Heterosis for biomass yield and related traits in five hybrids of Arabidopsis thaliana L. Heynh

Detecting directional epistasis and dominance from cross-line analyses in alpine populations of Arabidopsis thaliana

The contribution of non-additive genetic effects to the genetic architecture of fitness and to the evolutionary potential of populations has been a topic of theoretical and empirical interest for a long time. Yet, the empirical study of these effects in natural populations remains scarce, perhaps because measuring dominance and epistasis relies heavily on experimental line crosses. In this study, we explored the contribution of dominance and epistasis in natural alpine populations of Arabidopsis thaliana for 2 fitness traits, the dry biomass and the estimated number of siliques, measured in a greenhouse. We found that, on average, crosses between inbred lines of A. thaliana led to mid-parent heterosis for dry biomass but outbreeding depression for an estimated number of siliques. While heterosis for dry biomass was due to dominance, we found that outbreeding depression for an estimated number of siliques could be attributed to the breakdown of beneficial epistatic interactions. We simulated and discussed the implication of these results for the adaptive potential of the studied populations, as well as the use of line-cross analyses to detect non-additive genetic effects.

Morphological and metabolomics profiling of intraspecific Arabidopsis hybrids in relation to biomass heterosis

Heterosis contributes greatly to the worldwide agricultural yield. However, the molecular mechanism underlying heterosis remains unclear. This study took advantage of Arabidopsis intraspecific hybrids to identify heterosis-related metabolites. Forty-six intraspecific hybrids were used to examine parental effects on seed area and germination time. The degree of heterosis was evaluated based on biomass: combinations showing high heterosis of F₁ hybrids exhibited a biomass increase from 6.1 to 44% over the better parent value (BPV), whereas that of the low- and no-heterosis hybrids ranged from - 19.8 to 9.8% over the BPV. Metabolomics analyses of F₁ hybrids with high heterosis and those with low one suggested that changes in TCA cycle intermediates are key factors that control growth. Notably, higher fumarate/malate ratios were observed in the high heterosis F₁ hybrids, suggesting they provide metabolic support associated with the increased biomass. These hybrids may produce more energy-intensive biomass by speeding up the efficiency of TCA fluxes. However, the expression levels of TCA-process-related genes in F₁ hybrids were not associated with the intensity of heterosis, suggesting that the post-transcriptional or post-translational regulation of these genes may affect the productivity of the intermediates in the TCA cycle.

Heterosis for capsacinoids accumulation in chili pepper hybrids is dependent on parent-of-origin effect

Heterosis for agronomic traits is a widespread phenomenon that underpins hybrid crop breeding. However, heterosis at the level of cellular metabolites has not yet been fully explored. Some metabolites are highly sought after, like capsaicinoids found in peppers of the Capsicum genus, which confer the characteristic pungent ('hot') flavour of the fruits. We analysed the metabolic profile of the fruit placenta and pericarp of inter- and intra-specific hybrids of two species of Capsicum peppers, C. chinense (cv. Habanero and cv. Biquinho) and C. annuum var. annuum (cv. Jalapeño and cv. Cascadura Ikeda) in complete diallel crosses with reciprocals. The parents and hybrids were grown in a glasshouse and the profile of primary metabolites (sugars, amino acids and organic acids) and capsaicinoids was generated via gas chromatography-time of flight-mass spectrometry (GC-TOF-MS) and ultra-performance liquid chromatography coupled to a mass spectrometer (UPLC-MS), respectively. We found considerable heterotic effects specifically for capsaicinoids accumulation in the fruit placenta of the hybrids, including those derived from non-pungent parents. Furthermore, a large fraction of fruit primary metabolism was influenced by the specific cross combination, with marked parent-of-origin effects, i.e. whether a specific genotype was used as the pistillate or pollen parent. The differences in metabolite levels between the hybrids and their parents provide a snapshot of heterosis for primary and secondary metabolites and may contribute to explain the manifestation of whole-plant heterotic phenotypes.

From hybrid genomes to heterotic trait output: Challenges and opportunities

Heterosis (or hybrid vigor) has been widely used in crop seed breeding to improve many key economic traits. Nevertheless, the genetic and molecular basis of this important phenomenon has long remained elusive, constraining its flexible and effective exploitation. Advanced genomic approaches are efficient in characterizing the mechanism of heterosis. Here, we review how the omics approaches, including genomic, transcriptomic, and population genetics methods such as genome-wide association studies, can reveal how hybrid genomes outperform parental genomes in plants. This information opens up opportunities for genomic exploration and manipulation of heterosis in crop breeding.

Advances in lentil production through heterosis: Evaluating generations and breeding systems

Heterosis is defined as increased performance of the F1 hybrid relative to its parents. In the current study, a cohort of populations and parents were created to evaluate and understand heterosis across generations (i.e., F1 to F3) in lentil, a self-pollinated annual diploid (2n = 2× = 14) crop species. Lentil plants were evaluated for heterotic traits in terms of plant height, biomass fresh weight, seed number, yield per plant and 100 grain weight. A total of 47 selected lentil genotypes were cross hybridized to generate 72 F1 hybrids. The F1 hybrids from the top five crosses exhibited between 31%-62% heterosis for seed number with reference to the better parent. The five best performing heterotic crosses were selected with a negative control for evaluation at the subsequent F2 generation and only the tails of the distribution taken forward to be assessed in the F3 generation as a sub selection. Overall, heterosis decreases across the subsequent generations for all traits studied. However, some individual genotypes were identified at the F2 and sub-selected F3 generations with higher levels of heterosis than the best F1 mean value (hybrid mimics). The phenotypic data for the selected F2 and sub selected F3 hybrids were analysed, and the study suggested that 100 grain weight was the biggest driver of yield followed by seed number. A genetic diversity analysis of all the F1 parents failed to correlate genetic distance and divergence among parents with heterotic F1's. Therefore, genetic distance was not a key factor to determine heterosis in lentil. The study highlights the challenges associated with different breeding systems for heterosis (i.e., F1 hybrid-based breeding systems and/or via hybrid mimics) but demonstrates the potential significant gains that could be achieved in lentil productivity.

Fitness consequences of hybridization in a predominantly selfing species: insights into the role of dominance and epistatic incompatibilities

Studying the consequences of hybridization on plant performance is insightful to understand the adaptive potential of populations, notably at local scales. Due to reduced effective recombination, predominantly selfing species are organized in highly homozygous multi-locus-genotypes (or lines) that accumulate genetic differentiation both among- and within-populations. This high level of homozygosity facilitates the dissection of the genetic basis of hybrid performance in highly selfing species, which gives insights into the mechanisms of reproductive isolation between lines. Here, we explored the fitness consequences of hybridization events between natural inbred lines of the predominantly selfing species Medicago truncatula, at both within- and among-populations scales. We found that hybridization has opposite effects pending on studied fitness proxies, with dry mass showing heterosis, and seed production showing outbreeding depression. Although we found significant patterns of heterosis and outbreeding depression, they did not differ significantly for within- compared to among-population crosses. Family-based analyses allowed us to determine that hybrid differentiation was mostly due to dominance and epistasis. Dominance and/or dominant epistatic interactions increased dry mass, while decreasing seed production, and recessive epistatic interactions mostly had a positive effect on both fitness proxies. Our results illustrate how genetic incompatibilities can accumulate at a very local scale among multi-locus-genotypes, and how non-additive genetic effects contribute to heterosis and outbreeding depression.

