A large-scale screen for artificial selection in maize identifies candidate agronomic loci for domestication and crop improvement

Genetic dissection of internode length confers improvement for ideal plant architecture in maize

The optimal plant architecture, characterized by short stature, helps mitigate lodging, enables high-density planting, and facilitates mechanized harvesting. Internode length (IL), a crucial component of plant height in maize, plays a significant role in these processes. However, the genetic mechanisms underlying internode elongation remain poorly understood. In this study, we conducted a genome-wide association study to dissect the genetic architecture of IL in maize. The lengths of five internodes above and below the ear (referred as IL-related traits) were collected across multiple environments, revealing substantial variation. A total of 108 quantitative trait loci (QTL) were associated with 11 IL-related traits, with 17 QTL co-detected by different traits. Notably, three QTL have been selected in maize breeding progress. Three hundred and three genes associated with IL were found to operate through plant hormone signal transduction, receptor activity, and carbon metabolism pathways, influencing internode elongation. ZmIL1, which encodes alcohol dehydrogenase, exhibited a high expression level in internodes during the vegetative stage and has been selected in Chinese modern maize breeding. Additionally, ZmIL2 and ZmIL3 emerged as other crucial regulators of IL. Importantly, ZmIL1 has potential applications in maize varieties in the Huang-Huai-Hai region. This study represents the first comprehensive report on the genetic architecture of nearly all ILs in maize, providing profound insights into internode elongation mechanisms and genetic resources. These findings hold significant implications for dwarf breeding programs aimed at optimizing plant architecture for enhancing agronomic performance.

Cultivar-Specific Defense Responses in Wild and Cultivated Squash Induced by Belowground and Aboveground Herbivory

Plant domestication often alters plant traits, including chemical and physical defenses against herbivores. In squash, domestication leads to reduced levels of cucurbitacins and leaf trichomes, influencing interactions with insects. However, the impact of domestication on inducible defenses in squash remains poorly understood. Here, we investigated the chemical and physical defensive traits of wild and domesticated squash (Cucurbita argyrosperma), and compared their responses to belowground and aboveground infestation by the root-feeding larvae and the leaf-chewing adults of the banded cucumber beetle Diabrotica balteata (Coleoptera: Chrysomelidae). Wild populations contained cucurbitacins in roots and cotyledons but not in leaves, whereas domesticated varieties lacked cucurbitacins in all tissues. Belowground infestation by D. balteata larvae did not increase cucurbitacin levels in the roots but triggered the expression of cucurbitacin biosynthetic genes, irrespective of domestication status, although the response varied among different varieties. Conversely, whereas wild squash had more leaf trichomes than domesticated varieties, the induction of leaf trichomes in response to herbivory was greater in domesticated plants. Leaf herbivory varied among varieties but there was a trend of higher leaf damage on wild squash than domesticated varieties. Overall, squash plants responded to both belowground and aboveground herbivory by activating chemical defense-associated gene expression in roots and upregulating their physical defense in leaves, respectively. While domestication suppressed both chemical and physical defenses, our findings suggest that it may enhance inducible defense mechanisms by increasing trichome induction in response to herbivory.

Whole-genome sequencing and variant discovery of Citrus reticulata "Kinnow" from Pakistan

Citrus is a source of nutritional and medicinal advantages, cultivated worldwide with major groups of sweet oranges, mandarins, grapefruits, kumquats, lemons and limes. Pakistan produces all major citrus groups with mandarin (Citrus reticulata) being the prominent group that includes local commercial cultivars Feutral's Early, Dancy, Honey, and Kinnow. The present study designed to understand the genetic architecture of this unique variety of Citrus reticulata 'Kinnow.' The whole-genome resequencing and variant calling was performed to map the genomic variability that might be responsible for its particular characteristics like taste, seedlessness, juice content, thickness of peel, and shelf-life. A total of 139,436,350 raw sequence reads were generated with 20.9 Gb data in Fastq format having 98% effectiveness and 0.2% base call error rate. Overall, 3,503,033 SNPs, 176,949 MNPs, 323,287 INS, and 333,083 DEL were identified using the GATK4 variant calling pipeline against Citrus clementina. Furthermore, g:Profiler was applied for annotating the newly found variants, harbor genes/transcripts and their involved pathways. A total of 73,864 transcripts harbors 4,336,352 variants, most of the observed variants were predicted in non-coding regions and 1009 transcripts were found well annotated by different databases. Out of total aforementioned transcripts, 588 involved in biological processes, 234 in molecular functions and 167 transcripts in cellular components. In a nutshell, 18,153 high impact variants and 216 genic variants found in the current study, which may be used after its functional validation for marker-assisted breeding programs of "Kinnow" to propagate its valued traits for the improvement of contemporary citrus varieties in the region.

Genotyping of DNA pools identifies untapped landraces and genomic regions to develop next-generation varieties

Landraces, that is, traditional varieties, have a large diversity that is underexploited in modern breeding. A novel DNA pooling strategy was implemented to identify promising landraces and genomic regions to enlarge the genetic diversity of modern varieties. As proof of concept, DNA pools from 156 American and European maize landraces representing 2340 individuals were genotyped with an SNP array to assess their genome-wide diversity. They were compared to elite cultivars produced across the 20th century, represented by 327 inbred lines. Detection of selective footprints between landraces of different geographic origin identified genes involved in environmental adaptation (flowering times, growth) and tolerance to abiotic and biotic stress (drought, cold, salinity). Promising landraces were identified by developing two novel indicators that estimate their contribution to the genome of inbred lines: (i) a modified Roger's distance standardized by gene diversity and (ii) the assignation of lines to landraces using supervised analysis. It showed that most landraces do not have closely related lines and that only 10 landraces, including famous landraces as Reid's Yellow Dent, Lancaster Surecrop and Lacaune, cumulated half of the total contribution to inbred lines. Comparison of ancestral lines directly derived from landraces with lines from more advanced breeding cycles showed a decrease in the number of landraces with a large contribution. New inbred lines derived from landraces with limited contributions enriched more the haplotype diversity of reference inbred lines than those with a high contribution. Our approach opens an avenue for the identification of promising landraces for pre-breeding.

Genetic Diversification of Starch Branching Enzymes during Maize Domestication and Improvement

Elucidating the genetic basis of starch pasting and gelatinization properties is crucial for enhancing the quality of maize and its utility as feed and industrial raw material. In maize, ZmSBE genes encode important starch branching enzymes in the starch biosynthesis pathway. In this study, we re-sequenced the genomic sequences of ZmSBEI, ZmSBEIIa, ZmSBEIIb, and ZmSBEIII in three lines called 335 inbred lines, 68 landrace lines, and 32 teosinte lines. Analyses of nucleotide polymorphisms and haplotype diversity revealed differences in the selection patterns of ZmSBEI, ZmSBEIIa, ZmSBEIIb, and ZmSBEIII during maize domestication and improvement. A marker-trait association analysis of inbred lines detected 22 significant loci, including 18 SNPs and 4 indels significantly associated with three maize starch physicochemical properties. The allele frequencies of two variants (SNP17249C and SNP5055G) were examined in three lines. The frequency of SNP17249C in ZmSBEIIb was highest in teosinte lines, followed by landrace lines, and inbred lines, whereas there were no significant differences in the frequency of SNP5055G in ZmSBEIII among the three lines. These results suggest that ZmSBE genes play an important role in the phenotypic variations in the starch physicochemical properties in maize. The genetic variants detected in this study may be used to develop functional markers for improving maize starch quality.

