Chiasmata and the breeding system in wild populations of diploid wheats

Seven populations of the selfer Triticum longissimum (= Aegilops longissima) and five populations of the closely related outbreeder T. speltoides (= Ae. speltoides) were scored for chiasma frequencies in pollen mother cells. The populations of the selfer have significantly higher frequencies of chiasmata than the outbreeding populations. This difference becomes even clearer when interstitial chiasmata alone are compared. It is argued that an optimal degree of effective recombination is achieved by the balance between outbreeding and interstitial chiasmata. — There are wider differences between the selfing populations than between the outbreeding populations, but the differences between families (within populations) are small in both species. Variation between plants within families seems to be lower in the selfer, but nevertheless high enough to be inexplicable on the basis of selfing alone. — Small populations subject to hardship conditions show a higher frequency of chiasmata than others.     

Genes in diploid triticinae compensating for the low temperature regulating gene Ltp in chromosome 5D of Triticum aestivum

Summary Hybrids of Triticum aestivum (monosomic 5D or ditelosomic 5DL) x T. speltoides (= Ae. speltoides) showed that the genotype of T. speltoides carries gene(s) which can partially compensate for the expected decrease in chromosome association at low temperatures (10°C) in the absence of chromosome 5D. In hybrids of T. aestivum (normal, ditelosomic 3DL or ditelosomic 3DL-monosomic 5D) x T. longissimum (= Ae. sharonensis), this compensation was not observed.In normal F₁ hybrids of T. durum x T. longissimum partial chromosome association occurred at 10°C and this stabilizer effect may be explained by the presence of a Ltp-like gene on chromosome 5A. When a line of T. durum carrying a homozygous translocated 5B-5D chromosome was used in the crosses an even higher chromosome association was observed.These results suggest either the existence of a promoter gene for chromosome association in the 5D translocated segment or the loss of a weak suppressor gene in the removed segment of 5B. It was concluded that the translocated 5D segment did not carry the Ltp stabilizer gene.The work was supported by a fellowship of the Gulbenkian Foundation and partly carried out while the author was at the Department of Genetics, Agricultural University, Wageningen, The Netherlands.     

Studies on maternal inheritance in polyploid wheats with cytoplasmic DNAs as genetic markers

Summary Restriction fragment patterns of DNA fragments obtained after EcoRI cleavage of chloroplastic (cp) and mitochondrial (mt) DNAs isolated from different wheat species were compared. T. aestivum, T. timopheevi, Ae. speltoides, Ae. sharonensis and T. urartu gave species specific mt DNA patterns. Consequently, the cytoplasmic genomes of wheat cannot have originated from contemporary Ae. speltoides, Ae. sharonensis and T. urartu species. It is shown that cp and mt DNAs of Ae. ventricosa, a tetraploid used to transfer eyespot resistance into T. aestivum, contains cp and mt DNAs differing from DNAs isolated from T. aestivum and other wheats. In contrast, the cytoplasmic DNAs of Ae. ventricosa and Ae. squarrosa reveal an important homology, suggesting that Ae. squarrosa was the female parent of Ae. ventricosa. Disomic addition lines (T. aestivum — Ae. ventricosa) in both Ae. ventricosa cytoplasm and T. aestivum cytoplasm contained cytoplasmic DNAs identical to those of the maternal parent. Restriction patterns of the cp and mt DNAs isolated from eight lines of Triticale differing in their cytoplasm have been compared to those of the maternal parent. A strict maternal inheritance has been observed in each case.     

CHy-banding patterns and chromatin organization inAegilops andTriticum species (Poaceae)

The distribution of CHy-banded heterochromatin was studied in the chromosomes ofAegilops longissima, Ae. speltoides, Triticum monococcum, andT. turgidum. Interphase nuclei were measured after Feulgen staining at different thresholds of optical density; the curves so obtained indicated the relationship among the species with respect to the different fractions of the genomic DNA. The karyological and cytophotometric analyses indicate differences betweenAe. speltoides andAe. longissima, the latter species being enriched in heterochromatin. Similar results were demonstrated for the genusTriticum, in whichT. turgidum showed more heterochromatin when compared withT. monococcum. The results suggest that the B genome of the cultivated wheats possesses a type of heterochromatin that resembles the type present inAe. longissima.     