Biological pathway expression complementation contributes to biomass heterosis in Arabidopsis

The mechanisms underlying heterosis have long remained a matter of debate, despite its agricultural importance. How changes in transcriptional networks during plant development are relevant to the continuous manifestation of growth vigor in hybrids is intriguing and unexplored. Here, we present an integrated high-resolution analysis of the daily dynamic growth phenotypes and transcriptome atlases of young Arabidopsis seedlings (parental ecotypes [Col-0 and Per-1] and their F₁ hybrid). Weighted gene coexpression network analysis uncovered divergent expression patterns between parents of the network hub genes, in which genes related to the cell cycle were more highly expressed in one parent (Col-0), whereas those involved in photosynthesis were more highly expressed in the other parent (Per-1). Notably, the hybrid exhibited spatiotemporal high-parent-dominant expression complementation of network hub genes in the two pathways during seedling growth. This suggests that the integrated capacities of cell division and photosynthesis contribute to hybrid growth vigor, which could be enhanced by temporal advances in the progression of leaf development in the hybrid relative to its parents. Altogether, this study provides evidence of expression complementation between fundamental biological pathways in hybrids and highlights the contribution of expression dominance in heterosis.

In Arabidopsis thaliana Heterosis Level Varies among Individuals in an F1 Hybrid Population

Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior yield and biomass to parental lines and has been used to breed F₁ hybrid cultivars in many crops. A similar level of heterosis in all F₁ individuals is expected as they are genetically identical. However, we found variation in rosette size in individual F₁ plants from a cross between C24 and Columbia-0 accessions of Arabidopsis thaliana. Big-sized F₁ plants had 26.1% larger leaf area in the first and second leaves than medium-sized F₁ plants at 14 days after sowing in spite of the identical genetic background. We identified differentially expressed genes between big- and medium-sized F₁ plants by microarray; genes involved in the category of stress response were overrepresented. We made transgenic plants overexpressing 21 genes, which were differentially expressed between the two size classes, and some lines had increased plant size at 14 or 21 days after sowing but not at all time points during development. Change of expression levels in stress-responsive genes among individual F₁ plants could generate the variation in plant size of individual F₁ plants in A. thaliana.

Nonlinear phenotypic variation uncovers the emergence of heterosis in Arabidopsis thaliana

Heterosis describes the phenotypic superiority of hybrids over their parents in traits related to agronomic performance and fitness. Understanding and predicting nonadditive inheritance such as heterosis is crucial for evolutionary biology as well as for plant and animal breeding. However, the physiological bases of heterosis remain debated. Moreover, empirical data in various species have shown that diverse genetic and molecular mechanisms are likely to explain heterosis, making it difficult to predict its emergence and amplitude from parental genotypes alone. In this study, we examined a model of physiological dominance initially proposed by Sewall Wright to explain the nonadditive inheritance of traits like metabolic fluxes at the cellular level. We evaluated Wright's model for two fitness-related traits at the whole-plant level, growth rate and fruit number, using 450 hybrids derived from crosses among natural accessions of A. thaliana. We found that allometric relationships between traits constrain phenotypic variation in a nonlinear and similar manner in hybrids and accessions. These allometric relationships behave predictably, explaining up to 75% of heterosis amplitude, while genetic distance among parents at best explains 7%. Thus, our findings are consistent with Wright's model of physiological dominance and suggest that the emergence of heterosis on plant performance is an intrinsic property of nonlinear relationships between traits. Furthermore, our study highlights the potential of a geometric approach of phenotypic relationships for predicting heterosis of major components of crop productivity and yield.

A dissection model for mapping complex traits

Many quantitative traits are composites of other traits that contribute differentially to genetic variation. Quantitative trait locus (QTL) mapping of these composite traits can benefit by incorporating the mechanistic process of how their formation is mediated by the underlying components. We propose a dissection model by which to map these interconnected components traits under a joint likelihood setting. The model can test how a composite trait is determined by pleiotropic QTLs for its component traits or jointly by different sets of QTLs each responsible for a different component. The model can visualize the pattern of time-varying genetic effects for individual components and their impacts on composite traits. The dissection model was used to map two composite traits, stemwood volume growth decomposed into its stem height, stem diameter and stem form components for Euramerican poplar adult trees, and total lateral root length constituted by its average lateral root length and lateral root number components for Euphrates poplar seedlings. We found the pattern of how QTLs for different components contribute to phenotypic variation in composite traits. The detailed understanding of the genetic machineries of composite traits will not only help in the design of molecular breeding in plants and animals, but also shed light on the evolutionary processes of quantitative traits under natural selection.

Growth rate regulation is associated with developmental modification of source efficiency

Plants modulate their growth rate according to seasonal and environmental cues using a suite of growth repressors known to interact directly with cellular machinery controlling cell division and growth. Mutants lacking growth repressors show increased growth rates^1,2, but the mechanism by which these plants ensure source availability for faster growth is unclear. Here, we undertake a comprehensive analysis of the fast-growth phenotype of a quintuple growth-repressor mutant, using a combination of theoretical and experimental approaches to understand the physiological basis of source-sink coordination. Our results show that, in addition to the control of tissue growth rates, growth repressors also affect tissue composition and leaf thickness, modulating the efficiency of production of new photosynthetic capacity. Modelling suggests that increases in growth efficiency underlie growth-rate differences between the wild type and spatula della growth-repressor mutant, with spatula della requiring less carbon to synthesize a comparable photosynthetic capability to the wild type, and fixing more carbon per unit mass. We conclude that through control of leaf development, growth repressors regulate both source availability and sink strength to achieve growth-rate variation without risking a carbon deficit.

Assessment of heterosis in two Arabidopsis thaliana common-reference mapping populations

Hybrid vigour, or heterosis, has been of tremendous importance in agriculture for the improvement of both crops and livestock. Notwithstanding large efforts to study the phenomenon of heterosis in the last decades, the identification of common molecular mechanisms underlying hybrid vigour remain rare. Here, we conducted a systematic survey of the degree of heterosis in Arabidopsis thaliana hybrids. For this purpose, two overlapping Arabidopsis hybrid populations were generated by crossing a large collection of naturally occurring accessions to two common reference lines. In these Arabidopsis hybrid populations the range of heterosis for several developmental and yield related traits was examined, and the relationship between them was studied. The traits under study were projected leaf area at 17 days after sowing, flowering time, height of the main inflorescence, number of side branches from the main stem or from the rosette base, total seed yield, seed weight, seed size and the estimated number of seeds per plant. Predominantly positive heterosis was observed for leaf area and height of the main inflorescence, whereas mainly negative heterosis was observed for rosette branching. For the other traits both positive and negative heterosis was observed in roughly equal amounts. For flowering time and seed size only low levels of heterosis were detected. In general the observed heterosis levels were highly trait specific. Furthermore, no correlation was observed between heterosis levels and the genetic distance between the parental lines. Since all selected lines were a part of the Arabidopsis genome wide association (GWA) mapping panel, a genetic mapping approach was applied to identify possible regions harbouring genetic factors causal for heterosis, with separate calculations for additive and dominance effects. Our study showed that the genetic mechanisms underlying heterosis were highly trait specific in our hybrid populations and greatly depended on the genetic background, confirming the elusive character of heterosis.

