<Emphasis Type="Italic">VRN1</Emphasis> genes variability in tetraploid wheat species with a spring growth habit

Background

Vernalization genes VRN1 play a major role in the transition from vegetative to reproductive growth in wheat. In di-, tetra- and hexaploid wheats the presence of a dominant allele of at least one VRN1 gene homologue (Vrn-A1,?Vrn-B1, Vrn-G1 or Vrn-D1) determines the spring growth habit. Allelic variation between the Vrn-1 and vrn-1 alleles relies on mutations in the promoter region or the first intron. The origin and variability of the dominant VRN1 alleles, determining the spring growth habit in tetraploid wheat species have been poorly studied.

Results

Here we analyzed the growth habit of 228 tetraploid wheat species accessions and 25 % of them were spring type. We analyzed the promoter and first intron regions of VRN1 genes in 57 spring accessions of tetraploid wheats. The spring growth habit of most studied spring accessions was determined by previously identified dominant alleles of VRN1 genes. Genetic experiments proof the dominant inheritance of Vrn-A1d allele which was widely distributed across the accessions of Triticum dicoccoides. Two novel alleles were discovered and designated as Vrn-A1b.7 and Vrn-B1dic. Vrn-A1b.7 had deletions of 20 bp located 137 bp upstream of the start codon and mutations within the VRN-box when compared to the recessive allele of vrn-A1. So far the Vrn-A1d allele was identified only in spring accessions of the T. dicoccoides and T. turgidum species. Vrn-B1dic was identified in T. dicoccoides IG46225 and had 11 % sequence dissimilarity in comparison to the promoter of vrn-B1. The presence of Vrn-A1b.7 and Vrn-B1dic alleles is a predicted cause of the spring growth habit of studied accessions of tetraploid species. Three spring accessions T. aethiopicum K-19059, T. turanicum K-31693 and T. turgidum cv. Blancal possess recessive alleles of both VRN-A1 and VRN-B1 genes. Further investigations are required to determine the source of spring growth habit of these accessions.

Conclusions

New allelic variants of the VRN-A1 and VRN-B1 genes were identified in spring accessions of tetraploid wheats. The origin and evolution of VRN-A1 alleles in di- and tetraploid wheat species was discussed.

     

Large deletions within the first intron in <Emphasis Type="Italic">VRN-1</Emphasis> are associated with spring growth habit in barley and wheat

The broad adaptability of wheat and barley is in part attributable to their flexible growth habit, in that spring forms have recurrently evolved from the ancestral winter growth habit. In diploid wheat and barley growth habit is determined by allelic variation at the VRN-1 and/or VRN-2 loci, whereas in the polyploid wheat species it is determined primarily by allelic variation at VRN-1. Dominant Vrn-A1 alleles for spring growth habit are frequently associated with mutations in the promoter region in diploid wheat and in the A genome of common wheat. However, several dominant Vrn-A1, Vrn-B1, Vrn-D1 (common wheat) and Vrn-H1 (barley) alleles show no polymorphisms in the promoter region relative to their respective recessive alleles. In this study, we sequenced the complete VRN-1 gene from these accessions and found that all of them have large deletions within the first intron, which overlap in a 4-kb region. Furthermore, a 2.8-kb segment within the 4-kb region showed high sequence conservation among the different recessive alleles. PCR markers for these deletions showed that similar deletions were present in all the accessions with known Vrn-B1 and Vrn-D1 alleles, and in 51 hexaploid spring wheat accessions previously shown to have no polymorphisms in the VRN-A1 promoter region. Twenty-four tetraploid wheat accessions had a similar deletion in VRN-A1 intron 1. We hypothesize that the 2.8-kb conserved region includes regulatory elements important for the vernalization requirement. Epistatic interactions between VRN-H2 and the VRN-H1 allele with the intron 1 deletion suggest that the deleted region may include a recognition site for the flowering repression mediated by the product of the VRN-H2 gene of barley.     

