Hexaploid Parent Research Articles

The Physiological Basis of Improved Heat Tolerance in Selected Emmer-Derived Hexaploid Wheat Genotypes.

Wheat is sensitive to high-temperature stress with crop development significantly impaired depending on the severity and timing of stress. Various physiological mechanisms have been identified as selection targets for heat tolerance; however, the complex nature of the trait and high genotype × temperature interaction limits the selection process. A three-tiered phenotyping strategy was used to overcome this limitation by using wheat genotypes developed from the ancient domesticated wheat, emmer (Triticum dicoccon Schrank), which was considered to have a wide variation for abiotic stress tolerance. A contrasting pair of emmer-based hexaploid lines (classified as tolerant; G1 and susceptible; G2) developed from a backcross to the same recurrent hexaploid parent was chosen based on heat stress responses in the field and was evaluated under controlled glasshouse conditions. The same pair of contrasting genotypes was also subsequently exposed to a short period of elevated temperature (4 days) at anthesis under field conditions using in-field temperature-controlled chambers. The glasshouse and field-based heat chambers produced comparable results. G1 was consistently better adapted to both extended and short periods of heat stress through slow leaf senescence under heat stress, which extended the grain filling period, increased photosynthetic capacity, increased grain filling rates, and resulted in greater kernel weight and higher yield. The use of a combination of phenotyping methods was effective in identifying heat tolerant materials and the mechanisms involved.

Identification, introgression, and molecular marker genetic analysis and selection of a highly effective novel oat crown rust resistance from diploid oat, Avena strigosa.

Oat crown rust is one of the most damaging diseases of oat. We identified a new source of resistance and developed KASP and TaqMan markers for selection in breeding programs. A new highly effective resistance to oat crown rust (Puccinia coronata f. sp. avenae) was identified in the diploid oat Avena strigosa PI 258731 and introgressed into hexaploid cultivated oat. Young plants with this resistance show moderate susceptibility, whereas older plant tissues and adult plants are resistant with no virulent isolates encountered in over 8years of testing. Resistance was incorporated into hexaploid oat by embryo rescue, colchicine chromosome doubling followed by backcrosses with a hexaploid parent, and selection for stable transmission of resistance. To mitigate flag leaf and panicle chlorosis/necrosis associated with the resistance, crosses were made with derived resistant lines to breeding lines of divergent parentage followed by selection. Subsequently, two F2 sister lines, termed MNBT1020-1 and MNBT1021-1, were identified in which the chlorosis/necrosis was reduced. These two lines performed well in replicated multi-location state trials in 2015 and 2016 out-yielding all cultivar entries. Segregating F2:3 plants resulting from crosses of MNBT lines to susceptible parents were genotyped with the oat 6K SNP array, and SNP loci with close linkage to the resistance were identified. KASP assays generated from linked SNPs showed accurate discrimination of the resistance in derivatives of the resistant MNBT lines crossed to susceptible breeding lines. A TaqMan marker was developed and correctly identified homozygous resistance in over 95% of 379 F4 plants when rust was scored in F4:5 plants in the field. Thus, a novel highly effective resistance and associated molecular markers are available for use in breeding, genetic analysis, and functional studies.

Gene expression variation in natural populations of hexaploid and allododecaploid Spartina species (Poaceae)

Allopolyploidy is a peculiar process entailing the cohabitation of two (or more) divergent genomes. Consequences on plant genomes are varied and can utlimately alter gene expression and regulatory interactions. Most studies have explored polyploid expression evolution in experimental controlled conditions. Here, we analyzed global gene expression variation in natural populations of the Spartina polyploid complex including hexaploid parental species (S. maritima and S. alterniflora), two F1 hybrids (S. × townsendii and S. × neyrautii) and the allododecaploid S. anglica. In situ sampling and quantitative PCR were performed for comparing global expression of candidate genes involved in responses to abiotic stresses, lignin and cellulose metabolisms between five Spartina taxa. Illumina sequencing datasets and dedicated bioinformatic pipelines were employed to explore sequence heterogeneity in these highly duplicated genomes. Levels of gene expression were significantly higher in S. alterniflora (compared to the other hexaploid parent S. maritima) for seven of the analyzed genes. Effects of both hybridization and polyploidization are detected and consistent with previous global transcriptome analyses performed on Spartina plants grown in controlled conditions. Duplicated copies present in the hybrids and the allododecaploid were successfully assigned to either one of the parental genomes. Phylogenetic analyses identified for each of the parental hexaploid species, the presence of two distinct clades including two or more expressed copies. We provide here a comprehensive gene expression study based on individuals sampled in their natural habitat and detected the superimposed effect of environmental heterogeneity, hybridization and allopolyploidy.