To defend or to grow: lessons from Arabidopsis C24

The emergence of Arabidopsis as a model species and the availability of genetic and genomic resources have resulted in the identification and detailed characterization of abiotic stress signalling pathways. However, this has led only to limited success in engineering abiotic stress tolerance in crops. This is because there needs to be a deeper understanding of how to combine resistances to a range of stresses with growth and productivity. The natural variation and genomic resources of Arabidopsis thaliana (Arabidopsis) are a great asset to understand the mechanisms of multiple stress tolerances. One natural variant in Arabidopsis is the accession C24, and here we provide an overview of the increasing research interest in this accession. C24 is highlighted as a source of tolerance for multiple abiotic and biotic stresses, and a key accession to understand the basis of basal immunity to infection, high water use efficiency, and water productivity. Multiple biochemical, physiological, and phenological mechanisms have been attributed to these traits in C24, and none of them constrains productivity. Based on the uniqueness of C24, we postulate that the use of variation derived from natural selection in undomesticated species provides opportunities to better understand how complex environmental stress tolerances and resource use efficiency are co-ordinated.

Heterosis Is a Systemic Property Emerging From Non-linear Genotype-Phenotype Relationships: Evidence From in Vitro Genetics and Computer Simulations

Heterosis, the superiority of hybrids over their parents for quantitative traits, represents a crucial issue in plant and animal breeding as well as evolutionary biology. Heterosis has given rise to countless genetic, genomic and molecular studies, but has rarely been investigated from the point of view of systems biology. We hypothesized that heterosis is an emergent property of living systems resulting from frequent concave relationships between genotypic variables and phenotypes, or between different phenotypic levels. We chose the enzyme-flux relationship as a model of the concave genotype-phenotype (GP) relationship, and showed that heterosis can be easily created in the laboratory. First, we reconstituted in vitro the upper part of glycolysis. We simulated genetic variability of enzyme activity by varying enzyme concentrations in test tubes. Mixing the content of "parental" tubes resulted in "hybrids," whose fluxes were compared to the parental fluxes. Frequent heterotic fluxes were observed, under conditions that were determined analytically and confirmed by computer simulation. Second, to test this model in a more realistic situation, we modeled the glycolysis/fermentation network in yeast by considering one input flux, glucose, and two output fluxes, glycerol and acetaldehyde. We simulated genetic variability by randomly drawing parental enzyme concentrations under various conditions, and computed the parental and hybrid fluxes using a system of differential equations. Again we found that a majority of hybrids exhibited positive heterosis for metabolic fluxes. Cases of negative heterosis were due to local convexity between certain enzyme concentrations and fluxes. In both approaches, heterosis was maximized when the parents were phenotypically close and when the distributions of parental enzyme concentrations were contrasted and constrained. These conclusions are not restricted to metabolic systems: they only depend on the concavity of the GP relationship, which is commonly observed at various levels of the phenotypic hierarchy, and could account for the pervasiveness of heterosis.

Recent research on the mechanism of heterosis is important for crop and vegetable breeding systems

Heterosis or hybrid vigor is a phenomenon where hybrid progeny have superior performance compared to their parental inbred lines. This is important in the use of F₁ hybrid cultivars in many crops and vegetables. However, the molecular mechanism of heterosis is not clearly understood. Gene interactions between the two genomes such as dominance, overdominance, and epistasis have been suggested to explain the increased biomass and yield. Genetic analyses of F₁ hybrids in maize, rice, and canola have defined a large number of quantitative trait loci, which may contribute to heterosis. Recent molecular analyses of transcriptomes together with reference to the epigenome of the parents and hybrids have begun to uncover new facts about the generation of heterosis. These include the identification of gene expression changes in hybrids, which may be important for heterosis, the role of epigenetic processes in heterosis, and the development of stable high yielding lines.

Comprehensive Profiling and Inheritance Patterns of Metabolites in Foxtail Millet

Metabolomics aims at determining a sample's metabolites profile and hence provides a straight functional statement of an organism's physiological condition. Here, we investigated comprehensive profiling, natural variation and species-specific accumulation of both primary and secondary metabolites in foxtail millet using LC-MS, and inheritance patterns of metabolome in millet hybrids. The application of a broad target metabolomics method facilitated the simultaneous identification and quantification of more than 300 metabolites. The metabolic analysis of these compounds, such as flavonoids, phenolamides, hydrocinnamoyl derivatives, vitamins and LPCs, revealed their developmentally controlled accumulation, and natural variation in different tissues/varieties. Species-specific accumulation of secondary metabolites was observed based on a comparative metabolic analysis between millet and rice, such as flavonoid O-rutinosides/neohesperidosides and malonylated flavonoid O-glycosides. In analyzing the metabolic variations between hybrid progenies and their parental lines, including a photothermo-sensitive genic male sterility line and five Zhangzagu varieties, metabolic overdominant, and dominant patterns of inheritance could be observed. For example, hydrocinnamoyl derivatives and feruloylated flavonoids were identified as over-parent heterosis (overdominant) metabolites in milet hybrids. Our work paves the way for developing predictors of hybrid performance and the future analysis of the biosynthesis and regulation of relevant metabolic pathways in millet.

Epigenetic regulation of agronomical traits in Brassicaceae

Epigenetic regulation, covalent modification of DNA and changes in histone proteins are closely linked to plant development and stress response through flexibly altering the chromatin structure to regulate gene expression. In this review, we will illustrate the importance of epigenetic influences by discussing three agriculturally important traits of Brassicaceae. (1) Vernalization, an acceleration of flowering by prolonged cold exposure regulated through epigenetic silencing of a central floral repressor, FLOWERING LOCUS C. This is associated with cold-dependent repressive histone mark accumulation, which confers competency of consequence vegetative-to-reproductive phase transition. (2) Hybrid vigor, in which an F₁ hybrid shows superior performance to the parental lines. Combination of distinct epigenomes with different DNA methylation states between parental lines is important for increase in growth rate in a hybrid progeny. This is independent of siRNA-directed DNA methylation but dependent on the chromatin remodeler DDM1. (3) Self-incompatibility, a reproductive mating system to prevent self-fertilization. This is controlled by the S-locus consisting of SP11 and SRK which are responsible for self/non-self recognition. Because self-incompatibility in Brassicaceae is sporophytically controlled, there are dominance relationships between S haplotypes in the stigma and pollen. The dominance relationships in the pollen rely on de novo DNA methylation at the promoter region of a recessive allele, which is triggered by siRNA production from a flanking region of a dominant allele.

Genomic architecture of biomass heterosis in Arabidopsis

Heterosis is most frequently manifested by the substantially increased vigorous growth of hybrids compared with their parents. Investigating genomic variations in natural populations is essential to understand the initial molecular mechanisms underlying heterosis in plants. Here, we characterized the genomic architecture associated with biomass heterosis in 200 Arabidopsis hybrids. The genome-wide heterozygosity of hybrids makes a limited contribution to biomass heterosis, and no locus shows an obvious overdominance effect in hybrids. However, the accumulation of significant genetic loci identified in genome-wide association studies (GWAS) in hybrids strongly correlates with better-parent heterosis (BPH). Candidate genes for biomass BPH fall into diverse biological functions, including cellular, metabolic, and developmental processes and stimulus-responsive pathways. Important heterosis candidates include WUSCHEL, ARGOS, and some genes that encode key factors involved in cell cycle regulation. Interestingly, transcriptomic analyses in representative Arabidopsis hybrid combinations reveal that heterosis candidate genes are functionally enriched in stimulus-responsive pathways, including responses to biotic and abiotic stimuli and immune responses. In addition, stimulus-responsive genes are repressed to low-parent levels in hybrids with high BPH, whereas middle-parent expression patterns are exhibited in hybrids with no BPH. Our study reveals a genomic architecture for understanding the molecular mechanisms of biomass heterosis in Arabidopsis, in which the accumulation of the superior alleles of genes involved in metabolic and cellular processes improve the development and growth of hybrids, whereas the overall repressed expression of stimulus-responsive genes prioritizes growth over responding to environmental stimuli in hybrids under normal conditions.

