Comparative evolution of vegetative branching in sorghum.

Tillering and secondary branching are two plastic traits with high agronomic importance, especially in terms of the ability of plants to adapt to changing environments. We describe a quantitative trait analysis of tillering and secondary branching in two novel BC1F2 populations totaling 246 genotypes derived from backcrossing two Sorghum bicolor x S. halepense F1 plants to a tetraploidized S. bicolor. A two-year, two-environment phenotypic evaluation in Bogart, GA and Salina, KS permitted us to identify major effect and environment specific QTLs. Significant correlation between tillering and secondary branching followed by discovery of overlapping sets of QTLs continue to support the developmental relationship between these two organs and suggest the possibility of pleiotropy. Comparisons with two other populations sharing S. bicolor BTx623 as a common parent but sampling the breadth of the Sorghum genus, increase confidence in QTL detected for these two plastic traits and provide insight into the evolution of morphological diversity in the Eusorghum clade. Correspondence between flowering time and vegetative branching supports other evidence in suggesting a pleiotropic effect of flowering genes. We propose a model to predict biomass weight from plant architecture related traits, quantifying contribution of each trait to biomass and providing guidance for future breeding experiments.

Quantitative trait mapping of plant architecture in two BC1F2 populations of Sorghum Bicolor × S. halepense and comparisons to two other sorghum populations.

KEY MESSAGE: Comparing populations derived, respectively, from polyploid Sorghum halepense and its progenitors improved knowledge of plant architecture and showed that S. halepense harbors genetic novelty of potential value for sorghum improvement Vegetative growth and the timing of the vegetative-to-reproductive transition are critical to a plant's fitness, directly and indirectly determining when and how a plant lives, grows and reproduces. We describe quantitative trait analysis of plant height and flowering time in the naturally occurring tetraploid Sorghum halepense, using two novel BC1F2 populations totaling 246 genotypes derived from backcrossing two tetraploid Sorghum bicolor x S. halepense F1 plants to a tetraploidized S. bicolor. Phenotyping for two years each in Bogart, GA and Salina, KS allowed us to dissect variance into narrow-sense genetic (QTLs) and environmental components. In crosses with a common S. bicolor BTx623 parent, comparison of QTLs in S. halepense, its rhizomatous progenitor S. propinquum and S. bicolor race guinea which is highly divergent from BTx623 permit inferences of loci at which new alleles have been associated with improvement of elite sorghums. The relative abundance of QTLs unique to the S. halepense populations may reflect its polyploidy and subsequent 'diploidization' processes often associated with the formation of genetic novelty, a possibility further supported by a high level of QTL polymorphism within sibling lines derived from a common S. halepense parent. An intriguing hypothesis for further investigation is that polyploidy of S. halepense following 96 million years of abstinence, coupled with natural selection during its spread to diverse environments across six continents, may provide a rich collection of novel alleles that offer potential opportunities for sorghum improvement.

Transmission Genetics of a Sorghum bicolor × S. halepense Backcross Populations.

Despite a "ploidy barrier," interspecific crosses to wild and/or cultivated sorghum (Sorghum bicolor, 2n = 2x = 20) may have aided the spread across six continents of Sorghum halepense, also exemplifying risks of "transgene escape" from crops that could make weeds more difficult to control. Genetic maps of two BC1F1 populations derived from crosses of S. bicolor (sorghum) and S. halepense with totals of 722 and 795 single nucleotide polymorphism (SNP) markers span 37 and 35 linkage groups, with 2-6 for each of the 10 basic sorghum chromosomes due to fragments covering different chromosomal portions or independent segregation from different S. halepense homologs. Segregation distortion favored S. halepense alleles on chromosomes 2 (1.06-4.68 Mb, near a fertility restoration gene), 7 (1.20-6.16 Mb), 8 (1.81-5.33 Mb, associated with gene conversion), and 9 (47.5-50.1 Mb); and S. bicolor alleles on chromosome 6 (0-40 Mb), which contains both a large heterochromatin block and the Ma1 gene. Regions of the S. halepense genome that are recalcitrant to gene flow from sorghum might be exploited as part a multi-component system to reduce the likelihood of spread of transgenes or other modified genes. Its SNP profile suggests that chromosome segments from its respective progenitors S. bicolor and Sorghum propinquum have extensively recombined in S. halepense. This study reveals genomic regions that might discourage crop-to-weed gene escape, and provides a foundation for marker-trait association analysis to determine the genetic control of traits contributing to weediness, invasiveness, and perenniality of S. halepense.