Allelic variation at the <Emphasis Type="Italic">VRN-1</Emphasis> promoter region in polyploid wheat

Vernalization, the requirement of a long exposure to low temperatures to induce flowering, is an essential adaptation of plants to cold winters. We have shown recently that the vernalization gene VRN-1 from diploid wheat Triticum monococcum is the meristem identity gene APETALA1, and that deletions in its promoter were associated with spring growth habit. In this study, we characterized the allelic variation at the VRN-1 promoter region in polyploid wheat. The Vrn-A1a allele has a duplication including the promoter region. Each copy has similar foldback elements inserted at the same location and is flanked by identical host direct duplications (HDD). This allele was found in more than half of the hexaploid varieties but not among the tetraploid lines analyzed here. The Vrn-A1b allele has two mutations in the HDD region and a 20-bp deletion in the 5 UTR compared with the winter allele. The Vrn-A1b allele was found in both tetraploid and hexaploid accessions but at a relatively low frequency. Among the tetraploid wheat accessions, we found two additional alleles with 32 bp and 54 bp deletions that included the HDD region. We found no size polymorphisms in the promoter region among the winter wheat varieties. The dominant Vrn-A1 allele from two spring varieties from Afghanistan and Egypt (Vrn-A1c allele) and all the dominant Vrn-B1 and Vrn-D1 alleles included in this study showed no differences from their respective recessive alleles in promoter sequences. Based on these results, we concluded that the VRN-1 genes should have additional regulatory sites outside the promoter region studied here.     

Genetics of flowering time in bread wheat <Emphasis Type="Italic">Triticum aestivum</Emphasis>: complementary interaction between vernalization-insensitive and photoperiod-insensitive mutations imparts very early flowering habit to spring wheat

Time to flowering in the winter growth habit bread wheat is dependent on vernalization (exposure to cold conditions) and exposure to long days (photoperiod). Dominant Vrn-1 (Vrn-A1, Vrn-B1 and Vrn-D1) alleles are associated with vernalization independent spring growth habit. The semidominant Ppd-D1a mutation confers photoperiod-insensitivity or rapid flowering in wheat under short day and long day conditions. The objective of this study was to reveal the nature of interaction between Vrn-1 and Ppd-D1a mutations (active alleles of the respective genes vrn-1 and Ppd-D1b). Twelve Indian spring wheat cultivars and the spring wheat landrace Chinese Spring were characterized for their flowering times by seeding them every month for five years under natural field conditions in New Delhi. Near isogenic Vrn-1 Ppd-D1 and Vrn-1 Ppd-D1a lines constructed in two genetic backgrounds were also phenotyped for flowering time by seeding in two different seasons. The wheat lines of Vrn-A1a Vrn-B1 Vrn-D1 Ppd-D1a, Vrn-A1a Vrn-B1 Ppd-D1a and Vrn-A1a Vrn-D1 Ppd-D1a (or Vrn-1 Ppd-D1a) genotypes flowered several weeks earlier than that of Vrn-A1a Vrn-B1 Vrn-D1 Ppd-D1b, Vrn-A1b Ppd-D1b and Vrn-D1 Ppd-D1b (or Vrn-1 Ppd-D1b) genotypes. The flowering time phenotypes of the isogenic vernalization-insensitive lines confirmed that Ppd-D1a hastened flowering by several weeks. It was concluded that complementary interaction between Vrn-1 and Ppd-D1a active alleles imparted super/very-early flowering habit to spring wheats. The early and late flowering wheat varieties showed differences in flowering time between short day and long day conditions. The flowering time in Vrn-1 Ppd-D1a genotypes was hastened by higher temperatures under long day conditions. The ambient air temperature and photoperiod parameters for flowering in spring wheat were estimated at 25°C and 12 h, respectively.     

The occurrence of spring forms in tetraploid Timopheevi wheat is associated with variation in the first intron of the <Emphasis Type="Italic">VRN-A1</Emphasis> gene

Background

Triticum araraticum and Triticum timopheevii are tetraploid species of the Timopheevi group. The former includes both winter and spring forms with a predominance of winter forms, whereas T. timopheevii is considered a spring species. In order to clarify the origin of the spring growth habit in T. timopheevii, allelic variability of the VRN-1 gene was investigated in a set of accessions of both tetraploid species, together with the diploid species Ae. speltoides, presumed donor of the G genome to these tetraploids.

