.

Key Points Circadian rhythms in mammals are regulated by a master circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN), which coordinates rhythmic processes throughout the organism. Circadian clocks are cell autonomous and these cellular clocks are located in SCN neurons as well as in almost every cell in the body. The molecular mechanism of circadian clocks in mammals involves an autoregulatory transcriptional feedback loop involving the positive elements CLOCK and BMAL1, which transcriptionally activate the negative feedback elements period (PER) and cryptochrome (CRY), which inhibit their own transcription by repressing the CLOCK–BMAL1 complex. Post-translational regulation (for example, phosphorylation, acetylation and ubiquitylation) of clock proteins have important roles in regulating the stability, localization and turnover of clock components. The sleep disorder familial advanced sleep phase syndrome (FASPS) has been found to be caused by mutations in two core clock genes, period homologue 2 (PER2) and casein kinase 1 delta (CSNK1D), in humans. There is weak but emerging evidence for allelic variants in clock genes to be associated with diurnal preference, mood disorders, sleep and metabolic disorders. Peripheral circadian oscillators are controlled by signals arising from the SCN and from proximal signals related to feeding behaviour, hormonal signals and body-temperature fluctuations. In addition to their primary role in the generation of circadian rhythms, recent work has shown that circadian clock genes can affect a wide variety of other physiological processes. Emerging examples of circadian regulation of physiological pathways include diverse aspects of cellular metabolism, cell growth and DNA-damage control, xenobiotic responses, and the modulation of behavioural responses to drugs and alcohol. The knowledge that circadian clocks are cell autonomous and distributed throughout the body provide a new perspective to target central as well as peripheral circadian oscillators for therapeutic intervention. Many biological processes are regulated by circadian rhythms, which keep them in time with the Earth's 24-hour light–dark cycle. Elucidating the genetic control of circadian rhythms will help to understand the many diseases that can result when the clock goes wrong. Circadian cycles affect a variety of physiological processes, and disruptions of normal circadian biology therefore have the potential to influence a range of disease-related pathways. The genetic basis of circadian rhythms is well studied in model organisms and, more recently, studies of the genetic basis of circadian disorders has confirmed the conservation of key players in circadian biology from invertebrates to humans. In addition, important advances have been made in understanding how these molecules influence physiological functions in tissues throughout the body. Together, these studies set the scene for applying our knowledge of circadian biology to the understanding and treatment of a range of human diseases, including cancer and metabolic and behavioural disorders.

Mapping the methylome A newly developed method of characterizing an organism's 'methylome', that is the pattern of DNA methylation in the genome, has been used to generate a map of methylated cytosines in Arabidopsis to single base-pair resolution. The procedure, termed BS-Seq, combines bisulphite treatment of genomic DNA with ultra-high-throughput DNA sequencing to achieve a more precise and comprehensive result than previously possible. DNA methylation is an important factor in regulating gene expression, and this method, which can be applied to larger genomes like the mouse as well as to Arabidopsis , could prove a significant advance in the study of this form of gene regulation. In Arabidopsis , a map of methylated cytosines is generated at single base pair resolution by combining bisulphite treatment of genomic DNA with ultra-high-throughput sequencing. Cytosine DNA methylation is important in regulating gene expression and in silencing transposons and other repetitive sequences 1 , 2 . Recent genomic studies in Arabidopsis thaliana have revealed that many endogenous genes are methylated either within their promoters or within their transcribed regions, and that gene methylation is highly correlated with transcription levels 3 , 4 , 5 . However, plants have different types of methylation controlled by different genetic pathways, and detailed information on the methylation status of each cytosine in any given genome is lacking. To this end, we generated a map at single-base-pair resolution of methylated cytosines for Arabidopsis , by combining bisulphite treatment of genomic DNA with ultra-high-throughput sequencing using the Illumina 1G Genome Analyser and Solexa sequencing technology 6 . This approach, termed BS-Seq, unlike previous microarray-based methods, allows one to sensitively measure cytosine methylation on a genome-wide scale within specific sequence contexts. Here we describe methylation on previously inaccessible components of the genome and analyse the DNA methylation sequence composition and distribution. We also describe the effect of various DNA methylation mutants on genome-wide methylation patterns, and demonstrate that our newly developed library construction and computational methods can be applied to large genomes such as that of mouse.

Noscapine is an antitumor alkaloid from opium poppy that binds tubulin, arrests metaphase, and induces apoptosis in dividing human cells. Elucidation of the biosynthetic pathway will enable improvement in the commercial production of noscapine and related bioactive molecules. Transcriptomic analysis revealed the exclusive expression of 10 genes encoding five distinct enzyme classes in a high noscapine-producing poppy variety, HN1. Analysis of an F₂ mapping population indicated that these genes are tightly linked in HN1, and bacterial artificial chromosome sequencing confirmed that they exist as a complex gene cluster for plant alkaloids. Virus-induced gene silencing resulted in accumulation of pathway intermediates, allowing gene function to be linked to noscapine synthesis and a novel biosynthetic pathway to be proposed.

