Yehoshua Saranga

Repeated polyploidization of Gossypium genomes and the evolution of spinnable cotton fibres

Polyploidy often confers emergent properties, such as the higher fibre productivity and quality of tetraploid cottons than diploid cottons bred for the same environments. Here we show that an abrupt five- to sixfold ploidy increase approximately 60 million years (Myr) ago, and allopolyploidy reuniting divergent Gossypium genomes approximately 1–2 Myr ago, conferred about 30–36-fold duplication of ancestral angiosperm (flowering plant) genes in elite cottons (Gossypium hirsutum and Gossypium barbadense), genetic complexity equalled only by Brassica among sequenced angiosperms. Nascent fibre evolution, before allopolyploidy, is elucidated by comparison of spinnable-fibred Gossypium herbaceum A and non-spinnable Gossypium longicalyx F genomes to one another and the outgroup D genome of non-spinnable Gossypium raimondii. The sequence of a G. hirsutum AtDt (in which ‘t’ indicates tetraploid) cultivar reveals many non-reciprocal DNA exchanges between subgenomes that may have contributed to phenotypic innovation and/or other emergent properties such as ecological adaptation by polyploids. Most DNA-level novelty in G. hirsutum recombines alleles from the D-genome progenitor native to its New World habitat and the Old World A-genome progenitor in which spinnable fibre evolved. Coordinated expression changes in proximal groups of functionally distinct genes, including a nuclear mitochondrial DNA block, may account for clusters of cotton-fibre quantitative trait loci affecting diverse traits. Opportunities abound for dissecting emergent properties of other polyploids, particularly angiosperms, by comparison to diploid progenitors and outgroups.

Meta-analysis of Polyploid Cotton QTL Shows Unequal Contributions of Subgenomes to a Complex Network of Genes and Gene Clusters Implicated in Lint Fiber Development

QTL mapping experiments yield heterogeneous results due to the use of different genotypes, environments, and sampling variation. Compilation of QTL mapping results yields a more complete picture of the genetic control of a trait and reveals patterns in organization of trait variation. A total of 432 QTL mapped in one diploid and 10 tetraploid interspecific cotton populations were aligned using a reference map and depicted in a CMap resource. Early demonstrations that genes from the non-fiber-producing diploid ancestor contribute to tetraploid lint fiber genetics gain further support from multiple populations and environments and advanced-generation studies detecting QTL of small phenotypic effect. Both tetraploid subgenomes contribute QTL at largely non-homeologous locations, suggesting divergent selection acting on many corresponding genes before and/or after polyploid formation. QTL correspondence across studies was only modest, suggesting that additional QTL for the target traits remain to be discovered. Crosses between closely-related genotypes differing by single-gene mutants yield profoundly different QTL landscapes, suggesting that fiber variation involves a complex network of interacting genes. Members of the lint fiber development network appear clustered, with cluster members showing heterogeneous phenotypic effects. Meta-analysis linked to synteny-based and expression-based information provides clues about specific genes and families involved in QTL networks.

Lycopersicon esculentum lines containing small overlapping introgressions from L. pennellii

SummaryThe objective of this project was to introgress small overlapping chromosome segments which cover the genome of L. pennellii into Lycopersicon esculentum lines. The interspecific hybrid was backcrossed to L. esculentum, and a map of 981 cM, based on 146 molecular markers covering the entire genome, was produced. A similar backcross 1 population was selfed for six generations, under strong selection for cultivated tomato phenotypes, to produce 120 introgression lines. The introgression lines were assayed for the above-mentioned molecular markers, and 21 lines covering 936 cM of L. pennellii, with an average introgression of 86 cM, were selected to provide a resource for the mapping of new DNA clones. The rest of the lines have shorter introgressions consisting of specific regions with an average size of 38 cM. The proportion of the L. pennellii genome in the introgression lines was lower than expected (252 cM) because of strong selection against the wild-parent phenotype. The mean introgression rate for ends of linkage groups in the 120 lines was 3 times higher than for other regions of the genome. The introgression lines can assist in RFLP-based gene cloning by allowing the rapid selection of DNA markers that map to specific chromosome segments. The introgression lines also provide a base population for the mapping and breeding for quantitative traits such as salt and drought tolerance that characterize the wild species L. pennellii.

Genomic dissection of drought resistance in durum wheat × wild emmer wheat recombinant inbreed line population.