Results

The promoter region of the VRN-A1 locus in all studied tetraploid accessions of both T. araraticum and T. timopheevii represents the previously described allele VRN-A1f with a 50 bp deletion near the start codon. Three additional alleles were identified namely, VRN-A1f-del, VRN-A1f-ins and VRN-A1f-del/ins, which contained large mutations in the first (1^st) intron of VRN-A1. The first allele, carrying a deletion of 2.7 kb in a central part of intron 1, occurred in a few accessions of T. araraticum and no accessions of T. timopheevii. The VRN-A1f-ins allele, containing the insertion of a 0.4 kb MITE element about 0.4 kb upstream from the start of intron 1, and allele VRN-A1f-del/ins having this insertion coupled with a deletion of 2.7 kb are characteristic only for T. timopheevii. Allelic variation at the VRN-G1 locus includes the previously described allele VRN-G1a (with the insertion of a 0.2 kb MITE in the promoter) found in a few accessions of both tetraploid species. We showed that alleles VRN-A1f-del and VRN-G1a have no association with the spring growth habit, while in all accessions of T. timopheevii this habit was associated with the dominant VRN-A1f-ins and VRN-A1f-del/ins alleles. None of the Ae. speltoides accessions included in this study had changes in the promoter or 1^st intron regions of VRN-1 which might confer a spring growth habit. The VRN-1 promoter sequences analyzed herein and downloaded from databases have been used to construct a phylogram to assess the time of divergence of Ae. speltoides in relation to other wheat species.

Conclusions

Among accessions of T. araraticum, the preferentially winter predecessor of T. timopheevii, two large mutations were found in both VRN-A1 and VRN-G1 loci (VRN-A1f-del and VRN-G1a) that were found to have no effect on vernalization requirements. Spring tetraploid T. timopheevii had one VRN-1 allele in common for two species (VRN-G1a), and two that were specific (VRN-A1f-ins, VRN-A1f-del/ins). The latter alleles include mutations in the 1^st intron of VRN-A1 and also share a 0.4 kb MITE insertion near the start of intron 1. We suggested that this insertion resulted in a spring growth habit in a progenitor of T. timopheevii which has probably been selected during subsequent domestication. The phylogram constructed on the basis of the VRN-1 promoter sequences confirmed the early divergence (~3.5 MYA) of the ancestor(s) of the B/G genomes from Ae. speltoides.

     

Molecular characterization of vernalization response genes in Canadian spring wheat.

Vernalization response (Vrn) genes play a major role in determining the flowering/maturity times of spring-sown wheat. We characterized a representative set of 40 western Canadian adapted spring wheat cultivars/lines for 3 Vrn loci. The 40 genotypes were screened, along with 4 genotypes of known Vrn genes, using previously published genome-specific polymerase chain reaction primers designed for detecting the presence or absence of dominant or recessive alleles of the major Vrn loci: Vrn-A1, Vrn-B1, and Vrn-D1. The dominant promoter duplication allele Vrn-A1a was present in 34 of 40 cultivars/lines, whereas the promoter deletion allele Vrn-A1b was present in only 1 of the western Canadian cultivars (Triticum aestivum L. 'Rescue') and 2 of its derivative chromosomal substitution lines. The intron deletion allele Vrn-A1c was not present in any line tested. Only 4 of the western Canadian spring wheat cultivars tested here carry the recessive vrn-A1 allele. The dominant allele of Vrn-B1 was detected in 20 cultivars/lines. Fourteen cultivars/lines had dominant alleles of Vrn-A1a and Vrn-B1 in combination. All cultivars/lines carried the recessive allele for Vrn-D1. The predominance of the dominant allele Vrn-A1a in Canadian spring wheat appears to be due to the allele's vernalization insensitivity, which confers earliness under nonvernalizing growing conditions. Wheat breeders in western Canada have incorporated the Vrn-A1a allele into spring wheats mainly by selecting for early genotypes for a short growing season, thereby avoiding early and late season frosts. For the development of early maturing cultivars with high yield potential, different combinations of Vrn alleles may be incorporated into spring wheat breeding programs in western Canada.     