The first genetic map of a synthesized allohexaploid Brassica with A, B and C genomes based on simple sequence repeat markers.

We present the first genetic map of an allohexaploid Brassica species, based on segregating microsatellite markers in a doubled haploid mapping population generated from a hybrid between two hexaploid parents. This study reports the first genetic map of trigenomic Brassica. A doubled haploid mapping population consisting of 189 lines was obtained via microspore culture from a hybrid H16-1 derived from a cross between two allohexaploid Brassica lines (7H170-1 and Y54-2). Simple sequence repeat primer pairs specific to the A genome (107), B genome (44) and C genome (109) were used to construct a genetic linkage map of the population. Twenty-seven linkage groups were resolved from 274 polymorphic loci on the A genome (109), B genome (49) and C genome (116) covering a total genetic distance of 3178.8cM with an average distance between markers of 11.60cM. This is the first genetic framework map for the artificially synthesized Brassica allohexaploids. The linkage groups represent the expected complement of chromosomes in the A, B and C genomes from the original diploid and tetraploid parents. This framework linkage map will be valuable for QTL analysis and future genetic improvement of a new allohexaploid Brassica species, and in improving our understanding of the genetic control of meiosis in new polyploids.

Retention of D genome chromosomes in pentaploid wheat crosses

The transfer of genes between Triticum aestivum (hexaploid bread wheat) and T. turgidum (tetraploid durum wheat) holds considerable potential for genetic improvement of both these closely related species. Five different T. aestivum/T. turgidum ssp. durum crosses were investigated using Diversity Arrays Technology (DArT) markers to determine the inheritance of parental A, B and D genome material in subsequent generations derived from these crosses. The proportions of A, B and D chromosomal segments inherited from the hexaploid parent were found to vary significantly among individual crosses. F(2) populations retained widely varying quantities of D genome material, ranging from 99% to none. The relative inheritance of bread wheat and durum alleles in the A and B genomes of derived lines also varied among the crosses. Within any one cross, progeny without D chromosomes in general had significantly more A and B genome durum alleles than lines retaining D chromosomes. The ability to select for and manipulate this non-random segregation in bread wheat/durum crosses will assist in efficient backcrossing of selected characters into the recurrent durum or hexaploid genotype of choice. This study illustrates the utility of DArT markers in the study of inter-specific crosses to commercial crop species.

Vitamin C, flower color and ploidy variation of hybrids from a ploidy-unbalanced Actinidia interspecific cross and SSR characterization

Seedlings derived from an Actinidia interspecific cross between the hexaploid Actinidia chinensis var. deliciosa ‘Jinkui’ and the diploid male A. eriantha × A. chinensis var. chinensis ‘Chaohong’ hybrid were analyzed using flow cytometry, SSR markers and phenotypic observations. The results show that the leaf vitamin C content of this hybrid population has a mid-parent heterosis. Separation of flower color in the progeny was also observed, progeny with red flowers lighter than ‘Chaohong’, white flowers as in ‘Jinkui’ and intermediate types with a red base to the petals and white margins were all present. Flow cytometry analysis confirmed that most of the progeny were tetraploids, and molecular marker data showed that most of these tetraploid progeny had three alleles from the hexaploid parent and one allele from the diploid parent. UPGMA analysis based on the SSR markers showed that the diploid parent was completely separated from the hexaploid parent and all the progeny.

QTL mapping of multiple foliar disease and root-lesion nematode resistances in wheat

A genetic linkage map, based on a cross between the synthetic hexaploid CPI133872 and the bread wheat cultivar Janz, was established using 111 F1-derived doubled haploid lines. The population was phenotyped in multiple years and/or locations for seven disease resistance traits, namely, Septoria tritici blotch (Mycosphaeralla graminicola), yellow leaf spot also known as tan spot (Pyrenophora tritici-repentis), stripe rust (Puccinia striiformis f. sp. tritici), leaf rust (Puccinia triticina), stem rust (Puccinia graminis f. sp. tritici) and two species of root-lesion nematode (Pratylenchyus thornei and P. neglectus). The DH population was also scored for coleoptile colour and the presence of the seedling leaf rust resistance gene Lr24. Implementation of a multiple-QTL model identified a tightly linked cluster of foliar disease resistance QTL in chromosome 3DL. Major QTL each for resistance to Septoria tritici blotch and yellow leaf spot were contributed by the synthetic hexaploid parent CPI133872 and linked in repulsion with the coincident Lr24/Sr24 locus carried by parent Janz. This is the first report of linked QTL for Septoria tritici blotch and yellow leaf spot contributed by the same parent. Additional QTL for yellow leaf spot were detected in 5AS and 5BL. Consistent QTL for stripe rust resistance were identified in chromosomes 1BL, 4BL and 7DS, with the QTL in 7DS corresponding to the Yr18/Lr34 region. Three major QTL for P. thornei resistance (2BS, 6DS, 6DL) and two for P. neglectus resistance (2BS, 6DS) were detected. The recombinants combining resistance to Septoria tritici blotch, yellow leaf spot, rust diseases and root-lesion nematodes from parents CPI133872 and Janz constitute valuable germplasm for the transfer of multiple disease resistance into new wheat cultivars.