Genotyping by Sequencing of 393 Sorghum bicolor BTx623 × IS3620C Recombinant Inbred Lines Improves Sensitivity and Resolution of QTL Detection.

We describe a genetic map with a total of 381 bins of 616 genotyping by sequencing (GBS)-based SNP markers in a F6-F8 recombinant inbred line (RIL) population of 393 individuals derived from crossing S. bicolor BTx623 to S. bicolor IS3620C, a guinea line substantially diverged from BTx623. Five segregation distorted regions were found with four showing enrichment for S. bicolor alleles, suggesting possible selection during formation of this RIL population. A quantitative trait locus (QTL) study with this number of individuals, tripled relative to prior studies of this cross, provided resources, validated previous findings, and demonstrated improved power to detect plant height and flowering time related QTL relative to other published studies. An unexpected low correlation between flowering time and plant height permitted us to separate QTL for each trait and provide evidence against pleiotropy. Ten non- random syntenic regions conferring QTL for the same trait suggest that those QTL may represent alleles at genes functioning in the same manner since the 96 million year ago genome duplication that created these syntenic relationships, while syntenic regions conferring QTL for different trait may suggest sub-functionalization after duplication. Collectively, this study provides resources for marker-assisted breeding, as well as a framework for fine mapping and subsequent cloning of major genes for important traits such as plant height and flowering time in sorghum.

Multi-Phase US Spread and Habitat Switching of a Post-Columbian Invasive, Sorghum halepense.

Johnsongrass (Sorghum halepense) is a striking example of a post-Columbian founder event. This natural experiment within ecological time-scales provides a unique opportunity for understanding patterns of continent-wide genetic diversity following range expansion. Microsatellite markers were used for population genetic analyses including leaf-optimized Neighbor-Joining tree, pairwise FST, mismatch analysis, principle coordinate analysis, Tajima's D, Fu's F and Bayesian clusterings of population structure. Evidence indicates two geographically distant introductions of divergent genotypes, which spread across much of the US in <200 years. Based on geophylogeny, gene flow patterns can be inferred to have involved five phases. Centers of genetic diversity have shifted from two introduction sites separated by ~2000 miles toward the middle of the range, consistent with admixture between genotypes from the respective introductions. Genotyping provides evidence for a 'habitat switch' from agricultural to non-agricultural systems and may contribute to both Johnsongrass ubiquity and aggressiveness. Despite lower and more structured diversity at the invasion front, Johnsongrass continues to advance northward into cooler and drier habitats. Association genetic approaches may permit identification of alleles contributing to the habitat switch or other traits important to weed/invasive management and/or crop improvement.

Comparative Genetics of Seed Size Traits in Divergent Cereal Lineages Represented by Sorghum (Panicoidae) and Rice (Oryzoidae).