The Characterization and Geographical Distribution of the Genes Responsible for Vernalization Requirement in Chinese Bread Wheat

The frequency and distribution of the major vernalization requirement genes and their effects on growth habits were studied.Of the 551 bread wheat genotypes tested,seven allelic combinations of the three Vrn.1 genes were found to be responsible for the spring habit,three for the facultative habit and one for the winter habit.The three Vrn-1 genes behaved additively with the dominant allele of Vrn-A1 exerting the strongest effect.The allele combinations of the facultative genotypes and the discovery of spring genotypes with "winter" allele of Vrn-1 implied the presence of as yet unidentified alleles/genes for vernalization response.The dominant alleles of the three Vrn-1 genes were found in all ten ecological regions where wheat Is cultivated in China,with Vrn-D1 as the most common allele in nine and Vrn-A1 in one.The combination of vrn-A 1vrnB 1Vrn-D1 was the predominant genotype in seven of the regions.Compared with landraces,improved varieties contain a higher proportion of the spring type.This was attributed by a higher frequency of the dominant Vrn-A1 and Vrn-B1 alleles in the latter.Correlations between Vrn-1 allelic constitutions and heading date,spike length,plant type as well as cold tolerance were established.     

Genetic and Molecular Characterization of the VRN2 Loci in Tetraploid Wheat

Winter wheat (Triticum spp.) varieties require long exposures to low temperatures to flower, a process called vernalization. The VRN2 locus includes two completely linked zinc finger-CCT domain genes (ZCCT1 and ZCCT2) that act as flowering repressors down-regulated during vernalization. Deletions or mutations in these two genes result in the elimination of the vernalization requirement in diploid wheat (Triticum monococcum). However, natural allelic variation in these genes has not been described so far in polyploid wheat (tetraploid Triticum turgidum and hexaploid Triticum aestivum). A tetraploid wheat population segregating for both VRN-A2 and VRN-B2 loci facilitated the characterization of different alleles. Comparisons between functional and nonfunctional alleles revealed that both ZCCT1 and ZCCT2 genes are able to confer vernalization requirement and that different ZCCT genes are functional in different genomes. ZCCT1 and ZCCT2 proteins from nonfunctional vrn2 alleles have mutations at arginine amino acids at position 16, 35, or 39 of the CCT domain. These positions are conserved between CCT and HEME ACTIVATOR PROTEIN2 (HAP2) proteins, supporting a model in which the action of CCT domains is mediated by their interactions with HAP2/HAP3/HAP5 complexes. This study also revealed natural variation in gene copy number, including a duplication of the functional ZCCT-B2 gene and deletions or duplications of the complete VRN-B2 locus. Allelic variation at the VRN-B2 locus was associated with a partially dominant effect, which suggests that variation in the number of functional ZCCT genes can be used to expand allelic diversity for heading time in polyploid wheat and, hopefully, improve its adaptation to different environments.     

Evolution of <Emphasis Type="Italic">VRN-1</Emphasis> homoeologous loci in allopolyploids of <Emphasis Type="Italic">Triticum</Emphasis> and their diploid precursors

Background

The key gene in genetic system controlling the duration of the vegetative period in cereals is the VRN1 gene, whose product under the influence of low temperature (vernalization) promotes the transition of the apical meristem cells into a competent state for the development of generative tissues of spike. As early genetic studies shown, the dominant alleles of this gene underlie the spring forms of plants that do not require vernalization for this transition. In wheat allopolyploids various combinations of alleles of the VRN1 homoeologous loci (VRN1 homoeoalleles) provide diversity in such important traits as the time to heading, height of plants and yield. Due to genetical mapping of VRN1 loci it became possible to isolate the dominant VRN1 alleles and to study their molecular structure compared with the recessive alleles defining the winter type of plants. Of special interest is the process of divergence of VRN1 loci in the course of evolution from diploid ancestors to wheat allopolyploids of different levels of ploidy.

Results

Molecular analysis of VRN1 loci allowed to establish that various dominant alleles of these loci appeared as a result of mutations in two main regulatory regions: the promoter and the first intron. In the diploid ancestors of wheat, especially, in those of A- genome (T. boeoticum, T. urartu), the dominant VRN1 alleles are rare in accordance with a limited distribution of spring forms in these species. In the first allotetraploid wheat species including T. dicoccoides, T. araraticum (T. timopheevii), the spring forms were associated with a new dominant alleles, mainly, within the VRN-A1 locus. The process of accumulation of new dominant alleles at all VRN1 loci was significantly accelerated in cultivated wheat species, especially in common, hexaploid wheat T. aestivum, as a result of artificial selection of spring forms adapted to different climatic conditions and containing various combinations of VRN1 homoeoalleles.

