Wheat genome structure: translocations during the course of polyploidization

The genomic organization of Triticum timopheevii (2n=28, A^tA^tGG) was compared with hexaploid wheat T. aestivum (2n=42, AABBDD) by comparative mapping using microsatellites derived from bread wheat. Genetic maps for the two crosses T. timopheevii var. timopheevii × T. timopheevii var. typica and T. timopheevii K-38555×T. militinae were constructed. On the first population, 121 loci were mapped, and on the second population 103 loci. The transferability of the wheat markers to T. timopheevii was generally better for the A genome-specific markers (76–78% produced amplification products; 26 and 29% were polymorphic) than for B genome-specific markers (54% produced amplification products; 14 and 16% were polymorphic). Of the D genome-specific markers, one third produced amplification products in T. timopheevii, but only 5 and 2% were polymorphic in the corresponding mapping populations. The maps constructed confirmed the previously described translocation between chromosome arms 6A^tS and 1GS and revealed at least two yet unknown rearrangements on chromosomes 4A^t and 6A^t. The presence of other translocations and rearrangements between T. timopheevii and T. aestivum was demonstrated by a variety of markers mapping to nonhomoeologous positions.     

The Ph1 and Ph2 loci play different roles in the synaptic behaviour of hexaploid wheat Triticum aestivum

Triticum aestivum is an allohexaploid wheat (AABBDD) that shows diploid-like behaviour at metaphase-I. This behaviour is influenced by the action of several loci, Ph1 and Ph2 being the main loci involved. To study the effect of these two loci on chromosome pairing in T. aestivum we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late-zygotene, and at pachytene, of three different genotypes of cv Chinese Spring: standard line, ph1b and ph2b mutants. The analysis of the synaptic progression showed that only a few nuclei accomplish synapsis in the ph2b genotype, whereas most nuclei completed synapsis in the standard and ph1b genotypes. This result indicates that the Ph2 locus affects synaptic progression. The number of synaptonemal complex (SC) bivalents and of the different SC multivalent associations were determined in each nucleus. The mean number of lateral elements involved in SC multivalent associations (LEm) at mid- zygotene was relatively high and showed similar values in the three genotypes. These values decreased progressively between mid-zygotene and pachytene in the genotypes with the Ph1 locus because of the transformation of multivalents into bivalents. In the ph1b genotype, this value only decreased between late-zygotene and pachytene. Therefore, multivalent correction was more efficient in the presence than in the absence of the Ph1 locus.It is concluded that the Ph1 and Ph2 loci bring about diploidization of allohexaploid wheat via a different mechanism. Received: 31 July 2000 / Accepted: 15 November 2000     

Chromosome substitutions of Triticum timopheevii in common wheat and some observations on the evolution of polyploid wheat species

Whether the two tetraploid wheat species, the well known Triticum turgidum L. (macaroni wheat, AABB genomes) and the obscure T. timopheevii Zhuk. (A^tA^tGG), have monophyletic or diphyletic origin from the same or different diploid species presents an interesting evolutionary problem. Moreover, T. timopheevii and its wild form T. araraticum are an important genetic resource for macaroni and bread-wheat improvement. To study these objectives, the substitution and genetic compensation abilities of individual T. timopheevii chromosomes for missing chromosomes of T. aestivum Chinese Spring (AABBDD) were analyzed. Chinese Spring aneuploids (nullisomic-tetrasomics) were crossed with a T. timopheevii x Aegilops tauschii amphiploid to isolate T. timopheevii chromosomes in a monosomic condition. The F₁ hybrids were backcrossed one to four times to Chinese Spring aneuploids without selection for the T. timopheevii chromosome of interest. While spontaneous substitutions involving all A^t- and G-genome chromosomes were identified, the targeted T. timopheevii chromosome was not always recovered. Lines with spontaneous substitutions from T. timopheevii were chosen for further backcrossing. Six T. timopheevii chromosome substitutions were isolated: 6A^t (6A), 2G (2B), 3G (3B), 4G (4B), 5G (5B) and 6G (6B). The substitution lines had normal morphology and fertility. The 6A^t of T. timopheevii was involved in a translocation with chromosome 1G, resulting in the transfer of the group-1 gliadin locus to 6A^t. Chromosome 2G substituted for 2B at a frequency higher than expected and may carry putative homoeoalleles of gametocidal genes present on group-2 chromosomes of several alien species. Our data indicate a common origin for tetraploid wheat species, but from separate hybridization events because of the presence of a different spectrum of intergenomic translocations.     