Extraction of AADD component of Triticum aestivum (AABBDD)

A synthetic tetraploid, Triticum aegilopoides X Aegilops squarrosa (AADD), was crossed with seven varieties of Triticum aestivum and with one synthetic hexaploid, Triticum durum var. Carleton X Aegilops squarrosa. Two AADDB pentaploids were produced from 2 varieties of T. aestivum, Chinese Spring and A.T. 38 and a pentaploid AADDB hybrid was obtained from the cross involving the synthetic hexaploid, T. durum var. Carleton X Ae. squarrosa. The pentaploid involving var. Chinese Spring was backcrossed to the hexaploid parent and a general backcross was attempted with the pentaploid from A.T. 38, but no seed was obtained. The three generations of the synthetic tetraploid, T. aegilopoides X Ae. squarrosa were quite uniform morphologically, but there was marked variation in seed fertility. There is no evidence that variation in the fertility of the synthetic tetraploid was due to irregular meiosis or to the occurrence of aneuploids. The problems connected with the extraction of AADD component of T. aestivum are discussed.

Effectiveness of Interspecific Hybridization for Incorporating Birch Leafminer (Fenusa pusilla) Resistance into White-barked Betula

A significant pest affecting commonly planted Betula spp. is the birch leafminer (Fenusa pusilla Lepeletier), an insect that can be present in large populations in the landscape and can greatly reduce the vigor and ornamental value of these trees. Twenty-two interspecific crosses were performed between leafminer resistant and susceptible Betula species in an attempt to create the novel combination of ornamental white bark and significant leafminer resistance. Of the nine successful crosses, two produced resistant offspring. Progeny of the diploid × hexaploid cross B. turkestanica Litvin (2x) × B. alleghaniensis Britt. (6x) displayed a broad range of resistance levels, likely the result of segregating alleles contributed by the hexaploid parent. All crosses involving highly resistant individuals of B. costata Trautv. (2x) yielded leafminer susceptible progeny. These results suggest that the larval antibiosis demonstrated by B. alleghaniensis and B. costata is inherited as a recessive trait, and exhibits a gene dosage effect as evidenced by the B. turkestanica × B. alleghaniensis offspring. While most progeny of the B. populifolia Marsh (2x) × B. maximowicziana Regal (2x) cross were susceptible, a single resistant offspring, which was found to be triploid (3x), displayed a mechanism of resistance similar to that of a hypersensitive response. No strong intersectional barriers to hybridization were observed and all interploidy crosses were successful. The chromosome numbers of B. costata (2n = 2x = 28) and B. turkestanica (2n = 2x = 28) are reported here for the first time. The results of this study indicate that the potential exists for the development of insect resistant, ornamental white-barked birch clones through the implementation of a planned, systematic breeding program.

Introgressive hexaploid oats from the Avena abyssinica (♀) × A. sativa hybrid: performance, grain lipids and proteins.

A fertile hexaploid oat plant was obtained after several generations of selection for seed set and plant type from a colchicine-produced decaploid hybrid, Avena abyssinica(2n = 4x = 28, AABB) × A. sativa (2n = 6x = 42, AACCDD). The selected line proved to be stably fertile and in many characteristics equal or superior to the hexaploid parent. The grain protein fractions showed two qualitative differences from those of the pollen parent and several differences from the maternal parent. The fractionating extraction used was new for oats. The fatty acid composition of grains of the hybrid derivative was similar to that of the pollen parent, but different from that of the maternal parent. The maternal parent (A. abyssinica) had a relatively high 16:0 fatty acid content (ca. 20.5 mol%) compared with the level of the hexaploid parent and the hybrid derivation (ca. 17.5 mol% each) in field-grown grain. However, in grain produced in the greenhouse, the hexaploids had ca. 20.5 mol% of 16:0 fatty acid and a decrease in 18:1 fatty acid, whereas seed of the A. abyssinica parent showed only a slight increase (ca.21.5 mol%) in 16:0 fatty acid. These and other responses statistically significant may be due to adaptation to temperature conditions being wider in the hexaploids than in the East-African A. abyssinica. A new method of grain lipid extraction was introduced and showed good reproducibility. The derived hexaploid oat can be crossed with A. sativa for breeding purposes and due to its early maturity might also have direct use in northern or high-altitude cultivation.