Enhancement of Arabidopsis growth characteristics using genome interrogation with artificial transcription factors

The rapidly growing world population has a greatly increasing demand for plant biomass, thus creating a great interest in the development of methods to enhance the growth and biomass accumulation of crop species. In this study, we used zinc finger artificial transcription factor (ZF-ATF)-mediated genome interrogation to manipulate the growth characteristics and biomass of Arabidopsis plants. We describe the construction of two collections of Arabidopsis lines expressing fusions of three zinc fingers (3F) to the transcriptional repressor motif EAR (3F-EAR) or the transcriptional activator VP16 (3F-VP16), and the characterization of their growth characteristics. In total, six different 3F-ATF lines with a consistent increase in rosette surface area (RSA) of up to 55% were isolated. For two lines we demonstrated that 3F-ATF constructs function as dominant in trans acting causative agents for an increase in RSA and biomass, and for five larger plant lines we have investigated 3F-ATF induced transcriptomic changes. Our results indicate that genome interrogation can be used as a powerful tool for the manipulation of plant growth and biomass and that it might supply novel cues for the discovery of genes and pathways involved in these properties.

Genome-wide analyses of four major histone modifications in Arabidopsis hybrids at the germinating seed stage

BACKGROUND

Hybrid vigour (heterosis) has been used for decades in cropping agriculture, especially in the production of maize and rice, because hybrid varieties exceed their parents in plant biomass and seed yield. The molecular basis of hybrid vigour is not fully understood. Previous studies have suggested that epigenetic systems could play a role in heterosis.

RESULTS

In this project, we investigated genome-wide patterns of four histone modifications in Arabidopsis hybrids in germinating seeds. We found that although hybrids have similar histone modification patterns to the parents in most regions of the genome, they have altered patterns at specific loci. A small subset of genes show changes in histone modifications in the hybrids that correlate with changes in gene expression. Our results also show that genome-wide patterns of histone modifications in geminating seeds parallel those at later developmental stages of seedlings.

CONCLUSION

Ler/C24 hybrids showed similar genome-wide patterns of histone modifications as the parents at an early germination stage. However, a small subset of genes, such as FLC, showed correlated changes in histone modification and in gene expression in the hybrids. The altered patterns of histone modifications for those genes in hybrids could be related to some heterotic traits in Arabidopsis, such as flowering time, and could play a role in hybrid vigour establishment.

Early changes of gene activity in developing seedlings of Arabidopsis hybrids relative to parents may contribute to hybrid vigour

Hybrid vigour (heterosis) has been used for decades in crop industries, especially in the production of maize and rice. Hybrid varieties usually exceed their parents in plant biomass and seed yield. But the molecular basis of hybrid vigour is not fully understood. In this project, we studied heterosis at early stages of seedling development in Arabidopsis hybrids derived from crossing Ler and C24 accessions. We found that early heterosis is associated with non-additive gene expression that resulted from earlier changes in gene expression in the hybrids relative to the parents. The non-additively expressed genes are involved in metabolic pathways, including photosynthesis, critical for plant growth. The early increased expression levels of genes involved in energy production in hybrids is associated with heterosis in the young seedlings that could be essential for biomass heterosis at later developmental stages of the plant.

Role of DNA methylation in hybrid vigor in Arabidopsis thaliana

Hybrid vigor or heterosis refers to the superior performance of F1 hybrid plants over their parents. Heterosis is particularly important in the production systems of major crops. Recent studies have suggested that epigenetic regulation such as DNA methylation is involved in heterosis, but the molecular mechanism of heterosis is still unclear. To address the epigenetic contribution to heterosis in Arabidopsis thaliana, we used mutant genes that have roles in DNA methylation. Hybrids between C24 and Columbia-0 (Col) without RNA polymerase IV (Pol IV) or methyltransferase I (MET1) function did not reduce the level of biomass heterosis (as evaluated by rosette diameter). Hybrids with a mutation in decrease in dna methylation 1 (ddm1) showed a decreased heterosis level. Vegetative heterosis in the ddm1 mutant hybrid was reduced but not eliminated; a complete reduction could result if there was a change in methylation at all loci critical for generating the level of heterosis, whereas if only a proportion of the loci have methylation changes there may only be a partial reduction in heterosis.

Molecular and cellular characteristics of hybrid vigour in a commercial hybrid of Chinese cabbage

BACKGROUND

Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior performance compared to their parental inbred lines. Most commercial Chinese cabbage cultivars are F1 hybrids and their level of hybrid vigour is of critical importance and is a key selection criterion in the breeding system.

RESULTS

We have characterized the heterotic phenotype of one F1 hybrid cultivar of Chinese cabbage and its parental lines from early- to late-developmental stages of the plants. Hybrid cotyledons are larger than those of the parents at 4 days after sowing and biomass in the hybrid, determined by the fresh weight of leaves, is greater than that of the larger parent line by approximately 20% at 14 days after sowing. The final yield of the hybrid harvested at 63 days after sowing is 25% greater than the yield of the better parent. The larger leaves of the hybrid are a consequence of increased cell size and number of the photosynthetic palisade mesophyll cells and other leaf cells. The accumulation of plant hormones in the F1 was within the range of the parental levels at both 2 and 10 days after sowing. Two days after sowing, the expression levels of chloroplast-targeted genes in the cotyledon cells were upregulated in the F1 hybrid relative to their mid parent values. Shutdown of chlorophyll biosynthesis in the cotyledon by norflurazon prevented the increased leaf area in the F1 hybrid.

CONCLUSIONS

In the cotyledons of F1 hybrids, chloroplast-targeted genes were upregulated at 2 days after sowing. The increased activity levels of this group of genes suggested that their differential transcription levels could be important for establishing early heterosis but the increased transcription levels were transient. Inhibition of the photosynthetic process in the cotyledon reduced heterosis in later seedling stages. These observations suggest early developmental events in the germinating seedling of the hybrid may be important for later developmental vigour and yield advantage.

Genetic Components of Heterosis for Seedling Traits in an Elite Rice Hybrid Analyzed Using an Immortalized F2 Population

Utilization of heterosis has greatly contributed to rice productivity in China and many Asian countries. Superior hybrids usually show heterosis at two stages: canopy development at vegetative stage and panicle development at reproductive stage resulting in heterosis in yield. Although the genetic basis of heterosis in rice has been extensively investigated, all the previous studies focused on yield traits at maturity stage. In this study, we analyzed the genetic basis of heterosis at seedling stage making use of an "immortalized F2" population composed of 105 hybrids produced by intercrossing recombinant inbred lines (RILs) from a cross between Zhenshan 97 and Minghui 63, the parents of Shanyou 63, which is an elite hybrid widely grown in China. Eight seedling traits, seedling height, tiller number, leaf number, root number, maximum root length, root dry weight, shoot dry weight and total dry weight, were investigated using hydroponic culture. We analyzed single-locus and digenic genetic effects at the whole genome level using an ultrahigh-density SNP bin map obtained by population re-sequencing. The analysis revealed large numbers of heterotic effects for seedling traits including dominance, overdominance and digenic dominance (epistasis) in both positive and negative directions. Overdominance effects were prevalent for all the traits, and digenic dominance effects also accounted for a large portion of the genetic effects. The results suggested that cumulative small advantages of the single-locus effects and two-locus interactions, most of which could not be detected statistically, could explain the genetic basis of seedling heterosis of the F1 hybrid.