Pairing affinities of the B- and G-genome chromosomes of polyploid wheats with those of Aegilops speltoides.

Chromosome pairing at metaphase I was studied in different interspecific hybrids involving Aegilops speltoides (SS) and polyploid wheats Triticum timopheevii (AtAtGG), T. turgidum (AABB), and T. aestivum (AABBDD) to study the relationships between the S, G, and B genomes. Individual chromosomes and their arms were identified by means of C-banding. Pairing between chromosomes of the G and S genomes in T. timopheevii x Ae. speltoides (AtGS) hybrids reached a frequency much higher than pairing between chromosomes of the B and S genomes in T. turgidum x Ae. speltoides (ABS) hybrids and T. aestivum x Ae. speltoides (ABDS) hybrids, and pairing between B- and G-genome chromosomes in T. turgidum x T. timopheevii (AAtBG) hybrids or T. aestivum x T. timopheevii (AAtBGD) hybrids. These results support a higher degree of closeness of the G and S genomes to each other than to the B genome. Such relationships are consistent with independent origins of tetraploid wheats T. turgidum and T. timopheevii and with a more recent formation of the timopheevi lineage.     

Discovery and mapping of wheat Ph1 suppressors

Pairing between wheat (Triticum turgidum and T. aestivum) homeologous chromosomes is prevented by the expression of the Ph1 locus on the long arm of chromosome 5B. The genome of Aegilops speltoides suppresses Ph1 expression in wheat x Ae. speltoides hybrids. Suppressors with major effects were mapped as Mendelian loci on the long arms of Ae. speltoides chromosomes 3S and 7S. The chromosome 3S locus was designated Su1-Ph1 and the chromosome 7S locus was designated Su2-Ph1. A QTL with a minor effect was mapped on the short arm of chromosome 5S and was designated QPh.ucd-5S. The expression of Su1-Ph1 and Su2-Ph1 increased homeologous chromosome pairing in T. aestivum x Ae. speltoides hybrids by 8.4 and 5.8 chiasmata/cell, respectively. Su1-Ph1 was completely epistatic to Su2-Ph1, and the two genes acting together increased homeologous chromosome pairing in T. aestivum x Ae. speltoides hybrids to the same level as Su1-Ph1 acting alone. QPh.ucd-5S expression increased homeologous chromosome pairing by 1.6 chiasmata/cell in T. aestivum x Ae. speltoides hybrids and was additive to the expression of Su2-Ph1. It is hypothesized that the products of Su1-Ph1 and Su2-Ph1 affect pairing between homeologous chromosomes by regulating the expression of Ph1 but the product of QPh.ucd-5S may primarily regulate recombination between homologous chromosomes.     

C-banding pattern and polymorphism of Aegilops caudata and chromosomal constitutions of the amphiploid T. aestivum — Ae. caudata and six derived chromosome addition lines

Summary C-banding patterns were analysed in 19 different accessions of Aegilops caudata (= Ae. markgrafii, = Triticum dichasians) (2n = 14, genomically CC) from Turkey, Greece and the USSR, and a generalized C-banded karyotype was established. Chromosome specific C-bands are present in all C-genome chromosomes, allowing the identification of each of the seven chromosome pairs. While only minor variations in the C-banding pattern was observed within the accessions, a large amount of polymorphic variation was found between different accessions. C-banding analysis was carried out to identify Ae. caudata chromosomes in the amphiploid Triticum aestivum cv Alcedo — Ae. caudata and in six derived chromosome addition lines. The results show that the amphiploid carries the complete Ae. Caudate chromosome complement and that the addition lines I, II, III, IV, V and VIII carry the Ae. caudata chromosome pairs B, C, D, F, E and G, respectively. One of the two SAT chromosome pairs (A) is missing from the set. C-banding patterns of the added Ae. caudata chromosomes are identical to those present in the ancestor species, indicating that these chromosomes are not structurally rearranged. The results are discussed with respect to the homoeologous relationships of the Ae. caudata chromosomes.     