Drought is the major factor limiting wheat productivity worldwide. The gene pool of wild emmer wheat, Triticum turgidum ssp. dicoccoides, harbours a rich allelic repertoire for morpho-physiological traits conferring drought resistance. The genetic and physiological bases of drought responses were studied here in a tetraploid wheat population of 152 recombinant inbreed lines (RILs), derived from a cross between durum wheat (cv. Langdon) and wild emmer (acc# G18-16), under contrasting water availabilities. Wide genetic variation was found among RILs for all studied traits. A total of 110 quantitative trait loci (QTLs) were mapped for 11 traits, with LOD score range of 3.0-35.4. Several QTLs showed environmental specificity, accounting for productivity and related traits under water-limited (20 QTLs) or well-watered conditions (15 QTLs), and in terms of drought susceptibility index (22 QTLs). Major genomic regions controlling productivity and related traits were identified on chromosomes 2B, 4A, 5A and 7B. QTLs for productivity were associated with QTLs for drought-adaptive traits, suggesting the involvement of several strategies in wheat adaptation to drought stress. Fifteen pairs of QTLs for the same trait were mapped to seemingly homoeologous positions, reflecting synteny between the A and B genomes. The identified QTLs may facilitate the use of wild alleles for improvement of drought resistance in elite wheat cultivars.

Evaluation of different approaches for estimating and mapping crop water status in cotton with thermal imaging

Canopy temperature has long been recognized as an indicator of plant water status, therefore, a high-resolution thermal imaging system was used to map crop water status. Potential approaches for estimating crop water status from digital infrared images of the canopy were evaluated. The effect of time of day on leaf temperature measurements was studied: midday was found to be the optimal time for thermal image acquisition. Comparison between theoretical and empirical approaches for estimating leaf water potential showed that empirical temperature baselines were better than those obtained from energy balance equations. Finally, the effects of angle of view and spatial resolution of the thermal images were evaluated: water status was mapped by using angular thermal images. In spite of the different viewing angles and spatial resolution, the map provided a good representation of the measured leaf water potential.

Genetic analysis of wheat domestication and evolution under domestication

Wheat is undoubtedly one of the worlds major food sources since the dawn of Near Eastern agriculture and up to the present day. Morphological, physiological, and genetic modifications involved in domestication and subsequent evolution under domestication were investigated in a tetraploid recombinant inbred line population, derived from a cross between durum wheat and its immediate progenitor wild emmer wheat. Experimental data were used to test previous assumptions regarding a protracted domestication process. The brittle rachis (Br) spike, thought to be a primary characteristic of domestication, was mapped to chromosome 2A as a single gene, suggesting, in light of previously reported Br loci (homoeologous group 3), a complex genetic model involved in spike brittleness. Twenty-seven quantitative trait loci (QTLs) conferring threshability and yield components (kernel size and number of kernels per spike) were mapped. The large number of QTLs detected in this and other studies suggests that following domestication, wheat evolutionary processes involved many genomic changes. The Br gene did not show either genetic (co-localization with QTLs) or phenotypic association with threshability or yield components, suggesting independence of the respective loci. It is argued here that changes in spike threshability and agronomic traits (e.g. yield and its components) are the outcome of plant evolution under domestication, rather than the result of a protracted domestication process. Revealing the genomic basis of wheat domestication and evolution under domestication, and clarifying their inter-relationships, will improve our understanding of wheat biology and contribute to further crop improvement.

Genetic diversity for grain nutrients in wild emmer wheat: potential for wheat improvement

BACKGROUND AND AIMS Micronutrient malnutrition, particularly zinc and iron deficiency, afflicts over three billion people worldwide due to low dietary intake. In the current study, wild emmer wheat (Triticum turgidum ssp. dicoccoides), the progenitor of domesticated wheat, was tested for (1) genetic diversity in grain nutrient concentrations, (2) associations among grain nutrients and their relationships with plant productivity, and (3) the association of grain nutrients with the eco-geographical origin of wild emmer accessions. METHODS A total of 154 genotypes, including wild emmer accessions from across the Near Eastern Fertile Crescent and diverse wheat cultivars, were characterized in this 2-year field study for grain protein, micronutrient (zinc, iron, copper and manganese) and macronutrient (calcium, magnesium, potassium, phosphorus and sulphur) concentrations. KEY RESULTS Wide genetic diversity was found among the wild emmer accessions for all grain nutrients. The concentrations of grain zinc, iron and protein in wild accessions were about two-fold greater than in the domesticated genotypes. Concentrations of these compounds were positively correlated with one another, with no clear association with plant productivity, suggesting that all three nutrients can be improved concurrently with no yield penalty. A subset of 12 populations revealed significant genetic variation between and within populations for all minerals. Association between soil characteristics at the site of collection and grain nutrient concentrations showed negative associations between soil clay content and grain protein and between soil-extractable zinc and grain zinc, the latter suggesting that the greatest potential for grain nutrient minerals lies in populations from micronutrient-deficient soils. CONCLUSIONS Wild emmer wheat germplasm offers unique opportunities to exploit favourable alleles for grain nutrient properties that were excluded from the domesticated wheat gene pool.