Gene network simulations provide testable predictions for the molecular domestication syndrome

The domestication of plant species lead to repeatable morphological evolution, often referred to as the phenotypic domestication syndrome. Domestication is also associated with important genomic changes, such as the loss of genetic diversity compared to adequately large wild populations, and modifications of gene expression patterns. Here, we explored theoretically the effect of a domestication-like scenario on the evolution of gene regulatory networks. We ran population genetics simulations in which individuals were featured by their genotype (an interaction matrix encoding a gene regulatory network) and their gene expressions, representing the phenotypic level. Our domestication scenario included a population bottleneck and a selection switch mimicking human-mediated directional and canalizing selection, i.e., change in the optimal gene expression level and selection towards more stable expression across environments. We showed that domestication profoundly alters genetic architectures. Based on four examples of plant domestication scenarios, our simulations predict (i) a drop in neutral allelic diversity, (ii) a change in gene expression variance that depends upon the domestication scenario, (iii) transient maladaptive plasticity, (iv) a deep rewiring of the gene regulatory networks, with a trend towards gain of regulatory interactions, and (v) a global increase in the genetic correlations among gene expressions, with a loss of modularity in the resulting coexpression patterns and in the underlying networks. We provide empirically testable predictions on the differences of genetic architectures between wild and domesticated forms. The characterization of such systematic evolutionary changes in the genetic architecture of traits contributes to define a molecular domestication syndrome.

Haplotype Variations and Evolutionary Analysis of the Granule-Bound Starch Synthase I Gene in the Korean World Rice Collection

Granule-bound starch synthase I (GBSSI) is responsible for Waxy gene encoding the, which is involved in the amylose synthesis step of starch biosynthesis. We investigated the genotypic and haplotypic variations of GBSSI (Os06g0133000) gene, including its evolutionary relatedness in the nucleotide sequence level using single-nucleotide polymorphisms (SNPs), indels, and structural variations (SVs) from 475 Korean World Rice Collection (KRICE_CORE), which comprised 54 wild rice and 421 cultivated represented by 6 ecotypes (temperate japonica, indica, tropical japonica, aus, aromatic, and admixture) or in another way by 3 varietal types (landrace, weedy, and bred). The results revealed that 27 of 59 haplotypes indicated a total of 12 functional SNPs (fSNPs), identifying 9 novel fSNPs. According to the identified novel fSNPs, we classified the entire rice collection into three groups: cultivated, wild, and mixed (cultivated and wild) rice. Five novel fSNPs were localized in wild rice: four G/A fSNPs in exons 2, 9, and 12 and one T/C fSNP in exon 13. We also identified the three previously reported fSNPs, namely, a G/A fSNP (exon 4), an A/C fSNP (exon 6), and a C/T fSNP (exon 10), which were observed only in cultivated rice, whereas an A/G fSNP (exon 4) was observed exclusively in wild rice. All-against-all comparison of four varietal types or six ecotypes of cultivated rice with wild rice showed that the GBSSI diversity was higher only in wild rice (π = 0.0056). The diversity reduction in cultivated rice can be useful to encompass the origin of this gene GBSSI during its evolution. Significant deviations of positive (wild and indica under balancing selection) and negative (temperate and tropical japonica under purifying selection) Tajima's D values from a neutral model can be informative about the selective sweeps of GBSSI genome insights. Despite the estimation of the differences in population structure and principal component analysis (PCA) between wild and subdivided cultivated subgroups, an inbreeding effect was quantified by F _ST statistic, signifying the genetic relatedness of GBSSI. Our findings of a novel wild fSNPS can be applicable for future breeding of waxy rice varieties. Furthermore, the signatures of selective sweep can also be of informative into further deeper insights during domestication.

Population genomics of apricots unravels domestication history and adaptive events

Among crop fruit trees, the apricot (Prunus armeniaca) provides an excellent model to study divergence and adaptation processes. Here, we obtain nearly 600 Armeniaca apricot genomes and four high-quality assemblies anchored on genetic maps. Chinese and European apricots form two differentiated gene pools with high genetic diversity, resulting from independent domestication events from distinct wild Central Asian populations, and with subsequent gene flow. A relatively low proportion of the genome is affected by selection. Different genomic regions show footprints of selection in European and Chinese cultivated apricots, despite convergent phenotypic traits, with predicted functions in both groups involved in the perennial life cycle, fruit quality and disease resistance. Selection footprints appear more abundant in European apricots, with a hotspot on chromosome 4, while admixture is more pervasive in Chinese cultivated apricots. Our study provides clues to the biology of selected traits and targets for fruit tree research and breeding.

Insights into the genomic evolution of insects from cricket genomes

Most of our knowledge of insect genomes comes from Holometabolous species, which undergo complete metamorphosis and have genomes typically under 2 Gb with little signs of DNA methylation. In contrast, Hemimetabolous insects undergo the presumed ancestral process of incomplete metamorphosis, and have larger genomes with high levels of DNA methylation. Hemimetabolous species from the Orthopteran order (grasshoppers and crickets) have some of the largest known insect genomes. What drives the evolution of these unusual insect genome sizes, remains unknown. Here we report the sequencing, assembly and annotation of the 1.66-Gb genome of the Mediterranean field cricket Gryllus bimaculatus, and the annotation of the 1.60-Gb genome of the Hawaiian cricket Laupala kohalensis. We compare these two cricket genomes with those of 14 additional insects and find evidence that hemimetabolous genomes expanded due to transposable element activity. Based on the ratio of observed to expected CpG sites, we find higher conservation and stronger purifying selection of methylated genes than non-methylated genes. Finally, our analysis suggests an expansion of the pickpocket class V gene family in crickets, which we speculate might play a role in the evolution of cricket courtship, including their characteristic chirping.

Ylla, Extavour et al. use genomic data from crickets to investigate the evolution of large genome sizes and DNA methylation events in insects. Their findings indicate that transposable element activity drove genome expansion in hemimetabolous insects, such as crickets and grasshoppers, and that DNA methylation is predominant in conserved genes.

The genetic mechanism of heterosis utilization in maize improvement

BACKGROUND

In maize hybrid breeding, complementary pools of parental lines with reshuffled genetic variants are established for superior hybrid performance. To comprehensively decipher the genetics of heterosis, we present a new design of multiple linked F1 populations with 42,840 F1 maize hybrids, generated by crossing a synthetic population of 1428 maternal lines with 30 elite testers from diverse genetic backgrounds and phenotyped for agronomic traits.

RESULTS

We show that, although yield heterosis is correlated with the widespread, minor-effect epistatic QTLs, it may be resulted from a few major-effect additive and dominant QTLs in early developmental stages. Floral transition is probably one critical stage for heterosis formation, in which epistatic QTLs are activated by paternal contributions of alleles that counteract the recessive, deleterious maternal alleles. These deleterious alleles, while rare, epistatically repress other favorable QTLs. We demonstrate this with one example, showing that Brachytic2 represses the Ubiquitin3 locus in the maternal lines; in hybrids, the paternal allele alleviates this repression, which in turn recovers the height of the plant and enhances the weight of the ear. Finally, we propose a molecular design breeding by manipulating key genes underlying the transition from vegetative-to-reproductive growth.