Cytogenetic investigation of Triticum timopheevii (Zhuk.) Zhuk. and related species using the C-banding technique

Triticum timopheevii and related species T. militinae (2n=28, A^tG) and T. zhukovskyi (2n=42, A^mA^tG), hybrids T. kiharae, T. miguschovae, the amphidiploid T. timopheevii x T. tauschii (all 2n=42, A^tGD), T. fungicidum (ABA^tG) and T. timonovum (2n=56, A^tA^tGG) were analyzed using the C-banding technique. Chromosomes of the A^m and A^t genomes in the karyotype of T. zhukovskyi differed in their C-banding pattern. Partial substitutions of A^t-genome chromosomes and a complete substitution of the G-genome chromosomes by homoeologous chromosomes of an unidentified tetraploid wheat species with an AB genome composition were found in the T. timonovum karyotype. A^t- and G-genome chromosomes in the karyotypes of all studied species had similar C-banding patterns and were characterized by a low level of polymorphism. The comparative stability of the A^t and G genomes is determined by the origin and specifity of cultivation of studied species.     

The pattern of zygotene and pachytene pairing in allotetraploid Aegilops species sharing the D genome

Chromosome pairing behaviour of the allotetraploid Aegilops species sharing the D genome, Ae. crassa (DDMM), Ae. cylindrica (DDCC) and Ae. ventricosa (DDNN), was analyzed by electron microscopy in surfacespread prophase-I nuclei. Synaptonemal-complex analysis at zygotene and pachytene revealed that synapsis in the allotetraploids was mostly between homologous chromosomes, although a few multivalents were also formed. Only homologous bivalents were observed at metaphase-I. It is concluded that the mechanism controlling bivalent formation in these species acts mainly at zygotene by restricting pairing to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in homoeologous associations. These observations are discussed in relation to mechanisms of diploidization of polyploid meiosis.     

Structural chromosome differentiation between Triticum timopheevii and T. turgidum and T. aestivum

 Chromosome pairing at metaphase-I was analyzed in F₁ hybrids among T. turgidum (AABB), T. aestivum (AABBDD), and T. timopheevii (A^tA^tGG) to study the chromosome structure of T. timopheevii relative to durum (T. turgidum) and bread (T. aestivum) wheats. Individual chromosomes and their arms were identified by means of C-banding. Homologous pairing between the A-genome chromosomes was similar in the three hybrid types AA^tBG, AA^tBGD, and AABBD. However, associations of B-G were less frequent than B-B. Homoeologous associations were also observed, especially in the AA^tBGD hybrids. T. timopheevii chromosomes 1A^t, 2A^t, 5A^t, 7A^t, 2G, 3G, 5G, and 6G do not differ structurally from their counterpart in the A and B genomes. Thus, these three polyploid species inherited translocation 5AL/4AL from the diploid A-genome donor. Chromosome rearrangements that occurred at the tetraploid level were different in T. turgidum and T. timopheevii. Translocation 4AL/7BS and a pericentric inversion of chromosome 4A originated only in the T. turgidum lineage. The two lines of T. timophevii studied carry four different translocations, 6A^tS/1GS, 1GS/4GS, 4GS/4A^tL, and 4A^tL/3A^tL, which most likely arose in that sequence. These structural differences support a diphyletic origin of polyploid wheats. Received: 15 June 1998 / Accepted: 19 August 1998     

The synaptic behaviour of Triticum turgidum with variable doses of the Ph1 locus

Cultivated wheat Triticum turgidum is an allotetraploid (AABB) with diploid-like behaviour at metaphase I. This behaviour is mainly influenced by the action of the Ph1 locus. To study the effect of Ph1 on chromosome pairing in T. turgidum we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late-zygotene and at pachytene of three different genotypes: a standard line, ph1c mutant and a duplication mutant, with zero, two and four doses of Ph1, respectively. The number of synaptonemal complex (SC) bivalents and of the different SC multivalent associations were determined in each nucleus. The mean number of lateral elements involved in SC multivalent associations (LEm) at mid-zygotene was relatively high in all lines and was similar in two and zero doses of Ph1. These means changed little with the progression of zygotene but decreased at pachytene because of the transformation of multivalents into bivalents. Multivalent correction was more efficient in the presence than in the absence of Ph1. The four doses of Ph1 genotype showed a higher number of SC bivalents at mid-zygotene, and the frequency of multivalents decreased progressively throughout zygotene and pachytene. The results suggest that the main action of the Ph1 locus on the diploidisation mechanism would be related to a process of checking for homology operating during prophase I. Received: 27 February 2000 / Accepted: 12 July 2000     