Conclusions

This mini-review summarizes data on the molecular structure and distribution of various VRN1 homoeoalleles in wheat allopolyploids and their diploid predecessors.

     

Molecular characterization of vernalization loci VRN1 in wild and cultivated wheats

Background  

Variability of the VRN1 promoter region of the unique collection of spring polyploid and wild diploid wheat species together with diploid goatgrasses (donor of B and D genomes of polyploid wheats) were investigated. Accessions of wild diploid (T. boeoticum, T. urartu) and tetraploid (T. araraticum, T. timopheevii) species were studied for the first time.     

Assessment of genomic and species relationships in Triticum and Aegilops by PAGE and by differential staining of seed albumins and globulins

Summary Endosperm protein components from common bread wheats (Triticum aestivum L.) and related species were extracted with aluminum lactate, pH 3.2, and examined by electrophoresis in the same buffer. Electrophoretic patterns of the albumins and globulins were compared to evaluate the possibility that a particular species might have contributed its genome to tetraploid or hexaploid wheat. Together with protein component mobilities, differential band staining with Coomassie Brilliant Blue R250 was employed to test the identity or non-identity of bands. Eight species and 63 accessions, representative of Triticum and Aegilops were tested. Considerable intraspecific variation was observed for patterns of diploid but not for tetraploid or hexaploid species. Patterns of some accessions of Triticum urartu agreed closely with major parts of the patterns of Triticum dicoccoides and T. aestivum. A fast-moving, green band was found in all accessions of T. urartu and of Triticum boeoticum, however, that was not found in those of T. dicoccoides or T. aestivum. This band was present in all accessions of Triticum araraticum and Triticum zhukovskyi. Patterns of Aegilops longissima, which has been suggested as the donor of the B genome, differed substantially from those of T. dicoccoides and T. aestivum. Finally, two marker proteins of intermediate mobility were also observed and may be used to discriminate between accessions of T. araraticum/T. zhukovskyi and those of T. dicoccoides/T. aestivum.     

Storage-protein variation in wild emmer wheat (Triticum turgidum ssp.dicoccoides) from Jordan and Turkey. I. Electrophoretic characterization of genotypes

Seed storage-protein variation at theGlu-A1,Glu-B1 andGli-B1/Glu-B3 loci in the tetraploid wild progenitor of wheat,T. dicoccoides, was studied electrophoretically in 315 individuals representing nine populations from Jordan and three from Turkey. A total of 44 different HMW-glutenin patterns were identified, resulting from the combination of 15 alleles in the A genome and 19 in the B genome. Twenty-seven new allelic variants, 12 at theGlu-A1 locus and 15 at theGlu-B1 locus, were identified by comparing the mobilities of their subunits to those previously found in bread and durum wheats. The novel variants include six alleles at theGlu-A1 locus showing both x and y subunits. The genes coding for the 1Bx and 1By subunits showed no or very little (3%) inactivity, the 1Ax gene showed a moderate degree (6.3%) of inactivity whereas the gene coding for lAy showed the highest degree of inactivity (84.8%). A high level of polymorphism was also present for the omega- and gamma-gliadins and LMW-glutenin subunits encoded by genes at the linkedGli-B1 andGlu-B3 loci (19 alleles). Some Jordanian accessions were found to contain omega-gliadin 35, gamma-gliadin 45, and LMW-2 also present in cultivated durum wheats and related to good gluten viscoelasticity. The newly-discovered alleles enhance the genetic variability available for improving the technological quality of wheats. Additionally some of them may facilitate basic research on the relationship between industrial properties and the number and functionality of HMW- and LMW-glutenin subunits.     