CONCLUSION

The new population design is used to dissect the genetic basis of heterosis which accelerates maize molecular design breeding by diminishing deleterious epistatic interactions.

Variety Discrimination Power: An Appraisal Index for Loci Combination Screening Applied to Plant Variety Discrimination

Molecular marker technology is used widely in plant variety discrimination, molecular breeding, and other fields. To lower the cost of testing and improve the efficiency of data analysis, molecular marker screening is very important. Screening usually involves two phases: the first to control loci quality and the second to reduce loci quantity. To reduce loci quantity, an appraisal index that is very sensitive to a specific scenario is necessary to select loci combinations. In this study, we focused on loci combination screening for plant variety discrimination. A loci combination appraisal index, variety discrimination power (VDP), is proposed, and three statistical methods, probability-based VDP (P-VDP), comparison-based VDP (C-VDP), and ratio-based VDP (R-VDP), are described and compared. The results using the simulated data showed that VDP was sensitive to statistical populations with convergence toward the same variety, and the total probability of discrimination power (TDP) method was effective only for partial populations. R-VDP was more sensitive to statistical populations with convergence toward various varieties than P-VDP and C-VDP, which both had the same sensitivity; TDP was not sensitive at all. With the real data, R-VDP values for sorghum, wheat, maize and rice data begin to show downward tendency when the number of loci is 20, 7, 100, 100 respectively, while in the case of P-VDP and C-VDP (which have the same results), the number is 6, 4, 9, 19 respectively and in the case of TDP, the number is 6, 4, 4, 11 respectively. For the variety threshold setting, R-VDP values of loci combinations with different numbers of loci responded evenly to different thresholds. C-VDP values responded unevenly to different thresholds, and the extent of the response increased as the number of loci decreased. All the methods gave underestimations when data were missing, with systematic errors for TDP, C-VDP, and R-VDP going from smallest to biggest. We concluded that VDP was a better loci combination appraisal index than TDP for plant variety discrimination and the three VDP methods have different applications. We developed the software called VDPtools, which can calculate the values of TDP, P-VDP, C-VDP, and R-VDP. VDPtools is publicly available at https://github.com/caurwx1/VDPtools.git.

Enhancing grain-yield-related traits by CRISPR-Cas9 promoter editing of maize CLE genes

Several yield-related traits selected during crop domestication and improvement^1,2 are associated with increases in meristem size³, which is controlled by CLE peptide signals in the CLAVATA-WUSCHEL pathway^4-13. Here, we engineered quantitative variation for yield-related traits in maize by making weak promoter alleles of CLE genes, and a null allele of a newly identified partially redundant compensating CLE gene, using CRISPR-Cas9 genome editing. These strategies increased multiple maize grain-yield-related traits, supporting the enormous potential for genomic editing in crop enhancement.

Tomato Domestication Attenuated Responsiveness to a Beneficial Soil Microbe for Plant Growth Promotion and Induction of Systemic Resistance to Foliar Pathogens

Crop domestication events followed by targeted breeding practices have been pivotal for improvement of desirable traits and to adapt cultivars to local environments. Domestication also resulted in a strong reduction in genetic diversity among modern cultivars compared to their wild relatives, though the effect this could have on tripartite relationships between plants, belowground beneficial microbes and aboveground pathogens remains undetermined. We quantified plant growth performance, basal resistance and induced systemic resistance (ISR) by Trichoderma harzianum, a beneficial soil microbe against Botrytis cinerea, a necrotrophic fungus and Phytophthora infestans, a hemi-biotrophic oomycete, in 25 diverse tomato genotypes. Wild tomato related species, tomato landraces and modern commercial cultivars that were conventionally or organically bred, together, representing a domestication gradient were evaluated. Relationships between basal and ISR, plant physiological status and phenolic compounds were quantified to identify potential mechanisms. Trichoderma enhanced shoot and root biomass and ISR to both pathogens in a genotype specific manner. Moreover, improvements in plant performance in response to Trichoderma gradually decreased along the domestication gradient. Wild relatives and landraces were more responsive to Trichoderma, resulting in greater suppression of foliar pathogens than modern cultivars. Photosynthetic rate and stomatal conductance of some tomato genotypes were improved by Trichoderma treatment whereas leaf nitrogen status of the majority of tomato genotypes were not altered. There was a negative relationship between basal resistance and induced resistance for both diseases, and a positive correlation between Trichoderma-ISR to B. cinerea and enhanced total flavonoid contents. These findings suggest that domestication and breeding practices have altered plant responsiveness to beneficial soil microbes. Further studies are needed to decipher the molecular mechanisms underlying the differential promotion of plant growth and resistance among genotypes, and identify molecular markers to integrate selection for responsiveness into future breeding programs.

Identification of Ear Morphology Genes in Maize (Zea mays L.) Using Selective Sweeps and Association Mapping

The performance of maize hybrids largely depend on two parental inbred lines. Improving inbred lines using artificial selection is a key task in breeding programs. However, it is important to elucidate the effects of this selection on inbred lines. Altogether, 208 inbred lines from two maize heterosis groups, named Shaan A and Shaan B, were sequenced by the genotype-by-sequencing to detect genomic changes under selection pressures. In addition, we completed genome-wide association analysis in 121 inbred lines to identify candidate genes for ear morphology related traits. In a genome-wide selection scan, the inbred lines from Shaan A and Shaan B groups showed obvious population divergences and different selective signals distributed in 337 regions harboring 772 genes. Meanwhile, functional enrichment analysis showed those selected genes are mainly involved in regulating cell development. Interestingly, some ear morphology related traits showed significant differentiation between the inbred lines from the two heterosis groups. The genome-wide association analysis of ear morphology related traits showed that four associated genes were co-localized in the selected regions with high linkage disequilibrium. Our spatiotemporal pattern and gene interaction network results for the four genes further contribute to our understanding of the mechanisms behind ear and fruit length development. This study provides a novel insight into digging a candidate gene for complex traits using breeding materials. Our findings in relation to ear morphology will help accelerate future maize improvement.

Current State and Perspectives in Population Genomics of the Common Bean

* Correspondence: r [...].

Defining the Role of the MADS-Box Gene, Zea Agamous-like1, a Target of Selection During Maize Domestication

Genomic scans for genes that show the signature of past selection have been widely applied to a number of species and have identified a large number of selection candidate genes. In cultivated maize (Zea mays ssp. mays) selection scans have identified several hundred candidate domestication genes by comparing nucleotide diversity and differentiation between maize and its progenitor, teosinte (Z. mays ssp. parviglumis). One of these is a gene called zea agamous-like1 (zagl1), a MADS-box transcription factor, that is known for its function in the control of flowering time. To determine the trait(s) controlled by zagl1 that was (were) the target(s) of selection during maize domestication, we created a set of recombinant chromosome isogenic lines that differ for the maize versus teosinte alleles of zagl1 and which carry cross-overs between zagl1 and its neighbor genes. These lines were grown in a randomized trial and scored for flowering time and domestication related traits. The results indicated that the maize versus teosinte alleles of zagl1 affect flowering time as expected, as well as multiple traits related to ear size with the maize allele conferring larger ears with more kernels. Our results suggest that zagl1 may have been under selection during domestication to increase the size of the maize ear.