Selection for Spot Blotch Resistance in Four Spring Wheat Populations

Spot blotch of wheat (Triticum aestivum L.), caused by Bipolaris sorokiniana (Sacc.) Shoem, is serious constraint in South Asia, and leading cultivars have low levels of resistance. The response to selection for low and high area under disease progress curve (AUDPC) of spot blotch in four wheat populations, involving different Chinese hexaploid parents with high level of resistance and a commercial cultivar moderately resistant to spot blotch, was investigated. Selections were made in the F3 generation for low and high AUDPC of spot blotch and selected progenies evaluated in a replicated field test in the F4 generation at Rampur, Nepal, in 1994. Selection in the F3 for low and high AUDPC was effective in identifying F4 lines with low and high AUDPC, respectively. Low AUDPC resulted in higher biomass and grain yield, higher harvest index, and 1000‐kernel weight. On average, the low AUDPC lines headed later than the high AUDPC lines. Realized heritability estimates for AUDPC were intermediate to high in magnitude (0.48 to 0.76). AUDPC was negatively correlated with biomass (r = −0.195 to −0.451), grain yield (r = −0.169 to −0.452), harvest index (r = −0.256 to −0.597), days to heading (r =−0.319 to −0.570), and 1000‐kernel weight (r = −0.322 to −0.530). Results indicate that selection for low AUDPC of spot blotch in segregating generations would be effective in identifying wheat lines with high levels of resistance and would have positive effects on other characters.

Heritability estimates of field resistance to spot blotch in four spring wheat crosses

AbstractSpot blotch of wheat (Triticum aestivum L.), caused by Bipolaris sorokiniana (Sacc. in Sorok.) Shoem., is a major disease in South Asia. Popular commercial cultivars have low levels of resistance to spot blotch. Information on the inheritance of spot blotch resistance in wheat is lacking. Field studies were conducted in four wheat crosses, each involving a Chinese hexaploid parent with high levels of resistance and a commercial cultivar with low to intermediate levels of resistance to spot blotch. Data were recorded in the F2, F3 and F4 generations to estimate heritability. Field studies were conducted in three years (1992–94) at Rampur. Nepal, involving 150 lines in each cross. The spot blotch score was recorded as the percentage necrosis and associated chlorosis of the two upper most leaf surface. In the F2 generation three spot blotch readings on the flag leaf were taken whereas in the F3 and F4 generations four readings were recorded at 5‐day intervals on the flag and the penultimate leaves. The highest disease score (HDS) and the area under disease progress curve (AUDPC) were analysed. Heritability (h2) estimates for spot blotch resistance were intermediate to high measured in terms of HDS (0.47 < h2 < 0.67) and also AUDPC (0.58 < h2 < 0.77) both in F3 and F4 generations in each of the four crosses. Heritability values were somewhat higher for AUDPC than HDS. There were significant negative correlations (r) of days to heading with HDS (‐0.186 < r < ‐0.515) and AUDPC (‐0.218 < r < ‐0.623). One‐hundred kernel weight was significantly negatively correlated to AUDPC (‐0.245 < r < ‐0.454) in all crosses in each generation. The results suggest that selection for resistance to spot blotch could be effective in the segregating populations generated from hexaploid wheat parents having different levels of resistance. Although AUDPC appeared to be a better measure to determine genetic differences for spot blotch in wheat, HDS would be adequate in screening trials for resistance to spot blotch.

Inheritance of morphological and chemical characters in interspecific hybrids between Papaver bracteatum and Papaver pseudo-orientale

The alkaloid profiles and morphological traits of the capsules of Papaver bracteatum, P. pseudo-orientale, and their hybrids were studied. Dominance of the hexaploid parent P. pseudo-orientale was observed for various characters. A genetic model assuming allelic additive effects and polysomic inheritance was elaborated for the control of isothebaine content in the capsules. The distribution of thebaine content in the segregating generations, F2 and BCF1 was evidence of the transfer of genes from the diploid parent P. bracteatum in the gametes of the interspecific hybrid and their expression in its progenies. These findings indicate the potential use of inter-specific hybrids between the Oxytona species in the breeding of cultivars for industrial or ornamental purposes.