The effect of ph mutations on homoeolgous pairing in hybrids of wheat with Triticum longissimum

Homoeologous pairing at metaphase-I was analyzed in wild-type, ph2b, and ph1b hybrids of wheat and a low-pairing type of T. longissimum in order to study the effect of ph mutations on the pairing of T. longissimum chromosomes with wheat chromosomes. Chromosomes of both species, and their arms, were identified by C-banding. The three types of hybrids, with low-, intermediate-, and high-pairing levels, respectively, exhibited a very similar pairing pattern which was characterized by the existence of two types, A-D and B-S¹, of preferential pairing. These results confirm that the S¹ genome of T. longissimum is closely related to the B genome of wheat. The possible use of ph1b and ph2b mutations in the transfer to wheat of genes from related species is discussed.     

Microsatellite mapping of <Emphasis Type="Italic">Ae. speltoides</Emphasis> and map-based comparative analysis of the S,G, and B genomes of Triticeae species

The first microsatellite linkage map of Ae. speltoides Tausch (2n = 2x = 14, SS), which is a wild species with a genome closely related to the B and G genomes of polyploid wheats, was developed based on two F₂ mapping populations using microsatellite (SSR) markers from Ae. speltoides, wheat genomic SSRs (g-SSRs) and EST-derived SSRs. A total of 144 different microsatellite loci were mapped in the Ae. speltoides genome. The transferability of the SSRs markers between the related S, B, and G genomes allowed possible integration of new markers into the T. timopheevii G genome chromosomal maps and map-based comparisons. Thirty-one new microsatellite loci assigned to the genetic framework of the T. timopheevii G genome maps were composed of wheat g-SSR (genomic SSR) markers. Most of the used Ae. speltoides SSRs were mapped onto chromosomes of the G genome supporting a close relationship between the G and S genomes. Comparative microsatellite mapping of the S, B, and G genomes demonstrated colinearity between the chromosomes within homoeologous groups, except for intergenomic T6A^tS.1G, T4AL.5AL.7BS translocations. A translocation between chromosomes 2 and 6 that is present in the T. aestivum B genome was found in neither Ae. speltoides nor in T. timopheevii. Although the marker order was generally conserved among the B, S, and G genomes, the total length of the Ae. speltoides chromosomal maps and the genetic distances between homoeologous loci located in the proximal regions of the S genome chromosomes were reduced compared with the B, and G genome chromosomes.     

Construction and study of leaf rust-resistant common wheat lines with translocations of Aegilops speltoides Tausch

Genotyping was performed for the leaf rust-resistant line 73/00i (Triticum aestivum x Aegilops speltoides). Fluorescence in situ hybridization (FISH) with probes Spelt1 and pSc119.2 in combination with microsatellite analysis were used to determine the locations and sizes of the Ae. speltoides genetic fragments integrated into the line genome. Translocations were identified in the long arms of chromosomes 5B and 6B and in the short arm of chromosome 1B. The Spelt1 and pSc119.2 molecular cytological markers made it possible to rapidly establish lines with single translocation in the long arms of chromosomes 5B and 6B. The line carrying the T5BS x 5BL-5SL translocation was highly resistant to leaf rust, and the lines carrying the T6BS x 6BL-6SL translocation displayed moderate resistance. The translocations differed in chromosomal location from known leaf resistance genes transferred into common wheat from Ae. speltoides. Hence, it was assumed that new genes were introduced into the common wheat genome from Ae. speltoides. The locus that determined high resistance to leaf rust and was transferred into the common wheat genome from the long arm of Ae. speltoides chromosome 5S by the T5BS x 5BL-5SL translocation was preliminarily designated as LrAsp5.     

The effect of B chromosomes on meiotic and pre-meiotic spindles and chromosome pairing in Triticum/Aegilops hybrids

