Valorie H. Goff

Seed shattering in a wild sorghum is conferred by a locus unrelated to domestication

Suppression of seed shattering was a key step during crop domestication that we have previously suggested to be convergent among independent cereal lineages. Positional, association, expression, and mutant complementation data all implicate a WRKY transcription factor, SpWRKY, in conferring shattering to a wild sorghum relative, Sorghum propinquum. We hypothesize that SpWRKY functions in a manner analogous to Medicago and Arabidopsis homologs that regulate cell wall biosynthesis genes, with low expression toward the end of floral development derepressing downstream cell wall biosynthesis genes to allow deposition of lignin that initiates the abscission zone in the seed–pedicel junction. The recent discovery of a YABBY locus that confers shattering within Sorghum bicolor and other cereals validated our prior hypothesis that some parallel domestication may have been convergent. Ironically, however, the shattering allele of SpWRKY appears to be recently evolved in S. propinquum and illustrates a case in which the genetic control of a trait in a wild relative fails to extrapolate even to closely related crops. Remarkably, the SpWRKY and YABBY loci lie only 300 kb apart and may have appeared to be a single genetic locus in some sorghum populations.

Genetic Analysis of Recombinant Inbred Lines for Sorghum bicolor × Sorghum propinquum

We describe a recombinant inbred line (RIL) population of 161 F5 genotypes for the widest euploid cross that can be made to cultivated sorghum (Sorghum bicolor) using conventional techniques, S. bicolor × Sorghum propinquum, that segregates for many traits related to plant architecture, growth and development, reproduction, and life history. The genetic map of the S. bicolor × S. propinquum RILs contains 141 loci on 10 linkage groups collectively spanning 773.1 cM. Although the genetic map has DNA marker density well-suited to quantitative trait loci mapping and samples most of the genome, our previous observations that sorghum pericentromeric heterochromatin is recalcitrant to recombination is highlighted by the finding that the vast majority of recombination in sorghum is concentrated in small regions of euchromatin that are distal to most chromosomes. The advancement of the RIL population in an environment to which the S. bicolor parent was well adapted (indeed bred for) but the S. propinquum parent was not largely eliminated an allele for short-day flowering that confounded many other traits, for example, permitting us to map new quantitative trait loci for flowering that previously eluded detection. Additional recombination that has accrued in the development of this RIL population also may have improved resolution of apices of heterozygote excess, accounting for their greater abundance in the F5 than the F2 generation. The S. bicolor × S. propinquum RIL population offers advantages over early-generation populations that will shed new light on genetic, environmental, and physiological/biochemical factors that regulate plant growth and development.

Genetic analysis of inflorescence and plant height components in sorghum (Panicoidae) and comparative genetics with rice (Oryzoidae)

BackgroundDomestication has played an important role in shaping characteristics of the inflorescence and plant height in cultivated cereals. Taking advantage of meta-analysis of QTLs, phylogenetic analyses in 502 diverse sorghum accessions, GWAS in a sorghum association panel (n = 354) and comparative data, we provide insight into the genetic basis of the domestication traits in sorghum and rice.ResultsWe performed genome-wide association studies (GWAS) on 6 traits related to inflorescence morphology and 6 traits related to plant height in sorghum, comparing the genomic regions implicated in these traits by GWAS and QTL mapping, respectively. In a search for signatures of selection, we identify genomic regions that may contribute to sorghum domestication regarding plant height, flowering time and pericarp color. Comparative studies across taxa show functionally conserved ‘hotspots’ in sorghum and rice for awn presence and pericarp color that do not appear to reflect corresponding single genes but may indicate co-regulated clusters of genes. We also reveal homoeologous regions retaining similar functions for plant height and flowering time since genome duplication an estimated 70 million years ago or more in a common ancestor of cereals. In most such homoeologous QTL pairs, only one QTL interval exhibits strong selection signals in modern sorghum.ConclusionsIntersections among QTL, GWAS and comparative data advance knowledge of genetic determinants of inflorescence and plant height components in sorghum, and add new dimensions to comparisons between sorghum and rice.

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.

Alleles conferring improved fiber quality from EMS mutagenesis of elite cotton genotypes

Key messageGenetic improvements for many fiber traits are obtained by mutagenesis of elite cottons, mitigating genetic uniformity in this inbred polyploid by contributing novel alleles important to ongoing crop improvement.AbstractThe elite gene pool of cotton (Gossypium spp.) has less diversity than those of most other major crops, making identification of novel alleles important to ongoing crop improvement. A total of 3,164 M5 lines resulting from ethyl methanesulfonate (EMS) mutagenesis of two G. hirsutum breeding lines, TAM 94L-25 and Acala 1517-99, were characterized for basic components of fiber quality and selected yield components. Across all measured traits, the ranges of phenotypic values among the mutant lines were consistently larger than could be explained by chance (5.27–10.1 for TAM 94 L-25 and 5.29–7.94 standard deviations for Acala 1517-99-derived lines). Multi-year replicated studies confirmed a genetic basis for these differences, showing significant correlations between lines across years and environments. A subset of 157 lines selected for superior fiber qualities, including fiber elongation (22 lines), length (22), lint percent (17), fineness (23), Rd value (21), strength (19), uniformity (21) and multiple attributes in a selection index (26) were compared to 55 control lines in replicated trials in both Texas and Georgia. For all traits, mutant lines showing substantial and statistically significant improvements over control lines were found, in most cases from each of the two genetic backgrounds. This indicates that genetic improvements for a wide range of fiber traits may be obtained from mutagenesis of elite cottons. Indeed, lines selected for one fiber trait sometimes conferred additional attributes, suggesting pleiotropic effects of some mutations and offering multiple benefits for the incorporation of some alleles into mainstream breeding programs.

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.

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.

EMS-mutated cotton populations suggest overlapping genetic control of trichome and lint fiber variation

The complex genetic and environmental control of lint fiber yield and quality of cotton has long motivated interest in whether information from genetically-simpler trichome variations might contribute knowledge salient to cotton improvement. To investigate this question, from 3164 M5 lines resulting from EMS mutagenesis of two Gossypium hirsutum breeding lines, TAM 94L25 and Acala 1517-99, 106 lines with leaf and stem trichome variations and 55 control lines were further studied to investigate associations between trichome variation and lint fiber development. Although only weak correlation was found between stem/leaf trichome and fiber traits, we still found that among nine fiber traits measured in replicated trials, lines with mutations affecting stem trichome development had significant alterations for seven traits in the TAM 94L25 mutants, and six in the Acala 1517-99 mutants. While the small number of leaf trichome mutants found offered only minimal statistical power to resolve differences, mutant lines had significant alterations for three of the nine traits in the TAM 94L25 mutants, and two in the Acala 1517-99 mutants. In summary, mutants in leaf and/or stem trichome development often have altered lint fiber characteristics, supporting the hypothesis that there is considerable overlap in the sets of genetic factors acting in the development of these analogous organs. Moreover, visual selection of trichome mutants may be an effective screen to identify potential new alleles affecting lint fiber development.