Gene amplification in oocytes with 8 germinal vesicles from the tailed frog Ascaphus truei Stejneger

In the last 3 oogonial mitoses in Ascaphus truei all daughter nuclei remain in the same cell. The oocyte is 8-nucleate at the start of meiotic prophase and remains so until late in oogenesis when 7 of the nuclei disappear. All 8 nuclei in a single oocyte resemble one another with respect to size and chromatin distribution at all stages of meiotic prophase. Much of the Feulgen-positive material in pachytene nuclei is concentrated into one region of the nucleus. — All of the 8 germinal vesicles of yolky oocytes have a full set of lampbrush diplotene bivalents. Germinal vesicles from oocytes of up to 0.8 mm diameter have less than 100 nucleoli, some of which are multiple nucleoli in the sense that they have more than one core region. Each of the 8 nuclei in oocytes from one animal had about the same volume of nucleolar material. — Two values have been obtained for the amount of DNA in a diploid nucleus from Ascaphus. A biochemical estimate utilizing erythrocyte nuclei and the diphenylamine reaction yielded a value of 7.1 pg per nucleus. Microphotometry of erythrocyte nuclei stained with Feulgen's reagent gave a value of 8.2 pg per nucleus. — Microphotometric measurements of Feulgen-stained nuclei at various stages of meiotic prophase up to diplotene indicate that each nucleus synthesizes up to 5 pg of extrachromosomal DNA during and immediately after pachytene. This DNA is considered to be nucleolar. Autoradiography of nuclei from oocytes which had been incubated for 6h in ³H thymidine showed silver grains over pachytene and early diplotene nuclei only. In pachytene nuclei the silver grains overlaid that part of the nucleus where Feulgen-positive material was most concentrated. Most of the chromosomal material was unlabelled. — The significance of the 8-nucleate condition in Ascaphus oocytes is discussed, and the amount of nucleolar DNA synthesized at pachytene and of nucleolar material present in germinal vesicles is compared with corresponding situations in other amphibians.     

Chromosome synteny of the a genome of two evolutionary wheat lines

In order to estimate synteny between A^t and A polyploid wheat genomes belonging to different evolutionary lines (Timopheevi and Emmer), saturation of chromosome maps of Triticum timopheevii A^t genome by molecular markers has been conducted. Totally, 179 EST-SSR and 48 genomic SSR-markers have been used with the following integration of 13 and 7 markers correspondingly into chromosome maps of A^t genome. ESTSSR showed higher transferability and lower polymorphism than genomic SSR markers. The chromosome maps designed were compared to maps of homoeologous chromosome group of the T. aestivum A genome. No disturbances of colinearity, i.e., of the order of markers within the chromosome segments on which they had been previously mapped, were observed. According to the quantity assessment of markers amplifying in homoeologous chromosomes, the maximum divergence was detected in two groups (4A^t/4A and 3A^t/3A) among the seven chromosomes examined in the A ^t and A genomes. Comparison of molecular genetic mapping results with the published results of studying meiosis of F₁ hybrids and the frequency of chromosomes substitution in introgressive T. aestivum × T. timopheevii lines suggest that individual chromosomes of the A^t and A genomes evolve differently. Translocations were shown to introduce the major impact on the divergence of 4A^t/4A and 6A^t/6A chromosomes, while mutations of the primary DNA structure, on the divergence of homoeologous group 3 chromosomes. The level of reorganization of other chromosomes during the evolution in the A^t and A genomes was significantly lower.     