Diploid populations of Aegilops mutica and Aegilops speltoides containing B chromosomes have been used as male parents in crosses with aneuploid genotypes of Triticum aestivum to investigate the effect of B chromosomes on meiotic homologous and homoeologous chromosome pairing. F₁ hybrids of T. aestivum/Ae. mutica and T. aestivum/Ae. speltoides segregated into four classes with regard to the degree of meiotic chromosome pairing, irrespective of the presence of B chromosomes. The B chromosomes do not introduce factors altering the level of pairing other than that due to the natural allelic and gene variation occurring in the diploids. Similarly no reduction in pairing of homologous chromosomes was observed in genotypes in which pairs of homologues co-existed with B chromosomes. However, a significant drop in chiasma frequency was observed in F₁ hybrids of T. aestivum × Ae. mutica with B chromosomes and T. aestivum × Ae. mutica nullisomic for wheat chromosome 5D with B chromosomes, in temperature regimes of 12° C. No asynapsis occurred in similar hybrids in the absence of Mutica B chromosomes at low temperatures. The low-temperature sensitive phase lies early in the pre-meiotic interphase. In this instance the Mutica B chromosomes are interacting with specific gene loci of the A chromosomes. Synaptic pairing has been observed between A and B chromosomes in Ae. mutica. A high frequency of pollen mother cells with twice the number of chromosomes was observed in hybrids in the presence of Mutica B chromosomes due to failure of spindle formation at the last pre-meiotic mitosis. Meiotic spindle irregularities occurred in hybrids containing Speltoides B chromosomes. Hybrids of Ae. speltoides + B's X Ae. mutica + B's displayed the mitotic and meiotic spindle abnormalities introduced by the presence of the B chromosomes of each parent.     

The cytology of Brachycome

Over 1,000 plants of B. dichromosomatica have been counted. 10% of these carried one, two or three B chromosomes. The B chromosome is large, and though it is not heteropycnotic it condenses precociously at mitosis. It behaves regularly at mitosis, and when two are present they pair regularly at meiosis. Non-disjunction and preferential distribution of the B to the generative nucleus occurs at the first pollen grain mitosis, with very high frequency. This is corroborated by data from crosses, which also indicate that the transmission of the B through the female gamete is normal. — The frequency of B chromosomes in marginal populations of var. dichromosomatica is significantly higher than in central populations. In one population of var. alba the frequency of Bs increased significantly after two very dry seasons. It is suggested that both these cases of increased frequency were in response to a selective advantage of plants with Bs under arid conditions. — Plants with one B chromosome appear to be less fit than plants with 2 Bs. The combination of the calculated effects of the nondisjunction mechanism and the inferred relative fitness of the 0B, 1B and 2B plants, provides a reasonable explanation of the observed frequencies of the 0B, 1B and 2B plants in the populations studied.     

Variations in a hotspot region of chloroplast DNAs among common wheat and Aegilops revealed by nucleotide sequence analysis

The second largest BamHI fragment (B2) of the chloroplast DNA in Triticum (wheat) and Aegilops contains a highly variable region (a hotspot), resulting in four types of B2 of different size, i.e. B2l (10.5kb), B2m (10.2kb), B2 (9.6kb) and B2s (9.4kb). In order to gain a better understanding of the molecular nature of the variations in length and explain unexpected identity among B2 of Ae. ovata, Ae. speltoides and common wheat (T. aestivum), the nucleotide sequence between a stop codon of rbcL and a HindIII site in cemA in the hotspot was determined for Ae. ovata, Ae. speltoides, Ae. caudata and Ae. mutica. The total number of nucleotides in the region was 2808, 2810, 3302, and 3594 bp, for Ae. speltoides, Ae. ovata, Ae. caudata and Ae. mutica, respectively, and the sequences were compared with the corresponding ones of Ae. crassa 4x, T. aestivum and Ae. squarrosa. Compared with the largest B2l fragment of Ae. mutica, a 791bp and a 793 bp deletion were found in Ae. speltoides and Ae. ovata, respectively, and the possible site of deletion in the two species is the same as that of T. aestivum. However, a deleted segment in Ae. ovata is 2 bp longer than that of Ae. speltoides (and T. aestivum), demonstrating that recurrent deletions had occurred in the chloroplast genomes of both species. Comparison of the sequences from Ae. caudata and Ae. crassa 4x with that of Ae. mutica revealed a 289 bp and a 61 bp deletion at the same site in Ae. caudata and Ae. crassa 4x, respectively. Sequence comparison using wild Aegilops plants showed that the large length variations in a hotspot are fixed to each species. A considerable number of polymorphisms are observed in a loop in the 3' of rbcL. The study reveals the relative importance of the large and small indels and minute inversions to account for variations in the chloroplast genomes among closely related species.     