Cytological Studies in Cultivated Species of Camellia : III. Tetraploid Species and Hybrids between Diploid Species and Hexaploid Species

Pairing between the homologous chromosomes in some cultivated clones of Camellia gralethamialea was found to be complete and normal at metaphase I. The comparative study of meiosis revealed the presence of some univalents, trivalents, or quadrivalents in a tetraploid cultivar of C. japoleica and 10 tetraploid hybrids between diploid species and hexaploid species. Majority of the hybrid combinations showed similar patterns of chromosome associations at metaphase I: the most common configuration was 30u, and the mean numbers of chiasmata per bivalent were quite high, 1.9-2. O. Thus, among 30 bivalents 15 suggested allosyndetic pairing between one genome from diploid parent and one genome from hexaploid parent. Metaphase chromosome pairing in meiosis I in the tetraploid hybrids of Camellia studied was not sufficient to assign distinct genome formulas for the properties of different chromosome sets in various species. The presence of sterility and partial sterility shown in a tetraploid cultivar of C. japonica and 10 tetraploid hybrids might be due to both genetic and unstable meiotic chro-mosome configurations at metaphase I as well as developing petaloidy.

Cytogenetic Studies in <i>Avena</i>, XI. Chromosomal Constitution of the Inbred Progeny of Synthetic Amphiploid Oats (10x)

Cytological studies were made on the highly fertile progenies of the amphiploids (10x=70) produced from the interspecific hybrids between tetraploid (4x=28) and hexaploid (6x=42) species of Avena. These progeny lines were derived through ten to twelve generations of inbreeding and selection for fertility and vigor.Of fifteen lines examined, twelve had chromosome numbers varying from 2n=53 to 58, one had 50 to 52, and two had 42 chromosomes, as the result of chromosomal elimination in the preceding generations. The meiotic behavior of chromosomes was more regular in the plants with 42 chromosomes than in those with 54, 56 or 58 chromosomes. The progeny of the amphiploid oats tended to be cytogenetically stable on either octoploid or hexaploid level.Cytological observations in the F1 hybrids between different octoploid lines revealed that they are mutually different with respect to a few to several chromosomes. Pairing configurations of chromosomes in the F1 hybrids between the octoploid derivatives and the parental hexaploid species suggested that the octoploids possess all the three genomes of the natural hexaploids, although structural modifications have taken place probably in some of the chromosomes. One of the derived hexaploid lines was phenotypically similar to the parental hexaploid species, whereas the other possessed the characteristics of the both tetraploid and hexaploid parents. Cytological data from the testcrosses showed that the derived hexaploids have the same genome constitution with the natural hexaploids, except for one or two structurally modified chromosomes.The consequences of new synthetic oats are discussed in relation to the genome constitution.

Cytogenetic Studies in <i>Avena</i>, IX New Synthetic Oats in the Progenies of Induced Decaploid Interspecific Hybrids

For the pentaploid hybridization, Avena barbata, A. abyssinica and Tt-1 (a barbata like derivative from triploid hybrids, A. barbata×A. strigosa) were used as tetraploid parents and certain varieties of A. sativa and A. nuda as hexaploid ones. Young seedlings of the pentaploid hybrids were treated with 0.05% colchicine solution and their chromosome number was successfully doubled in some of them, producing decaploid amphiploids or chimera plants consisting of pentaploids and decaploids.Decaploid plants or panicles (C1) showed high seed fertility although the pentaploid ones set usually no seeds. Decaploids amphiploids, however, failed to maintain their high seed fertility. In C8-C12 generations, however, some amphiploid lines recovered fertility equal to the original parents but other lines still showed low or wide variation in fertility.Cytological investigations were attempted with certain lines in C9-C13. It is especially noted that certain lines have 2n=42, 54, 56, or 58, instead of the expected chromosome number, 70(2n) and those lines show regular meiotic behavior of chromosomes. In the other lines the chromosome number varied over a limited range (2-4) and still others over a wide range (4-8).Some characters such as plant height, tillering, and date of maturity were studied and compared with one of the parents, A. sativa var. Banner. Most of these synthetic oats are new forms possessing some characters from both tetraploid and hexaploid parents, and are much improved in certain agronomic characters.