A reconsideration of the domestication geography of tetraploid wheats

The domestication of tetraploid wheats started from their wild progenitor Triticum dicoccoides. In this paper, the geographical distribution of this progenitor is revised to include more sampling locations. The paper is based on a collection of wild and domesticated lines (226 accessions in total) analyzed by AFLP at 169 polymorphic loci. The collection includes the 69 wild lines considered by Mori et al. (2003) in their study on chloroplast DNA haplotypes of T. dicoccoides. The goal of the experiment was to reconsider which location thought to have generated the domesticated germplasm has the highest chance of being the actual site from which wild progenitors were sampled during domestication. Phylogenetic analysis of the nuclear AFLP databases indicates that two different genetic taxa of T. dicoccoides exist, the western one, colonizing Israel, Syria, Lebanon and Jordan, and the central-eastern one, which has been frequently sampled in Turkey and rarely in Iran and Iraq. It is the central-eastern race that played the role of the progenitor of the domesticated germplasm. This is supported by the cumulative results of the AFLP data from the collections of Ozkan et al. (2002) and of Mori et al. (2003), which indicate that the Turkish Karacadag population, intermixed with some Iraq-Iran lines, has a tree topology consistent with that of the progenitor of domesticated genotypes. The Turkish Kartal population belongs genetically to the central-eastern T. dicoccoides race but at the nuclear DNA level is less related to the domesticated gene pool. A general agreement between published work on tetraploid wheat domestication emerges from these results. A disagreement is nevertheless evident at the local geographical scale; the chloroplast DNA data indicate the Kartal mountains while AFLP fingerprinting points to the Karacadag Range as the putative site of tetraploid wheat domestication.     

Chromosomal location of genes for novel glutenin subunits and gliadins in wild emmer wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> L. var.<Emphasis Type="Italic"> dicoccoides</Emphasis>)

The glutenin and gliadin proteins of wild emmer wheat, Triticum turgidum L. var. dicoccoides, have potential for improvement of durum wheat (T. turgidum L. var. durum) quality. The objective of this study was to determine the chromosomes controlling the high molecular weight (HMW) glutenin subunits and gliadin proteins present in three T. turgidum var. dicoccoides accessions (Israel-A, PI-481521, and PI-478742), which were used as chromosome donors in Langdon durum- T. turgidum var. dicoccoides (LDN-DIC) chromosome substitution lines. The three T. turgidum var. dicoccoides accessions, their respective LDN-DIC substitution lines, and a number of controls with known HMW glutenin subunits were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), urea/SDS-PAGE, and acid polyacrylamide gel electrophoresis (A-PAGE). The results revealed that all three T. turgidum var. dicoccoides accessions possess Glu-A1 alleles that are the same as or similar to those reported previously. However, each T. turgidum var. dicoccoides accession had a unique Glu-B1 allele. PI-478742 had an unusual 1Bx subunit, which had mobility slightly slower than the 1Ax subunit in 12% SDS-PAGE gels. The subunits controlled by chromosome 1B of PI-481521 were slightly faster in mobility than the subunits of the Glu-B1n allele, and the 1By subunit was identified as band 8. The 1B subunits of Israel-A had similar mobility to subunits 14 and 16. The new Glu-B1 alleles were designated as Glu-B1be in Israel-A, Glu-B1bf in PI-481521, and Glu-B1bg in PI-478742. Results from A-PAGE revealed that PI-481521, PI-478742, and Israel-A had eight, 12, and nine unique gliadin bands, respectively, that were assigned to specific chromosomes. The identified glutenin subunits and gliadin proteins in the LDN-DIC substitution lines provide the basis for evaluating their effects on end-use quality, and they are also useful biochemical markers for identifying specific chromosomes or chromosome segments of T. turgidum var. dicoccoides.Communicated by B. Friebe     

Cold hardiness of wheat near-isogenic lines differing in vernalization alleles

Four major genes in wheat (Triticum aestivum L.), with the dominant alleles designated Vrn-A1, Vrn-B1, Vrn-D1, and Vrn4, are known to have large effects on the vernalization response, but the effects on cold hardiness are ambiguous. Near-isogenic experimental lines (NILs) in a Triple Dirk (TD) genetic background with different vernalization alleles were evaluated for cold hardiness. Although TD is homozygous dominant for Vrn-A1 (formerly Vrn1) and Vrn-B1 (formerly Vrn2), four of the lines are each homozygous dominant for a different vernalization gene, and one line is homozygous recessive for all four vernalization genes. Following establishment, the plants were initially acclimated for 6 weeks in a growth chamber and then stressed in a low temperature freezer from which they were removed over a range of temperatures as the chamber temperature was lowered 1.3°C h^–1. Temperatures resulting in no regrowth from 50% of the plants (LT₅₀) were determined by estimating the inflection point of the sigmoidal response curve by nonlinear regression. The LT₅₀ values were –6.7°C for cv. TD, –6.6°C for the Vrn-A1 and Vrn4 lines, –8.1°C for the Vrn-D1 (formerly Vrn3) line, –9.4°C for the Vrn-B1 line, and –11.7°C for the homozygous recessive winter line. The LT₅₀ of the true winter line was significantly lower than those of all the other lines. Significant differences were also observed between some, but not all, of the lines possessing dominant vernalization alleles. The presence of dominant vernalization alleles at one of the four loci studied significantly reduced cold hardiness following acclimation.     