Chromosomal location of genes for Rubisco small subunit and Rubisco-binding protein in common wheat

Summary The genes coding for the Rubisco small subunit (SSU) and for the -subunit of the Rubisco-binding protein were located to chromosome arms of common wheat. HindIII-digested total DNA from the hexaploid cultivar Chinese Spring and from ditelosomic and nullisomic-tetrasomic lines was probed with these two genes, whose chromosomal location was deduced from the disappearance of or from changes in the relative intensity of the relevant band(s). The Rubisco SSU pattern consisted of 14 bands, containing at least 21 different types of DNA fragments, which were allocated to two homoeologous groups: 15 to the short arm of group 2 chromosomes (4 to 2AS, 7 to 2BS, and 4 to 2DS) and 6 to the long arm of group 5 chromosomes (2 on each of arms 5AL, 5BL, and 5DL). The pattern of the Rubisco-binding protein consisted of three bands, each containing one type of fragment. These fragments were located to be on the short arm of group 2 chromosomes. The restriction fragment length polymorphism (RFLP) patterns of several hexaploid and tetraploid lines were highly conserved, whereas the patterns of several of their diploid progenitors were more variable. The variations found in the polyploid species were mainly confined to the B genome. The patterns of the diploids T. monococcum var. urartu and Ae. squarrosa were similar to those of the A and D genome, respectively, in polyploid wheats. The pattern of T. monococcum var. boeoticum was different from the patterns of the A genome, and the patterns of the diploids Ae. speltoides, Ae. longissima, and Ae. Searsii differed from that of the B genome.     

Comparative genetic maps reveal extreme crossover localization in the Aegilops speltoides chromosomes

A total of 137 loci were mapped in Aegilops speltoides, the closest extant relative of the wheat B genome, using two F₂ mapping populations and a set of wheat-Ae. speltoides disomic addition (DA) lines. Comparisons of Ae. speltoides genetic maps with those of Triticum monococcum indicated that Ae. speltoides conserved the gross chromosome structure observed across the tribe Triticeae. A putative inversion involving the short arm of chromosome 2 was detected in Ae. speltoides. A translocation between chromosomes 2 and 6, present in the wheat B genome, was absent. The ligustica/aucheri spike dimorphism behaved as allelic variation at a single locus, which was mapped in the centromeric region of chromosome 3. The genetic length of each chromosome arm was about 50 cM, irrespective of its physical length. Compared to T. monococcum genetic maps, recombination was virtually eliminated from the proximal 50–100 cM and was localized in short distal regions, which were often expanded compared to the T. monococcum maps. The wheat B genome and the genome of Ae. longissima, a close relative of Ae. speltoides, do not show the extreme localization of crossovers observed in Ae. speltoides.     

Additional evidence implicating Triticum searsii as the B-genome donor to wheat

In vitro DNA:DNA hybridizations and hydroxyapatite thermal-elution chromatography were employed to identify the diploid wheat species ancestral to the B genome of Triticum turgidum. ³H-T. turgidum DNA was hybridized to the unlabeled DNAs of T. urartu, T. speltoides, T. sharonensis, T. bicorne, T. longissimum, and T. searsii. ³H-Labeled DNAs of T. monococcum and a synthetic tetraploid AADD were hybridized with unlabeled DNAs of T. urartu and T. searsii to determine the relationship of the A genome of polyploid wheat and T. urartu. The heteroduplex thermal stabilities indicated that T. searsii was most closely related to the B genome of T. turgidum (AB) and that the genome of T. urartu and the A genome have a great deal of base-sequence homology. Thus, it appears that T. searsii is the B-genome donor to polyploid wheat or a major chromosome donor if the B genome is polyphyletic in origin.Published with the approval of the Director of The West Virginia Agricultural Experiment Station as Scientific Paper No. 1837.     

Genomic Instability in Wheat Induced by Chromosome 6b(s) of Triticum Speltoides

A massive restructuring of chromosomes was observed during the production of a substitution of chromosome 6B(s) from Triticum speltoides (Tausch) Gren. ex Richter for chromosome 6B of Chinese Spring wheat (Triticum aestivum L.). Deletions, translocations, ring chromosomes, dicentric chromosomes and a paracentric inversion were observed. Chromosome rearrangements occurred in both euchromatic and heterochromatic regions. Chromosome rearrangements were not observed either in the amphiploid between Chinese Spring and T. speltoides or in Chinese Spring. No chromosome rearrangements were observed in the backcross derivatives; however, after self-pollination of a monosomic substitution (2n = 41) of chromosome 6B(s) for wheat chromosome 6B, 49 of the 138 plants carried chromosome aberrations. Chromosome rearrangements were observed in both wheat and T. speltoides chromosomes. The frequency of chromosome rearrangements was high among the B-genome chromosomes, moderate among the A-genome chromosomes, and low among the D-genome chromosomes. In the B genome, the rearrangements were nonrandom, occurring most frequently in chromosomes 1B and 5B. Chromosome rearrangements were also frequent for the 6B(s) chromosome of T. speltoides. An intriguing aspect of these observations is that they indicate that wheat genomes can be subject to uneven rates of structural chromosome differentiation in spite of being in the same nucleus.     