Natural Variation and Domestication Selection of ZmPGP1 Affects Plant Architecture and Yield-Related Traits in Maize

ZmPGP1, involved in the polar auxin transport, has been shown to be associated with plant height, leaf angle, yield traits, and root development in maize. To explore natural variation and domestication selection of ZmPGP1, we re-sequenced the ZmPGP1 gene in 349 inbred lines, 68 landraces, and 32 teosintes. Sequence polymorphisms, nucleotide diversity, and neutral tests revealed that ZmPGP1 might be selected during domestication and improvement processes. Marker–trait association analysis in inbred lines identified 11 variants significantly associated with 4 plant architecture and 5 ear traits. SNP1473 was the most significant variant for kernel length and ear grain weight. The frequency of an increased allele T was 40.6% in teosintes, and it was enriched to 60.3% and 89.1% during maize domestication and improvement. This result revealed that ZmPGP1 may be selected in the domestication and improvement process, and significant variants could be used to develop functional markers to improve plant architecture and ear traits in maize.

Comparative population genomics unveils candidate genes for athletic performance in Hanoverians

Equine athletes have a genetic heritage that has been evolved for millions of years, which provides an opportunity to study the genetics of locomotion pattern and performance in mammals. The Hanoverian, a breed originating in Germany, is arguably among the most athletic of horse breeds, as well as possessing a balanced character and beautiful appearance. Here, we compared the whole genomes of Hanoverian with three other horse breeds (Akhal-Teke, Franches-Montagnes, and Standardbred), using the fixation index (Fst) and cross-population composite likelihood ratio (XP-CLR) methods for testing the multi-locus allele frequency differentiation between populations. We identified 299 and 485 positively selected genes using the Fst and XP-CLR methods, respectively. Further functional analyses showed that the ACTA1 gene is potentially involved in athletic performance in the Hanoverian breed, consistent with its role observed in human population. In addition, three other loci on chromosomes 1 and 20 were identified to be potentially involved in equine physical performance. The selected candidate genes identified in this study may be useful in current breeding efforts to develop improved breeds in regard to athletic performance.

Global status of 47 major wheat loci controlling yield, quality, adaptation and stress resistance selected over the last century

BACKGROUND

Wheat breeding over the last 100 years has increased productivity by adapting genotypes to local conditions, but the genomic changes and selection signals that caused phenotypic change during breeding are essentially unknown. Studying and understanding human selection of multiple important genes controlling key phenotypic traits will promote wheat molecular breeding.

RESULTS

A total of 1152 diverse global wheat materials were genotyped based on KASP markers from 47 genes controlling grain yield, grain quality, adaptation, and stress resistance. Significant phenotypic variations between landraces and modern cultivars were found in 11 adaptive and yield-related traits. Thirty-six improvement-selective favorable alleles, including 22 positive prolonged and 14 negative selection alleles, were identified through comparing frequency spectra. Sus1-7A-Hap-H, Sus1-7B-Hap-T, Sus2-2A-Hap-A, TGW6-A1a, Cwi-4A-Hap-C, vrn-A1, PHS1-PHS+ and Lr34+ were subjected to strong selection, and overwhelmingly strong selection had occurred before improvement selection at Psy-A1b, Psy-B1a or b, Psy-D1a and Cwi-5D-Hap-C. However, Rht-B1b, Rht-D1b and 1BL.1RS were rare or absent in Chinese landraces but present in modern Chinese cultivars and introduced accessions. Importantly, Lr68+, Fhb1+, Wx-B1b and Yr15+, currently existing at a low frequency, should be regarded as further major improvement targets in global wheat breeding. Gene flow analysis showed that introduced cultivars especially from the former USSR and Italy contributed to enriched genetic variation in modern Chinese cultivars.

CONCLUSIONS

This work objectively reports human selection on favorable alleles of multiple crucial genes in Asia, Europe, North America and CIMMYT, and traces the distribution of important genes in global wheat for molecular breeding.

Comparative mapping of quantitative trait loci for tassel-related traits of maize in F2:3 and RIL populations

Tassel architecture is an important trait in maize breeding and hybrid seed production. In this study, we investigated total tassel length (TTL) and tassel branch number (TBN) in 266 F_2:3 families across six environments and in 301 recombinant inbred lines (RILs) across three environments, where all the plants were derived from a cross between 08-641 and Ye478. We compared the genetic architecture of the two traits across two generations through combined analysis. In total, 27 quantitative trait loci (QTLs) (15 in F_2:3; 16 in RIL), two QTL × environment interactions (both in F_2:3), 11 pairs of epistatic interactions (seven in F_2:3; four in RIL) and four stable QTLs in both the F_2:3 and RILs were detected. The RIL population had higher detection power than the F_2:3 population. Nevertheless, QTL × environment interactions and epistatic interactions could be more easily detected in the F_2:3 population than in the RILs. Overall, the QTL mapping results in the F_2:3 and RILs were greatly influenced by genetic generations and environments. Finally, fine mapping for a novel and major QTL, qTTL-2-3 (bin 2.07), which accounted for over 8.49% of the phenotypic variation across different environments and generations, could be useful in marker-assisted breeding.

Fine-tuning the 'plant domestication-reduced defense' hypothesis: specialist vs generalist herbivores

Domesticated plants are assumed to have weakened chemical defenses. We argue, however, that artificial selection will have maintained defense traits against specialized herbivores that have coexisted with the crops throughout their domestication. We assessed the performance of eight species of insect herbivores from three feeding guilds on six European maize lines and six populations of their wild ancestor, teosinte. A metabolomics approach was used in an attempt to identify compounds responsible for observed differences in insect performance. Insects consistently performed better on maize than on teosinte. As hypothesized, this difference was greater for generalist herbivores that are normally not found on teosinte. We also found clear differences in defense metabolites among the different genotypes, but none that consistently correlated with differences in performance. Concentrations of benzoxazinoids, the main chemical defense in maize, tended to be higher in leaves of teosinte, but the reverse was true for the roots. It appears that chemical defenses that target specialized insects are still present at higher concentrations in cultivated maize than compounds that are more effective against generalists. These weakened broad-spectrum defenses in crops may explain the successes of novel pests.

QTL mapping and comparative genome analysis of agronomic traits including grain yield in winter rye

Genetic diversity in elite rye germplasm as well as F _2:3 testcross design enables fast QTL mapping to approach genes controlling grain yield, grain weight, tiller number and heading date in rye hybrids. Winter rye (Secale cereale L.) is a multipurpose cereal crop closely related to wheat, which offers the opportunity for a sustainable production of food and feed and which continues to emerge as a renewable energy source for the production of bioethanol and biomethane. Rye contributes to increase agricultural crop species diversity particularly in Central and Eastern Europe. In contrast to other small grain cereals, knowledge on the genetic architecture of complex inherited, agronomic important traits is yet limited for the outbreeding rye. We have performed a QTL analysis based on a F_2:3 design and testcross performance of 258 experimental hybrids in multi-environmental field trials. A genetic linkage map covering 964.9 cM based on SSR, conserved-orthologous set (COS), and mixed-phase dominant DArT markers allowed to describe 22 QTL with significant effects for grain yield, heading date, tiller number, and thousand grain weight across seven environments. Using rye COS markers, orthologous segments for these traits have been identified in the rice genome, which carry cloned and functionally characterized rice genes. The initial genome scan described here together with the existing knowledge on candidate genes provides the basis for subsequent analyses of the genetic and molecular mechanisms underlying agronomic important traits in rye.