Epistatic interaction between vernalization genes Vrn-Am1 and Vrn-Am2 in diploid wheat

Genes Vrn-A(m)1 and Vrn-A(m)2 control the vernalization requirement in diploid wheat (Triticum monococcum). The epistatic interaction between these two genes on flowering date was studied here using a factorial analysis of variance. One hundred and two F2 plants were classified according to their genotypes for molecular markers tightly linked to Vrn-A(m)1 and Vrn-A(m)2. Mean comparisons showed that the VrnA(m)2 allele for winter growth habit was dominant to the vrn-A(m)2 allele for spring growth habit and that the Vrn-A(m)1 allele for spring growth habit was dominant to the vrn-A(m)1 allele for winter growth habit. A significant interaction was found between these two genes, suggesting that they work in the same developmental pathway. Plants homozygous for the recessive vrn-A(m)2 allele for spring growth habit flowered earlier than plants from the Vrn-A(m)2 class independently of the alleles present at Vrn-A(m)1. However, differences in heading date between plants with the Vrn-A(m)1 allele and those with the vrn-A(m)1 allele were significant only when the dominant Vrn-A(m)2 allele was present. A genetic model for the action of these two vernalization genes is proposed in which the role of Vrn-A(m)1 is to counteract the Vrn-A(m)2-mediated delay of flowering.     

Heterochromatin differentiation and phylogenetic relationship of the A genomes in diploid and polyploid wheats

Summary Heterochromatin differentiation, including band size, sites, and Giemsa staining intensity, was analyzed by the HKG (HCl-KOH-Giemsa) banding technique in the A genomes of 21 diploid (Triticum urartu, T. boeoticum and T. monococcum), 13 tetraploid (T. araraticum, T. timopheevi, T. dicoccoides and T. turgidum var. Dicoccon, Polonicum), and 7 cultivars of hexaploid (T. aestivum) wheats from different germplasm collections. Among wild and cultivated diploid taxa, heterochromatin was located mainly at centromeric regions, but the size and staining intensity were distinct and some accessions' genomes had interstitial and telomeric bands. Among wild and cultivated polyploid wheats, heterochromatin exhibited bifurcated differentiation. Heterochromatinization occurred in chromosomes 4A^t and 7A^t and in smaller amounts in 2A^t, 3A^t, 5A^t, and 6A^t within the genomes of the tetraploid Timopheevi group (T. araraticum, and T. timopheevi) and vice versa within those of the Emmer group (T. dicoccoides and T. turgidum). Similar divergence patterns occurred among chromosome 4A^a and 7A^a of cultivars of hexaploid wheat (T. aestivum). These dynamic processes could be related to geographic distribution and to natural and artifical selection. Comparison of the A genomes of diploid wheats with those of polyploid wheats shows that the A genomes in existing diploid wheats could not be the direct donors of those in polyploid wheats, but that the extant taxa of diploids and polyploids probably have a common origin and share a common A-genomelike ancestor.Contribution of the College of Agricultural Sciences, Texas Tech Univ. Journal No. T-4-233.     