Reconsidering Domestication Of Legumes Versus Cereals In The Ancient Near East

In this paper, we discuss, from both biological and cultural perspectives, the ancient human-plant liaison that gave rise to Near Eastern agriculture. We explain the biological aspects of Near Eastern plant domestication by a comparative analysis of legume vs. cereal crop evolution. This comparison is illustrated by the natural distribution, ecological affinity, physiology, population structure, floral biology, growth habit, plant stature, seed dispersal mode, and seed dormancy of both wild and domesticated plants of these crop groups. We discuss the differences between Near Eastern legumes and cereals with regard to each of the above aspects, and we highlight the relevance of these differences with regard to Neolithic decision-making, adoption for farming, and subsequent evolution under domestication. We reached the following conclusions: (1) Near Eastern legumes underwent different evolutionary trajectories under domestication as compared with their companion cereals, despite apparent similarities between selection under domestication of both crop groups. (2) Careful comparison of pea, lentil, and chickpea shows that each of the Near Eastern legume crops has a unique evolutionary history in its own right, and this also holds true for the cereal crops. (3) The evolutionary history of each of the Near Eastern crops, prior to as well as after domestication, is well-reflected in its adaptation profile in present-day cropping systems, which determines each crops relative economic importance in different world regions (e.g., chickpea is a major pulse in the Indian subcontinent, and pea is a more important crop in temperate regions, while barley has the widest adaptation, extending from high-latitude temperate regions to semi-arid Mediterranean systems). (4) Ancient choice-making, as reflected in the founder crops repertoire, involved nutritional considerations that may have outweighed grain yield per area and/or time unit criteria.

Breeding tomatoes for salt tolerance: inheritance of salt tolerance and related traits in interspecific populations

SummaryInterspecific segregating populations derived from a cross between tomato (Lycopersicon esculentum) cv ‘M82-1 -8’ (M82) and the wild species L. pennellii accession LA-716 (Lpen716) were used to study the genetic basis of salt tolerance and its implications for breeding. BC1 (M82 x (M82 x Lpen716)) and BC1 S1 (progenies of selfed BC1 plants) populations were grown under arid field conditions and irrigated with water having electrical conductivities of 1.5 (control), 10 and 20 dSm-1. The evaluation of salt tolerance was based on total fruit yield (TY), total dry matter (TD) and TD under salinity relative to the control (RD). Sodium, potassium and chloride concentrations were measured in the leaves and stems. The methods for estimating heritability were adapted to BC1 plants and BC1S1 families. TY, TD and RD had heritability estimates of 0.3–0.45, indicating that salt tolerance can be improved by selection. Genetic correlations between traits indicated that high yield may be combined with salt tolerance and that ion contents are not likely to provide an efficient selection criteria for salt tolerance. Genetic correlations between performances under various salinity levels suggested that similar mechanisms affect the responses to salinity treatments of 10 and 20 dSm-1. Responses to “paper” selection confirmed that salt tolerance of the tomato may be improved by selection, and that this selection should be based on dry matter and yield parameters under salinity.

Carbon isotope ratio in cotton varies with growth stage and plant organ

Abstract Carbon isotope ratio ( 13 C/ 12 C, expressed with a differential notation as δ 13 C) has been proposed as an indirect selection criterion for plant water-use efficiency (WUE=total dry matter produced or yield harvested/water used). For efficient modification of WUE in breeding programs, it is essential to determine a sampling strategy, which will provide consistent genotypic ranking for δ 13 C and maximum differentiation between genotypes. The effects of growth stages and plant organs on δ 13 C values and their genotypic variations were studied in cotton cultivars grown in the field under two irrigation regimes. Values of δ 13 C in leaf remained stable during peak flowering and boll development stages and significantly increased at boll ripening stage, which could result from the effect of late-season water stress on WUE. δ 13 C varied significantly between plant organs, with lower values obtained in assimilating organs, leaf and bur, and higher values in non-assimilating organs, stem and fiber. This could possibly have resulted from carbon discrimination during secondary metabolism. A non-crossover interaction was found between growth stage and cultivar, whereas plant-organ effect did not interact with cultivars, indicating that a similar ranking of cultivars can be expected with δ 13 C at various growth stages and plant organs. Late season leaf samples and bur of the last boll revealed maximal differentiation among genotypes and water regimes, both among other growth stages as well as among other plant organs. These samples have also revealed the highest correlations between δ 13 C and WUE estimates. It is concluded that leaf sampling during boll ripening stage may be most effective for the assessment of δ 13 C as an indicator of WUE.