Differential expression of a WRKY gene between wild and cultivated soybeans correlates to seed size

Soybean (Glycine max) probably originated from the wild soybean (Glycine soja). Glycine max has a significantly larger seed size, but the underlying genomic changes are largely unknown. Candidate regulatory genes were preliminarily proposed by data co-localizing RNA sequencing with the quantitative loci (QTLs) for seed size. The soybean gene locus SoyWRKY15a and its orthologous genes from G. max (GmWRKY15a) and G. soja (GsWRKY15a) were analyzed in detail. The coding sequences were nearly identical between the two orthologs, but GmWRKY15a was significantly more highly expressed than GsWRKY15a. Four haplotypes (H1-H4) were found and they varied in the size of a CT-core microsatellite locus in the 5'-untranslated region of this gene. H1 (with six CT-repeats) was the only allelic version found in G. max, while H3 (with five CT-repeats) was the dominant G. soja allele. Differential expression of this gene in soybean pods was correlated with CT-repeat variation, and manipulation of the CT copy number altered the reporter gene expression, suggesting a regulatory role for the simple sequence repeats. Seed weight of wild soybeans harboring H1 was significantly greater than that of soybeans having haplotypes H2, H3, or H4, and seed weight was correlated with gene expression, suggesting the influence of GsWRKY15a in controlling seed size. However, the seed size might be refractory to increased SoyWRKY15a expression in cultivated soybeans. The evolutionary significance of SoyWRKY15a variation in soybean seed domestication is discussed.

Finding a Needle in a Haystack: Distinguishing Mexican Maize Landraces Using a Small Number of SNPs

In Mexico's territory, the center of origin and domestication of maize (Zea mays), there is a large phenotypic diversity of this crop. This diversity has been classified into “landraces.” Previous studies have reported that genomic variation in Mexican maize is better explained by environmental factors, particularly those related with altitude, than by landrace. Still, landraces are extensively used by agronomists, who recognize them as stable and discriminatory categories for the classification of samples. In order to investigate the genomic foundation of maize landraces, we analyzed genomic data (35,909 SNPs from Illumina MaizeSNP50 BeadChip) obtained from 50 samples representing five maize landraces (Comiteco, Conejo, Tehua, Zapalote Grande, and Zapalote Chico), and searched for markers suitable for landrace assignment. Landrace clusters could not be identified taking all the genomic information, but they become manifest taking only a subset of SNPs with high F_ST among landraces. Discriminant analysis of principal components was conducted to classify samples using SNP data. Two classification analyses were done, first classifying samples by landrace and then by altitude category. Through this classification method, we identified 20 landrace-informative SNPs and 14 altitude-informative SNPs, with only 6 SNPs in common for both analyses. These results show that Mexican maize phenotypic diversity can be classified in landraces using a small number of genomic markers, given the fact that landrace genomic diversity is influenced by environmental factors as well as artificial selection due to bio-cultural practices.

Genome-wide study of an elite rice pedigree reveals a complex history of genetic architecture for breeding improvement

Improving breeding has been widely utilized in crop breeding and contributed to yield and quality improvement, yet few researches have been done to analyze genetic architecture underlying breeding improvement comprehensively. Here, we collected genotype and phenotype data of 99 cultivars from the complete pedigree including Huanghuazhan, an elite, high-quality, conventional indica rice that has been grown over 4.5 million hectares in southern China and from which more than 20 excellent cultivars have been derived. We identified 1,313 selective sweeps (SSWs) revealing four stage-specific selection patterns corresponding to improvement preference during 65 years, and 1113 conserved Huanghuazhan traceable blocks (cHTBs) introduced from different donors and conserved in >3 breeding generations were the core genomic regions for superior performance of Huanghuazhan. Based on 151 quantitative trait loci (QTLs) identified for 13 improved traits in the pedigree, we reproduced their improvement process in silico, highlighting improving breeding works well for traits controlled by major/major + minor effect QTLs, but was inefficient for traits controlled by QTLs with complex interactions or explaining low levels of phenotypic variation. These results indicate long-term breeding improvement is efficient to construct superior genetic architecture for elite performance, yet molecular breeding with designed genotype of QTLs can facilitate complex traits improvement.

Genome-wide indel/SSR scanning reveals significant loci associated with excellent agronomic traits of a cabbage (Brassica oleracea) elite parental line '01-20'

Elite parental lines are of great significance to crop breeding. To discover unique genomic loci associated with excellent economic traits in the elite cabbage inbred-line ‘01–20’, we performed comparisons of phenotypes as well as whole-genome insertion-deletion/simple sequence repeat loci between ‘01–20’ and each of its five sister lines. ‘01–20’ has a range of excellent agronomic traits, including early-maturing, and improvements in plant type and leaf colour. Eight unique loci were discovered for ‘01–20’ and ‘01-07-258’, another elite line similar to ‘01–20’ at the whole-genome level. In addition, two excellent double-haploid lines derived from a cross of ‘01–20’ also inherited these loci. Based on the quantitative trait locus association results, five of these loci were found to be associated with important agronomic traits, which could explain why the elite parent ‘01–20’ possesses greener outer leaves, a more compact and upright plant-type, rounder head, shorter core length, and better taste. Additionally, some of these loci have clustering effects for quantitative trait loci associated with different traits; therefore, important genes in these regions were analysed. The obtained results should enable marker-assisted multi-trait selection at the whole-genome level in cabbage breeding and provide insights into significant genome loci and their breeding effects.

Evolution of meiotic recombination genes in maize and teosinte

Background

Meiotic recombination is a major source of genetic variation in eukaryotes. The role of recombination in evolution is recognized but little is known about how evolutionary forces affect the recombination pathway itself. Although the recombination pathway is fundamentally conserved across different species, genetic variation in recombination components and outcomes has been observed. Theoretical predictions and empirical studies suggest that changes in the recombination pathway are likely to provide adaptive abilities to populations experiencing directional or strong selection pressures, such as those occurring during species domestication. We hypothesized that adaptive changes in recombination may be associated with adaptive evolution patterns of genes involved in meiotic recombination.

Results

To examine how maize evolution and domestication affected meiotic recombination genes, we studied patterns of sequence polymorphism and divergence in eleven genes controlling key steps in the meiotic recombination pathway in a diverse set of maize inbred lines and several accessions of teosinte, the wild ancestor of maize. We discovered that, even though the recombination genes generally exhibited high sequence conservation expected in a pathway controlling a key cellular process, they showed substantial levels and diverse patterns of sequence polymorphism. Among others, we found differences in sequence polymorphism patterns between tropical and temperate maize germplasms. Several recombination genes displayed patterns of polymorphism indicative of adaptive evolution.

Conclusions

Despite their ancient origin and overall sequence conservation, meiotic recombination genes can exhibit extensive and complex patterns of molecular evolution. Changes in these genes could affect the functioning of the recombination pathway, and may have contributed to the successful domestication of maize and its expansion to new cultivation areas.