Microsatellite DNA polymorphism divergence in Triticum dicoccoides accessions highly resistant to yellow rust

 Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases of wheat throughout the world. Wild emmer wheat, Triticum dicoccoides, the progenitor of cultivated wheat, was found to be a valuable source for novel stripe-rust-resistance genes. The objective of the present study was to estimate the extent of genetic diversity among the wild emmer wheat accessions, previously identified as highly resistant to stripe rust, in order to select suitable parents for genetic-mapping studies. Twenty three wheat microsatellite (WMS) markers were used to detect DNA polymorphism among 21 accessions of T. dicoccoides, which included 19 resistant and two susceptible accessions originating mainly from the center of origin and diversity in the Upper Galilee and Hermon Mountain in northern Israel. In addition, two Triticum durum and one Triticum aestivum lines were also included in the analysis. The 23 WMS markers used were located on 23 chromosome arms, representing all 14 chromosomes of genomes A and B of wheat, and revealed a total of 230 alleles. The number of alleles ranged from 5 to 18, with an average of ten alleles per WMS. Genetic dissimilarity values between genotypes, calculated by the WMSderived data, were used to produce a dendrogram of the relationships among accessions using the unweighted pair-group method with arithmetic averages (UPGMA). The results showed that all of the wild emmer wheat accessions could be distinguished. Most of the resulting groups were strongly related to the ecogeographical origin of the accessions, indicating that the genetic diversity of T. dicoccoides is correlated with geographic distribution. The three major groups were the Rosh Pinna group (north of the Sea of Galilee), the Mount Hermon group (north of the Golan Heights) and Mount Kena’an group (Upper Galilee). The genetic similarity (GS) of the 21 T. dicoccoides accessions based on WMS results averaged 0.31. As expected, the T. durum and T. aestivum lines were grouped separately from the T. dicoccoides accessions. The results obtained suggest that a relatively small number of microsatellites can be used for the estimation of genetic diversity in wild material of T. dicoccoides. These results will be useful in the identification of suitable parents for the development of mapping populations for tagging yellow-rust resistance genes derived from T. dicoccoides. Furthermore, future work could test the adaptive evolutionary significance of microsatellites in natural populations of wild emmer wheat. Received: 8 August 1997 / Accepted: 25 August 1997     

Rationalising germplasm collections: a case study for wheat

In total 70 genebank accessions comprising 50 hexaploid, 12 tetraploid and 8 diploid wheats of the Gatersleben collection were selected based on the screening of the passport data for identical cultivar names or accession numbers of the donor genebanks. Twelve potential duplicate groups consisting of three to nine accessions with identical names/numbers were selected and analysed with DNA markers (microsatellites). A bootstrap approach based on re-sampling of both microsatellite markers and alleles within marker loci was used to test for homogeneity. Although several homogeneous groups were identified it became clear that cultivar name identity alone did not allow the determination of duplicates. A combination of SSR-analysis followed by the bootstrap method and database survey considering the botanical classification and other data (origin, growth habit and donor) available is recommended in order to determine duplicates. A procedure for the identification of duplicates and their further handling in ex situ genebanks is discussed.     

Low-temperature tolerance and genetic potential in wheat (Triticum aestivum L.): response to photoperiod, vernalization, and plant development

It is frequently observed that winter habit types are more low-temperature (LT) tolerant than spring habit types. This raises the question of whether this is due to pleiotropic effects of the vernalization loci or to the linkage of LT-tolerance genes to these vernalization loci. Reciprocal near-isogenic lines (NILs) for alleles at the Vrn-A1 locus, Vrn-A1 and vrn-A1, determining spring and winter habit respectively, in two diverse genetic backgrounds of wheat (Triticum aestivum L.) were used to separate the effects of vernalization, photoperiod, and development on identical, or near identical, genetic backgrounds. The vrn-A1 allele in the winter lines allowed full expression of genotype dependent LT tolerance potential. The winter allele (vrn-A1) in a very cold tolerant genetic background resulted in 11°C, or a 2.4-fold, greater LT tolerance compared to the spring allele. Similarly, the delay in development caused by short-day (SD) versus long-day (LD) photoperiod in the identical spring habit NIL resulted in an 8.5°C or 2.1-fold, increase in LT tolerance. The duration of time in early developmental stages was shown to underlie full expression of genetic LT-tolerance potential. Therefore, pleiotropic effects of the vernalization loci can explain the association of LT tolerance and winter habit irrespective of either the proposed closely linked Fr-A1 or the more distant Fr-A2 LT-tolerance QTLs. Plant development progressively reduced LT-acclimation ability, particularly after the main shoot meristem had advanced to the double ridge reproductive growth stage. The Vrn-1 genes, or other members of the flowering induction pathway, are discussed as possible candidates for involvement in LT-tolerance repression.