Seed size is closely related to fitness of wild plants, and its modification has been a key recurring element in domestication of seed/grain crops. In sorghum, a genomic and morphological model for panicoid cereals, a rich history of research into the genetics of seed size is reflected by a total of 13 likelihood intervals determined by conventional QTL (linkage) mapping in 11 nonoverlapping regions of the genome. To complement QTL data and investigate whether the discovery of seed size QTL is approaching "saturation," we compared QTL data to GWAS for seed mass, seed length, and seed width studied in 354 accessions from a sorghum association panel (SAP) that have been genotyped at 265,487 SNPs. We identified nine independent GWAS-based "hotspots" for seed size associations. Targeted resequencing near four association peaks with the most notable linkage disequilibrium provides further support of the role(s) of these regions in the genetic control of sorghum seed size and identifies two candidate causal variants with nonsynonymous mutations. Of nine GWAS hotspots in sorghum, seven have significant correspondence with rice QTL intervals and known genes for components of seed size on orthologous chromosomes. Identifying intersections between positional and association genetic data are a potentially powerful means to mitigate constraints associated with each approach, and nonrandom correspondence of sorghum (panicoid) GWAS signals to rice (oryzoid) QTL adds a new dimension to the ability to leverage genetic data about this important trait across divergent plants.

Comparative and evolutionary analysis of major peanut allergen gene families.

Peanut (Arachis hypogaea L.) causes one of the most serious food allergies. Peanut seed proteins, Arah1, Arah2, and Arah3, are considered to be among the most important peanut allergens. To gain insights into genome organization and evolution of allergen-encoding genes, approximately 617 kb from the genome of cultivated peanut and 215 kb from a wild relative were sequenced including three Arah1, one Arah2, eight Arah3, and two Arah6 gene family members. To assign polarity to differences between homoeologous regions in peanut, we used as outgroups the single orthologous regions in Medicago, Lotus, common bean, chickpea, and pigeonpea, which diverged from peanut about 50 Ma and have not undergone subsequent polyploidy. These regions were also compared with orthologs in many additional dicot plant species to help clarify the timing of evolutionary events. The lack of conservation of allergenic epitopes between species, and the fact that many different proteins can be allergenic, makes the identification of allergens across species by comparative studies difficult. The peanut allergen genes are interspersed with low-copy genes and transposable elements. Phylogenetic analyses revealed lineage-specific expansion and loss of low-copy genes between species and homoeologs. Arah1 syntenic regions are conserved in soybean, pigeonpea, tomato, grape, Lotus, and Arabidopsis, whereas Arah3 syntenic regions show genome rearrangements. We infer that tandem and segmental duplications led to the establishment of the Arah3 gene family. Our analysis indicates differences in conserved motifs in allergen proteins and in the promoter regions of the allergen-encoding genes. Phylogenetic analysis and genomic organization studies provide new insights into the evolution of the major peanut allergen-encoding genes.

A genome-wide BAC end-sequence survey of sugarcane elucidates genome composition, and identifies BACs covering much of the euchromatin.

BAC-end sequences (BESs) of hybrid sugarcane cultivar R570 are presented. A total of 66,990 informative BESs were obtained from 43,874 BAC clones. Similarity search using a variety of public databases revealed that 13.5 and 42.8 % of BESs match known gene-coding and repeat regions, respectively. That 11.7 % of BESs are still unmatched to any nucleotide sequences in the current public databases despite the fact that a close relative, sorghum, is fully sequenced, indicates that there may be many sugarcane-specific or lineage-specific sequences. We found 1,742 simple sequence repeat motifs in 1,585 BESs, spanning 27,383 bp in length. As simple sequence repeat markers derived from BESs have some advantages over randomly generated markers, these may be particularly useful for comparing BAC-based physical maps with genetic maps. BES and overgo hybridization information was used for anchoring sugarcane BAC clones to the sorghum genome sequence. While sorghum and sugarcane have extensive similarity in terms of genomic structure, only 2,789 BACs (6.4 %) could be confidently anchored to the sorghum genome at the stringent threshold of having both-end information (BESs or overgos) within 300 Kb. This relatively low rate of anchoring may have been caused in part by small- or large-scale genomic rearrangements in the Saccharum genus after two rounds of whole genome duplication since its divergence from the sorghum lineage about 7.8 million years ago. Limiting consideration to only low-copy matches, 1,245 BACs were placed to 1,503 locations, covering ~198 Mb of the sorghum genome or about 78 % of the estimated 252 Mb of euchromatin. BESs and their analyses presented here may provide an early profile of the sugarcane genome as well as a basis for BAC-by-BAC sequencing of much of the basic gene set of sugarcane.