Electronic supplementary material

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Different responses of soybean cyst nematode resistance between two RIL populations derived from Peking x 7605 under two ecological sites

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a plant-parasitic nematode that feeds on the roots of soybean and most economically devastating pathogen of soybean (Glycine max (L.) Merr.) worldwide. Host plant resistance is the most effective control method. To understand SCN resistance in different environments, two recombinant-inbred lines (RILs) populations NJ(RN)P7 (217 F2:8:11 lines) and JN(RN)P7 (248 F2:7:9 lines) were developed from the cross of the cultivars Peking x 7605 in Nanjing and Jinan, respectively, and examined in this study. Peking is resistant to SCN race 1 (HG types 2.5.7), while 7605 is highly susceptible. Chi-square test of frequency distribution of families' female index (FI) showed that resistance to SCN was significantly different between NJ(RN)P7 and JN(RN)P7 populations. Three recessive genes conditioned the inheritance of resistance to SCN race 1 in both populations, but significant difference was detected for the mean of FI on two populations (DM= -16.68, P< 0.01). This indicated that natural selection may affect resistance to SCN. By analysing the variation of phenotype, the genetic structure of the two populations was determined to be different. The inheritance and variation of resistance were confirmed by simple sequence repeat (SSR) markers. For the two populations, 10 SSR markers showed polymorphism of resistant and susceptible DNA bulks. Some markers associated with the resistance of SCN races 1, 2, 3 and 5, and two markers, Satt163 and Satt309, reportedly related to rgh1 were detected both in NJ(RN)P7 and JN(RN)P7 populations. The results support the view that a disease acts as a selective force on plant resistance characteristics, which may alter the relative fitness of resistance alleles.

Differentiation and description of aromatic short grain rice landraces of eastern Indian state of Odisha based on qualitative phenotypic descriptors

Background

Speciality rice, in general, and aromatic rice in particular, possess enormous market potential for enhancing farm profits. However, systematic characterization of the diversity present in this natural wealth is a major pre requisite for using it in the breeding programs. This study reports qualitative phenotypic trait based characterization of 126 short grain aromatic rice genotypes, collected from different areas of the state of Odisha, India.

Results

Out of the 24 descriptors employed, highest variability (8 different types) was observed for lemma-palea colour with a genetic diversity index (He) of 0.696. The principal component analysis reveals that the tip colour of lemma, colour of awn and colour of stigma, cumulatively explain 74 % of the total variation. The Population STRUCTURE analysis classified the population into two subpopulations which were subdivided further into four distinct groups. The western and southern districts of Odisha are endowed with maximum diversity in comparison to eastern and northern districts and at district level comparisons, Koraput and Puri districts are rich with a genetic diversity values of 0.324 and 0.303 respectively. With this set of morphological qualitative traits, based on ‘phenoprinting’, a newly proposed bar coding system, unique fingerprints of each genotype can be effectively generated that can help in easy identification of these genotypes.

Conclusion

Though aromatic rices represent a tiny fraction of the total rice germplasm, a small collection of 126 land races did exhibit rich diversity for all the qualitative traits. For lemma-palea colour, eight different types were detected while for tip colour of lemma, six different types were recorded, suggesting the presence of rich variability in short grain aromatic rices that are conserved in this region. The proposed ‘phenoprinting’ can be an effective descriptor with the unique finger prints generated for each genotype and coupled with molecular (DNA) finger printing, we can discriminate and identify each and every aromatic short grain rice genotype. The proposed system not only help in conservation but also can confer IPR protection to these specialty rices.

Electronic supplementary material

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A comprehensive study of the genomic differentiation between temperate Dent and Flint maize

BACKGROUND

Dent and Flint represent two major germplasm pools exploited in maize breeding. Several traits differentiate the two pools, like cold tolerance, early vigor, and flowering time. A comparative investigation of their genomic architecture relevant for quantitative trait expression has not been reported so far. Understanding the genomic differences between germplasm pools may contribute to a better understanding of the complementarity in heterotic patterns exploited in hybrid breeding and of mechanisms involved in adaptation to different environments.

RESULTS

We perform whole-genome screens for signatures of selection specific to temperate Dent and Flint maize by comparing high-density genotyping data of 70 American and European Dent and 66 European Flint inbred lines. We find 2.2 % and 1.4 % of the genes are under selective pressure, respectively, and identify candidate genes associated with agronomic traits known to differ between the two pools. Taking flowering time as an example for the differentiation between Dent and Flint, we investigate candidate genes involved in the flowering network by phenotypic analyses in a Dent-Flint introgression library and find that the Flint haplotypes of the candidates promote earlier flowering. Within the flowering network, the majority of Flint candidates are associated with endogenous pathways in contrast to Dent candidate genes, which are mainly involved in response to environmental factors like light and photoperiod. The diversity patterns of the candidates in a unique panel of more than 900 individuals from 38 European landraces indicate a major contribution of landraces from France, Germany, and Spain to the candidate gene diversity of the Flint elite lines.

CONCLUSIONS

In this study, we report the investigation of pool-specific differences between temperate Dent and Flint on a genome-wide scale. The identified candidate genes represent a promising source for the functional investigation of pool-specific haplotypes in different genetic backgrounds and for the evaluation of their potential for future crop improvement like the adaptation to specific environments.

Evolutionary Metabolomics Reveals Domestication-Associated Changes in Tetraploid Wheat Kernels

Domestication and breeding have influenced the genetic structure of plant populations due to selection for adaptation from natural habitats to agro-ecosystems. Here, we investigate the effects of selection on the contents of 51 primary kernel metabolites and their relationships in three Triticum turgidum L. subspecies (i.e., wild emmer, emmer, durum wheat) that represent the major steps of tetraploid wheat domestication. We present a methodological pipeline to identify the signature of selection for molecular phenotypic traits (e.g., metabolites and transcripts). Following the approach, we show that a reduction in unsaturated fatty acids was associated with selection during domestication of emmer (primary domestication). We also show that changes in the amino acid content due to selection mark the domestication of durum wheat (secondary domestication). These effects were found to be partially independent of the associations that unsaturated fatty acids and amino acids have with other domestication-related kernel traits. Changes in contents of metabolites were also highlighted by alterations in the metabolic correlation networks, indicating wide metabolic restructuring due to domestication. Finally, evidence is provided that wild and exotic germplasm can have a relevant role for improvement of wheat quality and nutritional traits.

Genome-wide view of genetic diversity reveals paths of selection and cultivar differentiation in peach domestication

Domestication and cultivar differentiation are requisite processes for establishing cultivated crops. These processes inherently involve substantial changes in population structure, including those from artificial selection of key genes. In this study, accessions of peach (Prunus persica) and its wild relatives were analysed genome-wide to identify changes in genetic structures and gene selections associated with their differentiation. Analysis of genome-wide informative single-nucleotide polymorphism loci revealed distinct changes in genetic structures and delineations among domesticated peach and its wild relatives and among peach landraces and modern fruit (F) and modern ornamental (O-A) cultivars. Indications of distinct changes in linkage disequilibrium extension/decay and of strong population bottlenecks or inbreeding were identified. Site frequency spectrum- and extended haplotype homozygosity-based evaluation of genome-wide genetic diversities supported selective sweeps distinguishing the domesticated peach from its wild relatives and each F/O-A cluster from the landrace clusters. The regions with strong selective sweeps harboured promising candidates for genes subjected to selection. Further sequence-based evaluation further defined the candidates and revealed their characteristics. All results suggest opportunities for identifying critical genes associated with each differentiation by analysing genome-wide genetic diversity in currently established populations. This approach obviates the special development of genetic populations, which is particularly difficult for long-lived tree crops.