Intergenomic chromosome substitutions in wheat interspecific hybrids and their use in the development of a genetic nomenclature of <Emphasis Type="Italic">Triticum timopheevii</Emphasis> chromosomes

The results of analysis of the genome formation in interspecific hybrids of Triticum aestivum with T. timopheevii are reviewed. The spectra of substitutions and rearrangements are shown to depend on the genotypes of the parental forms and on the direction of selection. The frequencies of substitutions of individual T. timopheevii chromosomes significantly vary and reflect the level of their divergence relative to the common wheat chromosomes. Some aspects of classification of the A^t- and G-genome chromosomes are discussed.     

Synaptonemal complex and recombination nodules in rye (Secale cereale)

Meiotic prophase in rye was investigated by serial-section reconstruction of pollen mother cell nuclei. In the mid-late zygotene nucleus, all lateral elements were continuous from telomere to telomere, and 9–20 pairing initiation sites per bivalent were observed. Chromosome and bivalent interlockings detected during zygotene were resolved at early pachytene when pairing was completed. In the three pachytene nuclei, the relative synaptonemal complex (SC) lengths and arm ratios were found to be in good correlation with light microscopic data of pachytene bivalents. Spatial tracing of the bivalents showed that they occupy separate areas in the nucleus. Three types of recombination nodules were observed: large, ellipsoïdal and small nodules at early pachytene and irregularly shaped nodules mainly associated with chromatin at late pachytene. Their number and position along the bivalents correlated well with the number and distribution of chiasmata. The classification of the seven bivalents was based on arm ratio and heterochromatic knob distribution.     

Cytogenetic Analysis of Hybrids Resistant to Yellow Rust and Powdery Mildew Obtained by Crossing Common Wheat (Triticum aestivum L., AABBDD) with Wheats of the Timopheevi Group (AtAtGG)

The karyotypes of 47 hybrid lines obtained from crosses of common wheat Triticum aestivum L. (cv. Rodina and line 353) with Triticum timopheevii(Zhuk.) Zhuk. (A ^t A ^t GG) and related species T. militinae Zhuk. et Migusch. (A ^t A ^t GG) and T. kiharae Dorof. et Migusch. (A ^t A ^t GGD ^sq D ^sq) were analyzed by C-banding. Most lines were resistant to yellow rust and powdery mildew. The introgression of alien genetic material to the common wheat genome was realized via substitutions of complete A ⁺-,G-, and D-genome chromosomes, chromosome arms, or their fragments. The pattern of chromosome substitutions in resistant lines differed from that in introgressive hybrids selected for other traits. Substitutions of chromosomes 6G, 2A^t, 2G, and 5G were revealed in 31, 23, 18, and 13 lines, respectively. Substitutions of chromosomes 4G, 4A^t, and 6A^t were not observed. In 15 lines, a 5BS.5BL-5GL translocation was identified. High frequency of substitutions of chromosomes 2A^t, 2G, 5G, and 6G indicate that they may carry the resistance genes and that they are closely related to the respective homoeologous chromosomes of common wheat that determines their high compensation ability.     

Meiotic chromosomal aberrations in wild populations of Podophyllum peltatum

Meiotic chromosomal aberrations observed in wild populations of the plant Podophyllum peltatum include incomplete homologous pairing, non-homologous pairing, and inversion heterozygosity in pachytene; univalents, asymmetrical bivalents, and translocation heterozygosity in metaphase-I; bridge and fragments in anaphase-I; and non-disjunction as detected in anaphase-II. Incomplete homologous pachytene pairing is believed to result in non-homologous pairing and in the formation of metaphase-I univalents. The unequal distribution and precocious division of univalents in anaphase-I leads to non-disjunction. Non-disjunction chromosomes (varying in frequency from 0.0 to 24.6%) appear to be distributed among the genome on the basis of chromosome length. Asymmetrical bivalents and anaphase-I side-arm bridges are believed to be caused by chromatid breakage and fusion rather than inversion heterozygosity. Of the 135 clones examined, 20 were found to be heterozygous for translocations. The possibility of widespread distribution of some translocations is suggested.     