Hormone-regulated defense and stress response networks contribute to heterosis in Arabidopsis F1 hybrids

Plant hybrids are extensively used in agriculture to deliver increases in yields, yet the molecular basis of their superior performance (heterosis) is not well understood. Our transcriptome analysis of a number of Arabidopsis F1 hybrids identified changes to defense and stress response gene expression consistent with a reduction in basal defense levels. Given the reported antagonism between plant immunity and growth, we suggest that these altered patterns of expression contribute to the greater growth of the hybrids. The altered patterns of expression in the hybrids indicate decreases to the salicylic acid (SA) biosynthesis pathway and increases in the auxin [indole-3-acetic acid (IAA)] biosynthesis pathway. SA and IAA are hormones known to control stress and defense responses as well as plant growth. We found that IAA-targeted gene activity is frequently increased in hybrids, correlating with a common heterotic phenotype of greater leaf cell numbers. Reduced SA concentration and target gene responses occur in the larger hybrids and promote increased leaf cell size. We demonstrated the importance of SA action to the hybrid phenotype by manipulating endogenous SA concentrations. Increasing SA diminished heterosis in SA-reduced hybrids, whereas decreasing SA promoted growth in some hybrids and phenocopied aspects of hybrid vigor in parental lines. Pseudomonas syringae infection of hybrids demonstrated that the reductions in basal defense gene activity in these hybrids does not necessarily compromise their ability to mount a defense response comparable to the parents.

Heterosis and inbreeding depression of epigenetic Arabidopsis hybrids

We have addressed the possible epigenetic contribution to heterosis using epigenetic inbred lines (epiRILs) with varying levels and distributions of DNA methylation. One line consistently displayed parent-of-origin heterosis for growth-related traits. Genome-wide transcription profiling followed by a candidate gene approach revealed 33 genes with altered regulation in crosses of this line that could contribute to the observed heterosis. Although none of the candidate genes could explain hybrid vigour, we detected intriguing, hybrid-specific transcriptional regulation of the RPP5 gene, encoding a growth suppressor. RPP5 displayed intermediate transcript levels in heterotic hybrids; surprisingly however, with global loss of fitness of their F2 progeny, we observed striking under-representation of the hybrid-like intermediate levels. Thus, in addition to genetic factors contributing to heterosis, our results strongly suggest that epigenetic diversity and epigenetic regulation of transcription play a role in hybrid vigour and inbreeding depression, and also in the absence of parental genetic diversity.

Salicylic acid biosynthesis is enhanced and contributes to increased biotrophic pathogen resistance in Arabidopsis hybrids

Heterosis, the phenotypic superiority of a hybrid over its parents, has been demonstrated for many traits in Arabidopsis thaliana, but its effect on defence remains largely unexplored. Here, we show that hybrids between some A. thaliana accessions show increased resistance to the biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Comparisons of transcriptomes between these hybrids and their parents after inoculation reveal that several key salicylic acid (SA) biosynthesis genes are significantly upregulated in hybrids. Moreover, SA levels are higher in hybrids than in either parent. Increased resistance to Pst DC3000 is significantly compromised in hybrids of pad4 mutants in which the SA biosynthesis pathway is blocked. Finally, increased histone H3 acetylation of key SA biosynthesis genes correlates with their upregulation in infected hybrids. Our data demonstrate that enhanced activation of SA biosynthesis in A. thaliana hybrids may contribute to their increased resistance to a biotrophic bacterial pathogen.

The molecular basis for heterosis, the phenomenon whereby hybrid plants show phenotypic superiority to their parents, remains poorly understood. Here, Yang et al. show that salicylic acid biosynthesis is enhanced in hybrids of Arabidopsis thaliana and correlates with heterosis for pathogen defence.

Heterosis and outbreeding depression in crosses between natural populations of Arabidopsis thaliana

Understanding the causes and architecture of genetic differentiation between natural populations is of central importance in evolutionary biology. Crosses between natural populations can result in heterosis if recessive or nearly recessive deleterious mutations have become fixed within populations because of genetic drift. Divergence between populations can also result in outbreeding depression because of genetic incompatibilities. The net fitness consequences of between-population crosses will be a balance between heterosis and outbreeding depression. We estimated the magnitude of heterosis and outbreeding depression in the highly selfing model plant Arabidopsis thaliana, by crossing replicate line pairs from two sets of natural populations (C↔R, B↔S) separated by similar geographic distances (Italy↔Sweden). We examined the contribution of different modes of gene action to overall differences in estimates of lifetime fitness and fitness components using joint scaling tests with parental, reciprocal F1 and F2, and backcross lines. One of these population pairs (C↔R) was previously demonstrated to be locally adapted, but locally maladaptive quantitative trait loci were also found, suggesting a role for genetic drift in shaping adaptive variation. We found markedly different genetic architectures for fitness and fitness components in the two sets of populations. In one (C↔R), there were consistently positive effects of dominance, indicating the masking of recessive or nearly recessive deleterious mutations that had become fixed by genetic drift. The other set (B↔S) exhibited outbreeding depression because of negative dominance effects. Additional studies are needed to explore the molecular genetic basis of heterosis and outbreeding depression, and how their magnitudes vary across environments.

The regulatory network mediated by circadian clock genes is related to heterosis in rice

Exploitation of heterosis in rice (Oryza sativa L.) has contributed greatly to global food security. In this study, we generated three sets of reciprocal F1 hybrids of indica and japonica subspecies to evaluate the relationship between yield heterosis and the circadian clock. There were no differences in trait performance or heterosis between the reciprocal hybrids, indicating no maternal effects on heterosis. The indica-indica and indica-japonica reciprocal F1 hybrids exhibited pronounced heterosis for chlorophyll and starch content in leaves and for grain yield/biomass. In contrast, the japonica-japonica F1 hybrids showed low heterosis. The three circadian clock genes investigated expressed in an above-high-parent pattern (AHP) at seedling stage in all the hybrids. The five genes downstream of the circadian clock, and involved in chlorophyll and starch metabolic pathways, were expressed in AHP in hybrids with strong better-parent heterosis (BPH). Similarly, three of these five genes in the japonica-japonica F1 hybrids showing low BPH were expressed in positive overdominance, but the other two genes were expressed in additive or negative overdominance. These results indicated that the expression patterns of circadian clock genes and their downstream genes are associated with heterosis, which suggests that the circadian rhythm pathway may be related to heterosis in rice.

Intraspecific Arabidopsis hybrids show different patterns of heterosis despite the close relatedness of the parental genomes

Heterosis is important for agriculture; however, little is known about the mechanisms driving hybrid vigor. Ultimately, heterosis depends on the interactions of specific alleles and epialleles provided by the parents, which is why hybrids can exhibit different levels of heterosis, even within the same species. We characterize the development of several intraspecific Arabidopsis (Arabidopsis thaliana) F1 hybrids that show different levels of heterosis at maturity. We identify several phases of heterosis beginning during embryogenesis and culminating in a final phase of vegetative maturity and seed production. During each phase, the hybrids show different levels and patterns of growth, despite the close relatedness of the parents. For instance, during the vegetative phases, the hybrids develop larger leaves than the parents to varied extents, and they do so by exploiting increases in cell size and cell numbers in different ratios. Consistent with this finding, we observed changes in the expression of genes known to regulate leaf size in developing rosettes of the hybrids, with the patterns of altered expression differing between combinations. The data show that heterosis is dependent on changes in development throughout the growth cycle of the hybrid, with the traits of mature vegetative biomass and reproductive yield as cumulative outcomes of heterosis at different levels, tissues, and times of development.