Comparative analysis of peanut NBS-LRR gene clusters suggests evolutionary innovation among duplicated domains and erosion of gene microsynteny.

• Plant genomes contain numerous disease resistance genes (R genes) that play roles in defense against pathogens. Scarcity of genetic polymorphism makes peanut (Arachis hypogaea) especially vulnerable to a wide variety of pathogens. • Here, we isolated and characterized peanut bacterial artificial chromosomes (BACs) containing a high density of R genes. Analysis of two genomic regions identified several TIR-NBS-LRR (Toll-interleukin-1 receptor, nucleotide-binding site, leucine-rich repeat) resistance gene analogs or gene fragments. We reconstructed their evolutionary history characterized by tandem duplications, possibly facilitated by transposon activities. We found evidence of both intergenic and intragenic gene conversions and unequal crossing-over, which may be driving forces underlying the functional evolution of resistance. • Analysis of the sequence mutations, protein secondary structure and three-dimensional structures, all suggest that LRR domains are the primary contributor to the evolution of resistance genes. The central part of LRR regions, assumed to serve as the active core, may play a key role in the resistance function by having higher rates of duplication and DNA conversion than neighboring regions. The assumed active core is characterized by significantly enriched leucine residue composition, accumulation of positively selected sites, and shorter beta sheets. • Homologous resistance gene analog (RGA)-containing regions in peanut, soybean, Medicago, Arabidopsis and grape have only limited gene synteny and microcollinearity.

Comparative analysis of Gossypium and Vitis genomes indicates genome duplication specific to the Gossypium lineage.

Genetic mapping studies have suggested that diploid cotton (Gossypium) might be an ancient polyploid. However, further evidence is lacking due to the complexity of the genome and the lack of sequence resources. Here, we used the grape (Vitis vinifera) genome as an out-group in two different approaches to further explore evidence regarding ancient genome duplication (WGD) event(s) in the diploid Gossypium lineage and its (their) effects: a genome-level alignment analysis and a local-level sequence component analysis. Both studies suggest that at least one round of genome duplication occurred in the Gossypium lineage. Also, gene densities in corresponding regions from Gossypium raimondii, V. vinifera, Arabidopsis thaliana and Carica papaya genomes are similar, despite the huge difference in their genome sizes and the different number of WGDs each genome has experienced. These observations fit the model that differences in plant genome sizes are largely explained by transposon insertions into heterochromatic regions.

A draft physical map of a D-genome cotton species (Gossypium raimondii).

BACKGROUND: Genetically anchored physical maps of large eukaryotic genomes have proven useful both for their intrinsic merit and as an adjunct to genome sequencing. Cultivated tetraploid cottons, Gossypium hirsutum and G. barbadense, share a common ancestor formed by a merger of the A and D genomes about 1-2 million years ago. Toward the long-term goal of characterizing the spectrum of diversity among cotton genomes, the worldwide cotton community has prioritized the D genome progenitor Gossypium raimondii for complete sequencing. RESULTS: A whole genome physical map of G. raimondii, the putative D genome ancestral species of tetraploid cottons was assembled, integrating genetically-anchored overgo hybridization probes, agarose based fingerprints and 'high information content fingerprinting' (HICF). A total of 13,662 BAC-end sequences and 2,828 DNA probes were used in genetically anchoring 1585 contigs to a cotton consensus genetic map, and 370 and 438 contigs, respectively to Arabidopsis thaliana (AT) and Vitis vinifera (VV) whole genome sequences. CONCLUSION: Several lines of evidence suggest that the G. raimondii genome is comprised of two qualitatively different components. Much of the gene rich component is aligned to the Arabidopsis and Vitis vinifera genomes and shows promise for utilizing translational genomic approaches in understanding this important genome and its resident genes. The integrated genetic-physical map is of value both in assembling and validating a planned reference sequence.