Landrace Germplasm for Improving Yield and Abiotic Stress Adaptation

Plant landraces represent heterogeneous, local adaptations of domesticated species, and thereby provide genetic resources that meet current and new challenges for farming in stressful environments. These local ecotypes can show variable phenology and low-to-moderate edible yield, but are often highly nutritious. The main contributions of landraces to plant breeding have been traits for more efficient nutrient uptake and utilization, as well as useful genes for adaptation to stressful environments such as water stress, salinity, and high temperatures. We propose that a systematic landrace evaluation may define patterns of diversity, which will facilitate identifying alleles for enhancing yield and abiotic stress adaptation, thus raising the productivity and stability of staple crops in vulnerable environments.

Genomic basis of the differences between cider and dessert apple varieties

Unraveling the genomic processes at play during variety diversification is of fundamental interest for understanding evolution, but also of applied interest in crop science. It can indeed provide knowledge on the genetic bases of traits for crop improvement and germplasm diversity management. Apple is one of the most important fruit crops in temperate regions, having both great economic and cultural values. Sweet dessert apples are used for direct consumption, while bitter cider apples are used to produce cider. Several important traits are known to differentiate the two variety types, in particular fruit size, biennial versus annual fruit bearing, and bitterness, caused by a higher content in polyphenols. Here, we used an Illumina 8k SNP chip on two core collections, of 48 dessert and 48 cider apples, respectively, for identifying genomic regions responsible for the differences between cider and dessert apples. The genome-wide level of genetic differentiation between cider and dessert apples was low, although 17 candidate regions showed signatures of divergent selection, displaying either outlier F_ST values or significant association with phenotypic traits (bitter versus sweet fruits). These candidate regions encompassed 420 genes involved in a variety of functions and metabolic pathways, including several colocalizations with QTLs for polyphenol compounds.

The Wukong Terminal-Repeat Retrotransposon in Miniature (TRIM) Elements in Diverse Maize Germplasm

TRIMs (terminal-repeat retrotransposons in miniature), which are characterized by their small size, have been discovered in all investigated vascular plants and even in animals. Here, we identified a highly conservative TRIM family referred to as Wukong elements in the maize genome. The Wukong family shows a distinct pattern of tandem arrangement in the maize genome suggesting a high rate of unequal crossing over. Estimation of insertion times implies a burst of retrotransposition activity of the Wukong family after the allotetraploidization of maize. Using next-generation sequencing data, we detected 87 new Wukong insertions in parents of the maize NAM population relative to the B73 reference genome and found abundant insertion polymorphism of Wukong elements in 75 re-sequenced maize lines, including teosinte, landraces, and improved lines. These results suggest that Wukong elements possessed a persistent retrotransposition activity throughout maize evolution. Moreover, the phylogenetic relationships among 76 maize inbreds and their relatives based on insertion polymorphisms of Wukong elements should provide us with reliable molecular markers for biodiversity and genetics studies.

Comparative transcriptomics uncovers alternative splicing changes and signatures of selection from maize improvement

Background

Alternative splicing (AS) is an important regulatory mechanism that greatly contributes to eukaryotic transcriptome diversity. A substantial amount of evidence has demonstrated that AS complexity is relevant to eukaryotic evolution, development, adaptation, and complexity. In this study, six teosinte and ten maize transcriptomes were sequenced to analyze AS changes and signatures of selection in maize domestication and improvement.

Results

In maize and teosinte, 13,593 highly conserved genes, including 12,030 multiexonic genes, were detected. By identifying AS isoforms from mutliexonic genes, we found that AS types were not significantly different between maize and teosinte. In addition, the two main AS types (intron retention and alternative acceptor) contributed to more than 60% of the AS events in the two species, but the average unique AS events per each alternatively spliced gene in maize (4.12) was higher than that in teosinte (2.26). Moreover, 94 genes generating 98 retained introns with transposable element (TE) sequences were detected in maize, which is far more than 9 retained introns with TEs detected in teosinte. This indicates that TE insertion might be an important mechanism for intron retention in maize. Additionally, the AS levels of 3864 genes were significantly different between maize and teosinte. Of these, 151 AS level-altered genes that are involved in transcriptional regulation and in stress responses are located in regions that have been targets of selection during maize improvement. These genes were inferred to be putatively improved genes.

Conclusions

We suggest that both maize and teosinte share similar AS mechanisms, but more genes have increased AS complexity during domestication from teosinte to maize. Importantly, a subset of AS level-increased genes that encode transcription factors and stress-responsive proteins may have been selected during maize improvement.

Electronic supplementary material

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Genetic Architecture of Ear Fasciation in Maize (Zea mays) under QTL Scrutiny

MAIZE EAR FASCIATION

Knowledge of the genes affecting maize ear inflorescence may lead to better grain yield modeling. Maize ear fasciation, defined as abnormal flattened ears with high kernel row number, is a quantitative trait widely present in Portuguese maize landraces.

MATERIAL AND METHODS

Using a segregating population derived from an ear fasciation contrasting cross (consisting of 149 F2:3 families) we established a two location field trial using a complete randomized block design. Correlations and heritabilities for several ear fasciation-related traits and yield were determined. Quantitative Trait Loci (QTL) involved in the inheritance of those traits were identified and candidate genes for these QTL proposed.

RESULTS AND DISCUSSION

Ear fasciation broad-sense heritability was 0.73. Highly significant correlations were found between ear fasciation and some ear and cob diameters and row number traits. For the 23 yield and ear fasciation-related traits, 65 QTL were identified, out of which 11 were detected in both environments, while for the three principal components, five to six QTL were detected per environment. Detected QTL were distributed across 17 genomic regions and explained individually, 8.7% to 22.4% of the individual traits or principal components phenotypic variance. Several candidate genes for these QTL regions were proposed, such as bearded-ear1, branched silkless1, compact plant1, ramosa2, ramosa3, tasselseed4 and terminal ear1. However, many QTL mapped to regions without known candidate genes, indicating potential chromosomal regions not yet targeted for maize ear traits selection.

CONCLUSIONS

Portuguese maize germplasm represents a valuable source of genes or allelic variants for yield improvement and elucidation of the genetic basis of ear fasciation traits. Future studies should focus on fine mapping of the identified genomic regions with the aim of map-based cloning.

The Draft Genome of Hop (Humulus lupulus), an Essence for Brewing

The female flower of hop (Humulus lupulus var. lupulus) is an essential ingredient that gives characteristic aroma, bitterness and durability/stability to beer. However, the molecular genetic basis for identifying DNA markers in hop for breeding and to study its domestication has been poorly established. Here, we provide draft genomes for two hop cultivars [cv. Saazer (SZ) and cv. Shinshu Wase (SW)] and a Japanese wild hop [H. lupulus var. cordifolius; also known as Karahanasou (KR)]. Sequencing and de novo assembly of genomic DNA from heterozygous SW plants generated scaffolds with a total size of 2.05 Gb, corresponding to approximately 80% of the estimated genome size of hop (2.57 Gb). The scaffolds contained 41,228 putative protein-encoding genes. The genome sequences for SZ and KR were constructed by aligning their short sequence reads to the SW reference genome and then replacing the nucleotides at single nucleotide polymorphism (SNP) sites. De novo RNA sequencing (RNA-Seq) analysis of SW revealed the developmental regulation of genes involved in specialized metabolic processes that impact taste and flavor in beer. Application of a novel bioinformatics tool, phylogenetic comparative RNA-Seq (PCP-Seq), which is based on read depth of genomic DNAs and RNAs, enabled the identification of genes related to the biosynthesis of aromas and flavors that are enriched in SW compared to KR. Our results not only suggest the significance of historical human selection process for enhancing aroma and bitterness biosyntheses in hop cultivars, but also serve as crucial information for breeding varieties with high quality and yield.