A classical genetic solution to enhance the biosynthesis of anticancer phytochemicals in Andrographis paniculata Nees

Andrographolides, the diterpene lactones, are major bioactive phytochemicals which could be found in different parts of the medicinal herb Andrographis paniculata. A number of such compounds namely andrographolide (AG), neoandrographolide (NAG), and 14-deoxy-11,12-didehydroandrographolide (DDAG) have already attracted a great deal of attention due to their potential therapeutic effects in hard-to-treat diseases such as cancers and HIV. Recently, they have also been considered as substrates for the discovery of novel pharmaceutical compounds. Nevertheless, there is still a huge gap in knowledge on the genetic pattern of the biosynthesis of these bioactive compounds. Hence, the present study aimed to investigate the genetic mechanisms controlling the biosynthesis of these phytochemicals using a diallel analysis. The high performance liquid chromatography analysis of the three andrographolides in 210 F₁ progenies confirmed that the biosynthesis of these andrographolides was considerably increased via intraspecific hybridization. The results revealed high, moderate and low heterosis for DDAG, AG and NAG, respectively. Furthermore, the preponderance of non-additive gene actions was affirmed in the enhancement of the three andrographolides contents. The consequence of this type of gene action was the occurrence of high broad-sense and low narrow-sense heritabilities for the above mentioned andrographolides. The prevalence of non-additive gene action suggests the suitability of heterosis breeding and hybrid seed production as a preferred option to produce new plant varieties with higher andrographolide contents using the wild accessions of A. paniculata. Moreover, from an evolutionary point of view, the occurrence of population bottlenecks in the Malaysian accessions of A. paniculata was unveiled by observing a low level of additive genetic variance (V_A) for all the andrographolides.

Genomic and epigenetic insights into the molecular bases of heterosis

Heterosis, also known as hybrid vigour, is widespread in plants and animals, but the molecular bases for this phenomenon remain elusive. Recent studies in hybrids and allopolyploids using transcriptomic, proteomic, metabolomic, epigenomic and systems biology approaches have provided new insights. Emerging genomic and epigenetic perspectives suggest that heterosis arises from allelic interactions between parental genomes, leading to altered programming of genes that promote the growth, stress tolerance and fitness of hybrids. For example, epigenetic modifications of key regulatory genes in hybrids and allopolyploids can alter complex regulatory networks of physiology and metabolism, thus modulating biomass and leading to heterosis. The conceptual advances could help to improve plant and animal productivity through the manipulation of heterosis.

Extensive genomic characterization of a set of near-isogenic lines for heterotic QTL in maize (Zea mays L.)

Background

Despite the crucial role that heterosis has played in crop improvement, its genetic and molecular bases are still elusive. Several types of structured populations were used to discover the genetic architecture underlying complex phenotypes, and several QTL related to heterosis were detected. However, such analyses generally lacked the statistical power required for the detailed characterization of individual QTL. Currently, QTL introgression into near-isogenic materials is considered the most effective strategy to this end, despite such materials inevitably contain a variable, unknown and undesired proportion of non-isogenic genome.

An introgression program based on residual heterozygous lines allowed us to develop five pairs of maize (Zea mays L.) near-isogenic lines (NILs) suitable for the fine characterization of three major heterotic QTL previously detected. Here we describe the results of the detailed genomic characterization of these NILs that we undertook to establish their genotypic structure, to verify the presence of the expected genotypes within target QTL regions, and to determine the extent and location of residual non-isogenic genomic regions.

Results

The SNP genotyping approach allowed us to determine the parent-of-origin allele for 14,937 polymorphic SNPs and to describe in detail the genotypic structure of all NILs. The correct introgression was confirmed for all target QTL in the respective NIL and several non-isogenic regions were detected genome-wide. Possible linkage drag effects associated to the specific introgressed regions were observed. The extent and position of other non-isogenic regions varied among NIL pairs, probably deriving from random segregating sections still present at the separation of lineages within pairs.

Conclusions

The results of this work strongly suggest that the actual isogenicity and the genotypic architecture of near-isogenic materials should be monitored both during the introgression procedure and on the final materials as a paramount requisite for a successful mendelization of target QTL. The information here gathered on the genotypic structure of NILs will be integrated in future experimental programs aimed at the fine mapping and isolation of major heterotic QTL, a crucial step towards the understanding of the molecular bases of heterosis in maize.

The inheritance pattern of 24 nt siRNA clusters in arabidopsis hybrids is influenced by proximity to transposable elements

Hybrids often display increased size and growth, and thus are widely cultivated in agriculture and horticulture. Recent discoveries demonstrating the important regulatory roles of small RNAs have greatly improved our understanding of many basic biological questions, and could illuminate the molecular basis for the enhanced growth and size of hybrid plants. We profiled small RNAs by deep sequencing to characterize the inheritance patterns of small RNA levels in reciprocal hybrids of two Arabidopsis thaliana accessions, Columbia and Landsberg erecta. We find 24-nt siRNAs predominate among those small RNAs that are differentially expressed between the parents. Following hybridization, the transposable element (TE)-derived siRNAs are often inherited in an additive manner, whereas siRNAs associated with protein-coding genes are often down-regulated in hybrids to the levels observed for the parent with lower relative siRNA levels. Among the protein-coding genes that exhibit this pattern, genes that function in pathogen defense, abiotic stress tolerance, and secondary metabolism are significantly enriched. Small RNA clusters from protein-coding genes where a TE is present within one kilobase show a different predominant inheritance pattern (additive) from those that do not (low-parent dominance). Thus, down-regulation in the form of low-parent dominance is likely the default pattern of inheritance for genic siRNA, and a different inheritance mechanism for TE siRNA is suggested.

Heterosis manifestation during early Arabidopsis seedling development is characterized by intermediate gene expression and enhanced metabolic activity in the hybrids

Heterosis-associated cellular and molecular processes were analyzed in seeds and seedlings of Arabidopsis thaliana accessions Col-0 and C24 and their heterotic hybrids. Microscopic examination revealed no advantages in terms of hybrid mature embryo organ sizes or cell numbers. Increased cotyledon sizes were detectable 4 days after sowing. Growth heterosis results from elevated cell sizes and numbers, and is well established at 10 days after sowing. The relative growth rates of hybrid seedlings were most enhanced between 3 and 4 days after sowing. Global metabolite profiling and targeted fatty acid analysis revealed maternal inheritance patterns for a large proportion of metabolites in the very early stages. During developmental progression, the distribution shifts to dominant, intermediate and heterotic patterns, with most changes occurring between 4 and 6 days after sowing. The highest incidence of heterotic patterns coincides with establishment of size differences at 4 days after sowing. In contrast, overall transcript patterns at 4, 6 and 10 days after sowing are characterized by intermediate to dominant patterns, with parental transcript levels showing the largest differences. Overall, the results suggest that, during early developmental stages, intermediate gene expression and higher metabolic activity in the hybrids compared to the parents lead to better resource efficiency, and therefore enhanced performance in the hybrids.