Intraspecific karyotype divergence inTriticum araraticum (Poaceae)

Karyotypes of 185 accessions ofTriticum araraticum Jakubz. (2n = 28 = 4x = A^tA^tGG) from Iraq, Iran, Turkey, and Transcaucasia were analyzed using C-banding technique. All accessions showed a certain degree of C-banding polymorphism and further karyotypic diversity was generated by structural rearrangements, mainly translocations. Eighty-one accessions had the normal karyotype similar to that ofT. timopheevii (cultivation), i.e., they showed C-banding polymorphism but no chromosomal rearrangements based on the resolving power of the C-banding technique. One-hundred four accessions showed 34 karyotypic variants, 31 had reciprocal translocations with the breakpoints in the centromeric regions of chromosomes. Three showed reciprocal translocations with the breakpoints in intercalary regions of chromosomes. A paracentric inversion for 7A^t chromosome was observed in some accessions. The rearranged karyotypes differed from the normal by one translocation in 21 variants, by two in 9 variants, by three in 1 variant, and by four in 2 variants of karyotypes. Translocations occurred more frequenty in the chromosomes of G-genome than of A^t-genome. Individual chromosomes differed in the frequencies of their involvement in translocations. Each geographical region contained a unique spectrum of translocations. Karyotypic diversity was the highest in Iraq followed by Transcaucasia and Turkey. Iran showed little karyotypic variation. Based on karyotypic analysis, Iraq should be considered as a centre of origin and primary centre of diversity ofT. araraticum.     

The cytogenetics of a Triticum turgidum x Psathyrostachys juncea hybrid and its backcross derivatives

Psathyrostachys juncea (2n = 2x = 14, NN), a source of barley yellow dwarf (BYDV) virus resistance with tolerance to drought and salinity, has been successfully hybridized in its autotetraploid form (2n = 4x = 28, NNNN) as the pollen parent to durum wheat (Triticum turgidum L.). The 2n = 4x = 28 (ABNN) F₁ hybrid has a mean meiotic metaphase-I configuration of 20.29 univalents + 0.29 ring bivalents + 3.36 rod bivalents + 0.14 trivalents. Spike length, internode length, glume awn length and lemma awn length, as well as the general spike morphology of the F₁ hybrid, are intermediate with those of the two parents. Pollinating the ABNN F₁ hybrid has given backcross (BC)-I derivatives of an amphiploid (AABBNN) that expresses limited self-fertility. BC-2 derivatives have been obtained from these plants. Direct transfers of useful genes from Ps. juncea to wheat would require substantial genetic manipulation strategies. Both conventional and novel methodologies, which may complement each other, and so facilitate reaching an agricultural objective end point, are addressed.     

Marker-assisted development and characterization of a set of Triticum aestivum lines carrying different introgressions from the T. timopheevii genome

A set of common wheat introgression lines carrying one or two introgressions from Triticum timopheevii was produced by means of marker-assisted backcross selection. The starting material consisted of two BC1F20 (T. aestivum*2/T. timopheevii) lines with resistance to leaf rust, stem rust, powdery mildew, spot blotch, and loose smut and containing multiple 1A^t, 2A^t, 2G, 3A^tL, 3GL, 4GL, 5A^tL, 5GL, and 6G T. timopheevii chromosome fragments. The two lines were crossed with, and backcrossed three times to common wheat cultivar Saratovskaya 29. In total, 275 BC2F1 and BC3F2 plants were characterized by microsatellite markers and in situ hybridization. Molecular and cytological analyses revealed 38 plants with a single introgression from chromosomes 2G, 5GL, or 6G of T. timopheevii and 72 plants, each with two introgressions, among them three plants carrying a T. timopheevii translocation involving the D genome (2DS.2GL). It was observed that the lengths of fragments introgressed from the A^t genome were more than halved in the BC2 generation, while the lengths of 2G and 5GL introgressed fragments were only slightly reduced after the third backcross. The introgression lines were tested for resistance to the native Puccinia triticina population of the Western Siberian region of Russia. Lines with a single introgressed 5GL region carrying the major leaf rust resistance locus, QLr.icg-5B, were completely resistant. The presence of two minor resistance loci, QLr.icg-2A and QLr.icg-1A, suppressed disease development and reduced the number of urediniospores by up to 25 % but did not lead to a hypersensitive response. The introgression lines therefore constitute promising sources of new resistance to Puccinia triticina.     

The synaptic behaviour of the wild forms of Triticum turgidum and T. timopheevii.