The origin and evolution of maize in the Southwestern United States

The origin of maize (Zea mays mays) in the US Southwest remains contentious, with conflicting archaeological data supporting either coastal(1-4) or highland(5,6) routes of diffusion of maize into the United States. Furthermore, the genetics of adaptation to the new environmental and cultural context of the Southwest is largely uncharacterized(7). To address these issues, we compared nuclear DNA from 32 archaeological maize samples spanning 6,000 years of evolution to modern landraces. We found that the initial diffusion of maize into the Southwest about 4,000 years ago is likely to have occurred along a highland route, followed by gene flow from a lowland coastal maize beginning at least 2,000 years ago. Our population genetic analysis also enabled us to differentiate selection during domestication for adaptation to the climatic and cultural environment of the Southwest, identifying adaptation loci relevant to drought tolerance and sugar content.

Genome re-sequencing of semi-wild soybean reveals a complex Soja population structure and deep introgression

Semi-wild soybean is a unique type of soybean that retains both wild and domesticated characteristics, which provides an important intermediate type for understanding the evolution of the subgenus Soja population in the Glycine genus. In this study, a semi-wild soybean line (Maliaodou) and a wild line (Lanxi 1) collected from the lower Yangtze regions were deeply sequenced while nine other semi-wild lines were sequenced to a 3-fold genome coverage. Sequence analysis revealed that (1) no independent phylogenetic branch covering all 10 semi-wild lines was observed in the Soja phylogenetic tree; (2) besides two distinct subpopulations of wild and cultivated soybean in the Soja population structure, all semi-wild lines were mixed with some wild lines into a subpopulation rather than an independent one or an intermediate transition type of soybean domestication; (3) high heterozygous rates (0.19-0.49) were observed in several semi-wild lines; and (4) over 100 putative selective regions were identified by selective sweep analysis, including those related to the development of seed size. Our results suggested a hybridization origin for the semi-wild soybean, which makes a complex Soja population structure.

Detecting SNPs underlying domestication-related traits in soybean

BACKGROUND

Cultivated soybean (Glycine max) experienced a severe genetic bottleneck during its domestication and a further loss in diversity during its subsequent selection. Here, a panel of 65 wild (G. soja) and 353 cultivated accessions was genotyped at 552 single-nucleotide polymorphism loci to search for signals of selection during and after domestication.

RESULTS

The wild and cultivated populations were well differentiated from one another. Application of the Fst outlier test revealed 64 loci showing evidence for selection. Of these, 35 related to selection during domestication, while the other 29 likely gradually became monomorphic as a result of prolonged selection during post domestication. Two of the SNP locus outliers were associated with testa color.

CONCLUSIONS

Identifying genes controlling domestication-related traits is important for maintaining the diversity of crops. SNP locus outliers detected by a combined forward genetics and population genetics approach can provide markers with utility for the conservation of wild accessions and for trait improvement in the cultivated genepool.

Molecular evolution of candidate genes for crop-related traits in sunflower (Helianthus annuus L.)

Evolutionary analyses aimed at detecting the molecular signature of selection during crop domestication and/or improvement can be used to identify genes or genomic regions of likely agronomic importance. Here, we describe the DNA sequence-based characterization of a pool of candidate genes for crop-related traits in sunflower. These genes, which were identified based on homology to genes of known effect in other study systems, were initially sequenced from a panel of improved lines. All genes that exhibited a paucity of sequence diversity, consistent with the possible effects of selection during the evolution of cultivated sunflower, were then sequenced from a panel of wild sunflower accessions an outgroup. These data enabled formal tests for the effects of selection in shaping sequence diversity at these loci. When selection was detected, we further sequenced these genes from a panel of primitive landraces, thereby allowing us to investigate the likely timing of selection (i.e., domestication vs. improvement). We ultimately identified seven genes that exhibited the signature of positive selection during either domestication or improvement. Genetic mapping of a subset of these genes revealed co-localization between candidates for genes involved in the determination of flowering time, seed germination, plant growth/development, and branching and QTL that were previously identified for these traits in cultivated × wild sunflower mapping populations.

Whole-genome sequencing reveals untapped genetic potential in Africa's indigenous cereal crop sorghum

Sorghum is a food and feed cereal crop adapted to heat and drought and a staple for 500 million of the world’s poorest people. Its small diploid genome and phenotypic diversity make it an ideal C₄ grass model as a complement to C₃ rice. Here we present high coverage (16–45 × ) resequenced genomes of 44 sorghum lines representing the primary gene pool and spanning dimensions of geographic origin, end-use and taxonomic group. We also report the first resequenced genome of S. propinquum, identifying 8 M high-quality SNPs, 1.9 M indels and specific gene loss and gain events in S. bicolor. We observe strong racial structure and a complex domestication history involving at least two distinct domestication events. These assembled genomes enable the leveraging of existing cereal functional genomics data against the novel diversity available in sorghum, providing an unmatched resource for the genetic improvement of sorghum and other grass species.

Sorghum is a drought-resistant food and feed cereal crop used by over half a billion of the world’s poorest people. Here the authors present high-coverage resequencing genome data of 44 sorghum lines of varying geographic and taxonomic origin, which include a number of sorghum wild relatives.

Genetic and biochemical differences in populations bred for extremes in maize grain methionine concentration

BACKGROUND

Methionine is an important nutrient in animal feed and several approaches have been developed to increase methionine concentration in maize (Zea mays L.) grain. One approach is through traditional breeding using recurrent selection. Using divergent selection, genetically related populations with extreme differences in grain methionine content were produced. In order to better understand the molecular mechanisms controlling grain methionine content, we examined seed proteins, transcript levels of candidate genes, and genotypes of these populations.

RESULTS

Two populations were selected for high or low methionine concentration for eight generations and 40 and 56% differences between the high and low populations in grain methionine concentration were observed. Mean values between the high and low methionine populations differed by greater than 1.5 standard deviations in some cycles of selection. Other amino acids and total protein concentration exhibited much smaller changes. In an effort to understand the molecular mechanisms that contribute to these differences, we compared transcript levels of candidate genes encoding high methionine seed storage proteins involved in sulfur assimilation or methionine biosynthesis. In combination, we also explored the genetic mechanisms at the SNP level through implementation of an association analysis. Significant differences in methionine-rich seed storage protein genes were observed in comparisons of high and low methionine populations, while transcripts of seed storage proteins lacking high levels of methionine were unchanged. Seed storage protein levels were consistent with transcript levels. Two genes involved in sulfur assimilation, Cys2 and CgS1 showed substantial differences in allele frequencies when two selected populations were compared to the starting populations. Major genes identified across cycles of selection by a high-stringency association analysis included dzs18, wx, dzs10, and zp27.

CONCLUSIONS

We hypothesize that transcriptional changes alter sink strength by altering the levels of methionine-rich seed storage proteins. To meet the altered need for sulfur, a cysteine-rich seed storage protein is altered while sulfur assimilation and methionine biosynthesis throughput is changed by selection for certain alleles of Cys2 and CgS1.