Ploidy and Hybridity Effects on Growth Vigor and Gene Expression in Arabidopsis thaliana Hybrids and Their Parents

Both ploidy and hybridity affect cell size and growth vigor in plants and animals, but the relative effects of genome dosage and hybridization on biomass, fitness, and gene expression changes have not been systematically examined. Here we performed the first comparative analysis of seed, cell, and flower sizes, starch and chlorophyll content, biomass, and gene expression changes in diploid, triploid, and tetraploid hybrids and their respective parents in three Arabidopsis thaliana ecotypes: Columbia, C24, and Landsberg erecta (Ler). Ploidy affects many morphological and fitness traits, including stomatal size, flower size, and seed weight, whereas hybridization between the ecotypes leads to altered expression of central circadian clock genes and increased starch and chlorophyll content, biomass, and seed weight. However, varying ploidy levels has subtle effects on biomass, circadian clock gene expression, and chlorophyll and starch content. Interestingly, biomass, starch content, and seed weight are significantly different between the reciprocal hybrids at all ploidy levels tested, with the lowest and highest levels found in the reciprocal triploid hybrids, suggesting parent-of-origin effects on biomass, starch content, and seed weight. These findings provide new insights into molecular events of polyploidy and heterosis, as well as complex agronomic traits that are important to biomass and seed production in hybrid and polyploid crops.

Life history shapes trait heredity by accumulation of loss-of-function alleles in yeast

A fundamental question in biology is whether variation in organisms primarily emerges as a function of adaptation or as a function of neutral genetic drift. Trait variation in the model organism baker's yeast follows population bottlenecks rather than environmental boundaries suggesting that it primarily results from genetic drift. Based on the yeast life history, we hypothesized that population-specific loss-of-function mutations emerging in genes recently released from selection is the predominant cause of trait variation within the species. As retention of one functional copy of a gene in diploid yeasts is typically sufficient to maintain completely unperturbed performance, we also conjectured that a crossing of natural yeasts from populations with different loss-of-function mutations would provide a further efficient test bed for this hypothesis. Charting the first species-wide map of trait inheritance in a eukaryotic organism, we found trait heredity to be strongly biased toward diploid hybrid performance exactly mimicking the performance of the best of the parents, as expected given a complete dominance of functional over nonfunctional alleles. Best parent heterosis, partial dominance, and negative nonadditivity were all rare phenomena. Nonadditive inheritance was observed primarily in crosses involving at least one very poor performing parent, most frequently of the West African population, and when molecularly dissected, loss-of-function alleles were identified as the underlying cause. These findings provide support for that population-specific loss-of-function mutations do have a strong impact on genotype-phenotype maps and underscores the role of neutral genetic drift as a driver for trait variation within species.

An Evaluation of Arabidopsis thaliana Hybrid Traits and Their Genetic Control

Heterosis is an important phenomenon in agriculture. However, heterosis often greatly varies among hybrids and among traits. To investigate heterosis across a large number of traits and numerous genotypes, we evaluated 12 life history traits on parents and hybrids derived from five Arabidopsis thaliana ecotypes (Col, Ler-0, Cvi, Ws, and C24) by using a complete diallel analysis containing 20 hybrids. Parental contributions to heterosis were hybrid and trait specific with a few reciprocal differences. Most notably, C24 generated hybrids with flowering time, biomass, and reproductive traits that often exceeded high-parent values. However, reproductive traits of C24 and Col hybrids and flowering time traits of C24 and Ler hybrids had no heterosis. We investigated whether allelic variation at flowering time genes FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) could explain the genotype- and trait-specific contribution of C24 to hybrid traits. We evaluated both Col and Ler lines introgressed with various FRI and FLC alleles and hybrids between these lines and C24. Hybrids with functional FLC differed from hybrids with nonfunctional FLC for 21 of the 24 hybrid-trait combinations. In most crosses, heterosis was fully or partially explained by FRI and FLC. Our results describe the genetic diversity for heterosis within a sample of A. thaliana ecotypes and show that FRI and FLC are major factors that contribute to heterosis in a genotype and trait specific fashion.

Paternal effects in Arabidopsis indicate that offspring can influence their own size

The existence of genetic variation in offspring size in plants and animals is puzzling because offspring size is often strongly associated with fitness and expected to be under stabilizing selection. An explanation for variation in seed size is conflict between parents and between parents and offspring. However, for this hypothesis to be true, it must be shown that the offspring genotype can affect its own size. The existence of paternal effects would support this hypothesis, but these have rarely been shown. Using a diallel cross among four natural accessions of Arabidopsis thaliana we show that maternal, paternal and positional effects jointly influence seed size, number and the frequency of seed abortion. We found that seed abortion (%) depends on the combination of maternal and paternal genotypes, suggesting the existence of mate choice or epistatic incompatibility among accessions of A. thaliana. In addition, since paternal genotype explains approximately 10 per cent of the variation in seed size, we propose that A. thaliana's offspring must influence the amount of resources allocated to themselves. Identification of paternal effects in Arabidopsis should facilitate dissection of the genetic mechanisms involved in paternal effects.

Genetic and transformation studies reveal negative regulation of ERS1 ethylene receptor signaling in Arabidopsis

BACKGROUND

Ethylene receptor single mutants of Arabidopsis do not display a visibly prominent phenotype, but mutants defective in multiple ethylene receptors exhibit a constitutive ethylene response phenotype. It is inferred that ethylene responses in Arabidopsis are negatively regulated by five functionally redundant ethylene receptors. However, genetic redundancy limits further study of individual receptors and possible receptor interactions. Here, we examined the ethylene response phenotype in two quadruple receptor knockout mutants, (ETR1) ers1 etr2 ein4 ers2 and (ERS1) etr1 etr2 ein4 ers2, to unravel the functions of ETR1 and ERS1. Their functions were also reciprocally inferred from phenotypes of mutants lacking ETR1 or ERS1. Receptor protein levels are correlated with receptor gene expression. Expression levels of the remaining wild-type receptor genes were examined to estimate the receptor amount in each receptor mutant, and to evaluate if effects of ers1 mutations on the ethylene response phenotype were due to receptor functional compensation. As ers1 and ers2 are in the Wassilewskija (Ws) ecotype and etr1, etr2, and ein4 are in the Columbia (Col-0) ecotype, possible effects of ecotype mixture on ethylene responses were also investigated.

RESULTS

Ethylene responses were scored based on seedling hypocotyl measurement, seedling and rosette growth, and relative Chitinase B (CHIB) expression. Addition of ers1 loss-of-function mutations to any ETR1-containing receptor mutants alleviated ethylene growth inhibition. Growth recovery by ers1 mutation was reversed when the ers1 mutation was complemented by ERS1p:ERS1. The addition of the ers2-3 mutation to receptor mutants did not reverse the growth inhibition. Overexpressing ERS1 receptor protein in (ETR1 ERS1)etr2 ein4 ers2 substantially elevated growth inhibition and CHIB expression. Receptor gene expression analyses did not favor receptor functional compensation upon the loss of ERS1.

CONCLUSIONS

Our results suggest that ERS1 has dual functions in the regulation of ethylene responses. In addition to repressing ethylene responses, ERS1 also promotes ethylene responses in an ETR1-dependent manner. Several lines of evidence support the argument that ecotype mixture does not reverse ethylene responses. Loss of ERS1 did not lead to an increase in total receptor gene expression, and functional compensation was not observed. The inhibitory effects of ERS1 on the ethylene signaling pathway imply negative receptor collaboration.

Predicting Arabidopsis freezing tolerance and heterosis in freezing tolerance from metabolite composition

Heterosis, or hybrid vigor, is one of the most important tools in plant breeding and has previously been demonstrated for plant freezing tolerance. Freezing tolerance is an important trait because it can limit the geographical distribution of plants and their agricultural yield. Plants from temperate climates increase in freezing tolerance during exposure to low, non-freezing temperatures in a process termed 'cold acclimation'. Metabolite profiling has indicated a major reprogramming of plant metabolism in the cold, but it has remained unclear in previous studies which of these changes are related to freezing tolerance. In the present study, we have used metabolic profiling to discover combinations of metabolites that predict freezing tolerance and its heterosis in Arabidopsis thaliana. We identified compatible solutes and, in particular, the pathway leading to raffinose as crucial statistical predictors for freezing tolerance and its heterosis, while some TCA cycle intermediates contribute only to predicting the heterotic phenotype. This indicates coordinate links between heterosis and metabolic pathways, suggesting that a limited number of regulatory genes may determine the extent of heterosis in this complex trait. In addition, several unidentified metabolites strongly contributed to the prediction of both freezing tolerance and its heterosis and we present an exemplary analysis of one of these, identifying it as a hexose conjugate.