Different wild allopolyploid species of Triticeae show extensive bivalent formation at zygotene while a considerable number of multivalents is present in cultivated polyploid wheats. To study the chromosome behaviour at early meiotic stages in wild forms of tetraploid wheats Triticum turgidum and T timopheevii (2n = 4x = 28) we have analysed the synaptic pattern in fully traced spread nuclei at mid- and late zygotene and at pachytene of wild accessions of these species. The mean number of synaptonemal complex (SC) bivalents at mid-zygotene ranged from 12.22 to 13.14 among the accessions studied indicating a strong restriction of synapsis initiation to homologous chromosomes. The mean of bivalents increased at pachytene because of the transformation of multivalents into bivalents. Ring bivalents observed at metaphase I support that SC bivalents were formed by homologous chromosomes. The average values of SC bivalents at mid-zygotene in the wild forms are much higher than the average values observed in the cultivated tetraploid wheats but similar to that of a mutant line of T turgidum with a duplication that includes Ph1, the major homoeologous pairing suppressor locus. These results suggest that the efficiency of the mechanism operating in the homologous recognition for synapsis is higher in wild wheat populations than in cultivated varieties. Apparently, a relatively detrimental modification of the pairing regulating genetic system accompanied the domestication of the wild wheat forms.     

Electron microscopy of meiosis in Drosophila melanogaster females

Complete reconstruction of the synaptonemal complex in 12 pachytene (defined here as that stage in which the synaptonemal complex is continuous throughout the bivalents) nuclei from one wild-type germarium has permitted the following observations. 1) Drosophila melanogaster bivalents at pachytene exhibit a chromocentral arrangement; the pericentric heterochromatin of all bivalents lies in one region of the nucleus, the chromocenter. Telomeric ends do not appear to abutt the nuclear envelope. 2) Synaptonemal complex is present in the pericentric heterochromatin; however, it is morphologically distinct from that present in the euchromatic portion of the bivalents. 3) Length of the synaptonemal complex of the bivalent arms is greatest at early pachytene; the synaptonemal complex then becomes progressively shorter. Minimum length is approximately one-half of the maximum. 4) Decrease in length of synaptonemal complex is accompanied by an increase in thickness. Reconstruction of 20 pachytene nuclei from an additional 8 germaria suggests that these observations are typical. Correlations between these cytological observations and genetic observations (e.g., patterns of crossing-over) are discussed.     

Ultrastructural studies of late meiotic prophase nuclei of spermatocytes in Ascaris suum

Post pachytene stages of meiotic prophase in males of Ascaris suum have been analyzed with the electron microscope. No synaptonemal-like polycomplexes (PCs) have been observed in the nucleoplasm or cytoplasm during the period from pachytene to diakinesis. From Serially sectioned diplotene nuclei it was found that the bivalents are located near the periphery of the nuclei, the central part of the nuclei being vacant. Each nucleus contains one nucleolus. Up to 1 m long stretches of unpaired lateral elements (LEs) are found in some of the diplotene bivalents. These LEs are morphologically similar to unpaired LEs in early zygotene nuclei. Partial 3-dimensional reconstruction of two nuclei shows that the bivalents contain some small stretches of synaptonemal complex (SC) up to 1.9 m long. Some bivalents at diakinesis show remnants of SCs. At this stage chromosomes are fibrous, condensed, attached to the nuclear envelope and mostly with a rounded profile in cross section. The synchronous development of the spermatocytes and small bivalents at diplotene in A. suum make this system a good object for the study of localization of SC remnants.     

Mapping of pachytene bivalents of female codling moth Cydia pomonella (Linnaeus) (Lep., Tortricidae)

Spread pachytene nuclei of codling moth Cydia pomonella (Linnaeus) (Lep., Tortricidae) females of a Syrian strain (SY) were used to investigate chromomere patterns of chromosome bivalents and determine their length. The karyotype of female codling moths consists of 28 chromosome bivalents, of which seven are clearly distinguishable using chromosome length and the number and size of the chromomeres in the pachytene stage. One autosome bivalent has two nucleolar organizing regions (NORs) that are located at the opposite ends of the chromosome and appear as distinct structural landmarks. In female codling moths, the WZ sex chromosome bivalent was easily identified in pachytene oocytes according to the heterochromatic thread of the W chromosome. This study contributed to the knowledge and identification of pachytene chromosomes of female codling moths.