How plants handle heat stress The transcription of repetitive elements such as retrotransposons — mobile genetic elements constituting more than 40% and 60% of the human and maize (corn) genomes, respectively — is normally repressed, to prevent their unchecked dissemination throughout the genome. Ito et al . show that heat stress in Arabidopsis plants induces transcription of the ONSEN retroelement. Accumulation of ONSEN is suppressed by small interfering RNAs (siRNAs). In the absence of siRNAs, new ONSEN insertions appear in the progeny, having transposed during differentiation. These results imply a memory of stress that is counteracted by siRNAs, providing a way of preventing transgenerational retrotransposition in plants facing environmental stress. The transcription of repetitive elements such as retrotransposons is normally repressed, to prevent their unchecked dissemination throughout the genome. This study shows that heat stress induces the transcription of the ONSEN retroelement. The accumulation of ONSEN is suppressed by small interfering RNAs. When siRNAs were absent, new ONSEN insertions were found in the progeny of heat-stressed plants, having transposed during differentiation. These results imply a memory of stress that can be counteracted by siRNAs. Eukaryotic genomes consist to a significant extent of retrotransposons that are suppressed by host epigenetic mechanisms, preventing their uncontrolled propagation 1 , 2 . However, it is not clear how this is achieved. Here we show that in Arabidopsis seedlings subjected to heat stress, a copia -type retrotransposon named ONSEN (Japanese ‘hot spring’) not only became transcriptionally active but also synthesized extrachromosomal DNA copies. Heat-induced ONSEN accumulation was stimulated in mutants impaired in the biogenesis of small interfering RNAs (siRNAs); however, there was no evidence of transposition occurring in vegetative tissues. After stress, both ONSEN transcripts and extrachromosomal DNA gradually decayed and were no longer detected after 20–30 days. Surprisingly, a high frequency of new ONSEN insertions was observed in the progeny of stressed plants deficient in siRNAs. Insertion patterns revealed that this transgenerational retrotransposition occurred during flower development and before gametogenesis. Therefore in plants with compromised siRNA biogenesis, memory of stress was maintained throughout development, priming ONSEN to transpose during differentiation of generative organs. Retrotransposition was not observed in the progeny of wild-type plants subjected to stress or in non-stressed mutant controls, pointing to a crucial role of the siRNA pathway in restricting retrotransposition triggered by environmental stress. Finally, we found that natural and experimentally induced variants in ONSEN insertions confer heat responsiveness to nearby genes, and therefore mobility bursts may generate novel, stress-responsive regulatory gene networks.

A restricted range in height and phenology of the elite Seri/Babax recombinant inbred line (RIL) population makes it ideal for physiological and genetic studies. Previous research has shown differential expression for yield under water deficit associated with canopy temperature (CT). In the current study, 167 RILs plus parents were phenotyped under drought (DRT), hot irrigated (HOT), and temperate irrigated (IRR) environments to identify the genomic regions associated with stress-adaptive traits. In total, 104 QTL were identified across a combination of 115 traits × 3 environments × 2 years, of which 14, 16, and 10 QTL were associated exclusively with DRT, HOT, and IRR, respectively. Six genomic regions were related to a large number of traits, namely 1B-a, 2B-a, 3B-b, 4A-a, 4A-b, and 5A-a. A yield QTL located on 4A-a explained 27 and 17% of variation under drought and heat stress, respectively. At the same location, a QTL explained 28% of the variation in CT under heat, while 14% of CT variation under drought was explained by a QTL on 3B-b. The T1BL.1RS (rye) translocation donated by the Seri parent was associated with decreased yield in this population. There was no co-location of consistent yield and phenology or height-related QTL, highlighting the utility of using a population with a restricted range in anthesis to facilitate QTL studies. Common QTL for drought and heat stress traits were identified on 1B-a, 2B-a, 3B-b, 4A-a, 4B-b, and 7A-a confirming their generic value across stresses. Yield QTL were shown to be associated with components of other traits, supporting the prospects for dissecting crop performance into its physiological and genetic components in order to facilitate a more strategic approach to breeding.

Blast disease is a devastating fungal disease of rice, one of the world's staple foods. Race-specific resistance to blast disease has usually not been durable. Here, we report the cloning of a previously unknown type of gene that confers non-race-specific resistance and its successful use in breeding. Pi21 encodes a proline-rich protein that includes a putative heavy metal-binding domain and putative protein-protein interaction motifs. Wild-type Pi21 appears to slow the plant's defense responses, which may support optimization of defense mechanisms. Deletions in its proline-rich motif inhibit this slowing. Pi21 is separable from a closely linked gene conferring poor flavor. The resistant pi21 allele, which is found in some strains of japonica rice, could improve blast resistance of rice worldwide.