BAC libraries of Triticum urartu, Aegilops speltoides and Ae. tauschii, the diploid ancestors of polyploid wheat

Triticum urartu, Aegilops speltoides and Ae. tauschii are respectively the immediate diploid sources, or their closest relatives, of the A, B and D genomes of polyploid wheats. Here we report the construction and characterization of arrayed large-insert libraries in a bacterial artificial chromosome (BAC) vector, one for each of these diploid species. The libraries are equivalent to 3.7, 5.4 and 4.1 of the T. urartu, Ae. speltoides, Ae. tauschii genomes, respectively. The predicted levels of genome coverage were confirmed by library hybridization with single-copy genes. The libraries were used to estimate the proportion of known repeated nucleotide sequences and gene content in each genome by BAC-end sequencing. Repeated sequence families previously detected in Triticeae accounted for 57, 61 and 57% of the T. urartu, Ae. speltoides and Ae. tauschii genomes, and coding regions accounted for 5.8, 4.5 and 4.8%, respectively.     

An immunochemical approach to species relationship in Triticum and some related species

Summary An immunological reaction, precipitation in gel, was produced using a rabbit antiserum directed to a specific protein constantly present in bread wheats (T. aestivum, genome AABBDD), but absent in durum wheat (T. durum Desf., genome AABB). This protein was isolated in the soluble-protein fraction of bread wheat caryopses by combined biochemical and immunological techniques.The availability of such a specific anti-bread wheat serum made possible the analysis of a series of varieties and species of wheat and of some closely related (Secale, Aegilops) and less closely related (Hordeum, Haynaldia) taxa to determine whether the protein was present or absent. Hordeum vulgare, Haynaldia villosa, Triticum monoccocum and Triticum turgidum gave a negative result, while positive results were obtained in T. aestivum, T. timopheevi, T. zhukovskyi, Secale cereale, Aegilops speltoides, Ae. mutica, Ae. comosa, Ae. caudata, Ae. umbellulata, Ae. squarrosa, and also in the artificial amphiploids (Ae. speltoides x T. monococcum) and (Ae. caudata x T. monococcum).It is concluded that these results agree closely with the classification of Triticum proposed by MacKey in 1966. The investigated protein not only permits the differentiation of T. aestivum from T. turgidum, but also T. turgidum from T. timopheevi at tetraploid level and T. monococcum from all the diploid species of Aegilops.     

The application of wheat microsatellites to identify disomic Triticum aestivum-Aegilops markgrafii addition lines

 We describe the use of wheat microsatellites for the discrimination of Aegilops markgrafii chromosomes. Twenty out of eighty eight wheat microsatellites (WMS) tested were able to distinguish Triticum aestivum-Ae. markgrafii addition lines. Six, three, three, one and six of 18 WMS can be used as markers for single Ae. markgrafii chromosomes B, C, D, F and G, respectively. Addition line A is not available but additional bands, appearing only in Ae. markgrafii and the T. aestivum-Ae. markgrafii amphiploid and not in any of the available addition lines, indicate that three WMS detect markers for Ae. markgrafii chromosomes A. Addition line E could not be detected by any of the WMS markers applied, although the 20 WMS represented all the homologous groups of wheat. All three WMS located on the short arm of group-2 chromosomes were located on Ae. markgrafii chromosome B; three of four WMS, located on the long arm of wheat group-2 chromosomes, were specific to Ae. markgrafii chromosome G and three of four WMS, specific to group-5 chromosomes, were markers for Ae. markgrafii chromosome C, indicating the homoeology of these wheat chromosome arms with the respective Ae. markgrafii chromosomes. Received: 29 May 1997 / Accepted: 10 September 1997