QTL analysis of early stage heterosis for biomass in Arabidopsis

The main objective of this study was to identify genomic regions involved in biomass heterosis using QTL, generation means, and mode-of-inheritance classification analyses. In a modified North Carolina Design III we backcrossed 429 recombinant inbred line and 140 introgression line populations to the two parental accessions, C24 and Col-0, whose F (1) hybrid exhibited 44% heterosis for biomass. Mid-parent heterosis in the RILs ranged from -31 to 99% for dry weight and from -58 to 143% for leaf area. We detected ten genomic positions involved in biomass heterosis at an early developmental stage, individually explaining between 2.4 and 15.7% of the phenotypic variation. While overdominant gene action was prevalent in heterotic QTL, our results suggest that a combination of dominance, overdominance and epistasis is involved in biomass heterosis in this Arabidopsis cross.

Heterosis is prevalent for multiple traits in diverse maize germplasm

BACKGROUND

Heterosis describes the superior phenotypes observed in hybrids relative to their inbred parents. Maize is a model system for studying heterosis due to the high levels of yield heterosis and commercial use of hybrids.

METHODS

The inbred lines from an association mapping panel were crossed to a common inbred line, B73, to generate nearly 300 hybrid genotypes. Heterosis was evaluated for seventeen phenotypic traits in multiple environments. The majority of hybrids exhibit better-parent heterosis in most of the hybrids measured. Correlations between the levels of heterosis for different traits were generally weak, suggesting that the genetic basis of heterosis is trait-dependent.

CONCLUSIONS

The ability to predict heterosis levels using inbred phenotype or genetic distance between the parents varied for the different traits. For some traits it is possible to explain a significant proportion of the heterosis variation using linear modeling while other traits are more difficult to predict.

Unraveling epistasis with triple testcross progenies of near-isogenic lines

Libraries of near-isogenic lines (NILs) are a powerful plant genetic resource to map quantitative trait loci (QTL). Nevertheless, QTL mapping with NILs is mostly restricted to genetic main effects. Here we propose a two-step procedure to map additive-by-additive digenic epistasis with NILs. In the first step, a generation means analysis of parents, their F(1) hybrid, and one-segment NILs and their triple testcross (TTC) progenies is used to identify in a one-dimensional scan loci exhibiting QTL-by-background interactions. In a second step, one-segment NILs with significant additive-by-additive background interactions are used to produce particular two-segment NILs to test for digenic epistatic interactions between these segments. We evaluated our approach by analyzing a random subset of a genomewide Arabidopsis thaliana NIL library for growth-related traits. The results of our experimental study illustrated the potential of the presented two-step procedure to map additive-by-additive digenic epistasis with NILs. Furthermore, our findings suggested that additive main effects as well as additive-by-additive digenic epistasis strongly influence the genetic architecture underlying growth-related traits of A. thaliana.

Heterosis in the freezing tolerance, and sugar and flavonoid contents of crosses between Arabidopsis thaliana accessions of widely varying freezing tolerance

Heterosis is defined as the increased vigour of hybrids in comparison to their parents. We investigated 24 F(1) hybrid lines of Arabidopsis thaliana generated by reciprocally crossing either C24 or Col with six other parental accessions (Can, Co, Cvi, Ler, Rsch, Te) that differ widely in their freezing tolerance. The crosses differed in the degree of heterosis for freezing tolerance, both in the non-acclimated state and after a 14 d cold acclimation period. Crosses with C24 showed more heterosis than crosses with Col, and heterosis was stronger in acclimated than in non-acclimated plants. Leaf content of soluble sugars and proline showed more deviation from mid-parent values in crosses involving C24 than in those involving Col, and deviations were larger in acclimated than in non-acclimated plants. There were significant correlations between the content of different sugars and leaf freezing tolerance, as well as between heterosis effects in freezing tolerance and sugar content. Flavonoid content and composition varied between accessions, and between non-acclimated and acclimated plants. In the crosses, large deviations from the mid-parent values in the contents of different flavonols occurred, and there were strikingly strong correlations between both flavonol content and freezing tolerance, and between heterosis effects in freezing tolerance and flavonol content.

Heterosis for biomass-related traits in Arabidopsis investigated by quantitative trait loci analysis of the triple testcross design with recombinant inbred lines

Arabidopsis thaliana has emerged as a leading model species in plant genetics and functional genomics including research on the genetic causes of heterosis. We applied a triple testcross (TTC) design and a novel biometrical approach to identify and characterize quantitative trait loci (QTL) for heterosis of five biomass-related traits by (i) estimating the number, genomic positions, and genetic effects of heterotic QTL, (ii) characterizing their mode of gene action, and (iii) testing for presence of epistatic effects by a genomewide scan and marker x marker interactions. In total, 234 recombinant inbred lines (RILs) of Arabidopsis hybrid C24 x Col-0 were crossed to both parental lines and their F1 and analyzed with 110 single-nucleotide polymorphism (SNP) markers. QTL analyses were conducted using linear transformations Z1, Z2, and Z3 calculated from the adjusted entry means of TTC progenies. With Z1, we detected 12 QTL displaying augmented additive effects. With Z2, we mapped six QTL for augmented dominance effects. A one-dimensional genome scan with Z3 revealed two genomic regions with significantly negative dominance x additive epistatic effects. Two-way analyses of variance between marker pairs revealed nine digenic epistatic interactions: six reflecting dominance x dominance effects with variable sign and three reflecting additive x additive effects with positive sign. We conclude that heterosis for biomass-related traits in Arabidopsis has a polygenic basis with overdominance and/or epistasis being presumably the main types of gene action.

Genetic basis of heterosis for growth-related traits in Arabidopsis investigated by testcross progenies of near-isogenic lines reveals a significant role of epistasis

Epistasis seems to play a significant role in the manifestation of heterosis. However, the power of detecting epistatic interactions among quantitative trait loci (QTL) in segregating populations is low. We studied heterosis in Arabidopsis thaliana hybrid C24 x Col-0 by testing near-isogenic lines (NILs) and their triple testcross (TTC) progenies. Our objectives were to (i) provide the theoretical basis for estimating different types of genetic effects with this experimental design, (ii) determine the extent of heterosis for seven growth-related traits, (iii) map the underlying QTL, and (iv) determine their gene action. Two substitution libraries, each consisting of 28 NILs and covering approximately 61 and 39% of the Arabidopsis genome, were assayed by 110 single-nucleotide polymorphism (SNP) markers. With our novel generation means approach 38 QTL were detected, many of which confirmed heterotic QTL detected previously in the same cross with TTC progenies of recombinant inbred lines. Furthermore, many of the QTL were common for different traits and in common with the 58 QTL detected by a method that compares triplets consisting of a NIL, its recurrent parent, and their F(1) cross. While the latter approach revealed mostly (75%) overdominant QTL, the former approach allowed separation of dominance and epistasis by analyzing all materials simultaneously and yielded substantial positive additive x additive effects besides directional dominance. Positive epistatic effects reduced heterosis for growth-related traits in our materials.