Polyploid Wheat Species Research Articles

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(t)A(t)GG), 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.

Engineering of interstitial foreign chromosome segments containing the K + /Na + selectivity gene Kna1 by sequential homoeologous recombination in durum wheat

Targeted homoeologous recombination mediated by the absence of the Ph1 locus is currently the most efficient technique by which foreign genes can be introgressed into polyploid wheat species. Because intra-arm homoeologous double cross-overs are rare, introgressed foreign genes are usually on terminal foreign chromosome segments. Since the minimum length of such a segment is determined by the position of a gene in the chromosome, large chromosome segments with undesirable genetic effects are often introgressed. Introgression of foreign genes on short interstitial segments based on two cycles of homoeologous recombination is described here. The utility of the technique is demonstrated by the introgression of the Kna1 locus, which controls K+/Na+ selectivity in T. aesivum L., on short interstitial segments of chromosome 4D into chromosome 4B of Triticum turgidum L. The level of recombination in a homoeologous segment is not significantly affected by a juxtaposed proximal homologous segment in the absence of the Ph1 locus.

Engineering of interstitial foreign chromosome segments containing the K+/Na+ selectivity gene Kna1 by sequential homoeologous recombination in durum wheat

Targeted homoeologous recombination mediated by the absence of the Ph1 locus is currently the most efficient technique by which foreign genes can be introgressed into polyploid wheat species. Because intra-arm homoeologous double cross-overs are rare, introgressed foreign genes are usually on terminal foreign chromosome segments. Since the minimum length of such a segment is determined by the position of a gene in the chromosome, large chromosome segments with undesirable genetic effects are often introgressed. Introgression of foreign genes on short interstitial segments based on two cycles of homoeologous recombination is described here. The utility of the technique is demonstrated by the introgression of the Kna1 locus, which controls K(+)/Na(+) selectivity in T. aesivum L., on short interstitial segments of chromosome 4D into chromosome 4B of Triticum turgidum L. The level of recombination in a homoeologous segment is not significantly affected by a juxtaposed proximal homologous segment in the absence of the Ph1 locus.

Relationships betweenNor-loci from differentTriticeae species

TheNor-loci of polyploid wheats and their putative diploid progenitor species were assayed by probing isolated nuclear DNA with ribosomal DNA spacer sequences (spacer rDNA sequences, isolated by cloning), from theNor-loci of genomes B (Triticum aestivum), G (T. timopheevi), B (syn. S,T. speltoides), A (T. monococcum) and V (Dasypyrum villosum). DNA samples for analysis were digested with the restriction endonuclease Taq 1 and assayed by DNA-DNA hybridization under standard (37°C) and high stringency (64°C) conditions. The assay procedure emphasized differences between the divergent spacer sequences of the polyploid species and allowed relative homologies to the respective sequences in diploid species to be established. — The studies indicated thatT. timopheevi andT. speltoides contain different sets of spacer rDNA sequences which were readily distinguishable and, in the case ofT. timopheevi, assigned toNor-loci on different chromosomes. This contrast with the spacer rDNA sequences of the majorNor-loci on chromosomes 1 B and 6 B inT. aestivum, which were difficult to distinguish and were deduced to contain very similar sequences. Among the diploid progenitor species only the spacer rDNA fromT. speltoides shared close homology with polyploid wheat species. OneNor-locus inT. timopheevi (on chromosome 6 G) did not show close homology with any of the rDNA spacer probes available. — The data suggestsT. speltoides was the origin of someNor-loci for both theT. timopheevi andT. turgidum lines of tetraploid wheats. The possibility that the 6GNor-locus inT. timopheevi may have derived from an unknown diploid species by introgressive hybridization is discussed. The spacer rDNA sequence probe fromT. monococcum shared good homology with some accessions ofD. villosum and a line ofT. dicoccoides; the implications of this finding for evolution of present-day wheats are discussed.

Linear differentiation of cereal chromosomes

The BSG test was used in a comparative study of the linear chromosome differentiation and the idiograms of T. Macha ssp. tubalicum v. 'letschchumicum ' Dek. et Men., T. georgicum Dek., T. timopheevi. Zhuk., T. carthlicum Nevski, T. dicoccum Schrank, v. 'rufum', T. durum Desf. v. 'Arnautka' were compiled.The karyotype of each polyploid wheat species consists of two groups of chromosomes. The first is formed by ten pairs of "constant" chromosomes occurring almost in all species and the second by all the rest of the "variable" chromosomes that are either fully specific for the species in question or occur only in a few species. T. timopheevi largely differs from other species of polyploid wheats in the high level and specific localization of structural heterochromatin on chromosomes. The rols of introgression in wheat evolution and the necessity of establishing a General Cytological Nomenclature of Cereal Chromosomes are discussed.

Deoxyribonucleic Acid and High Temperature Sensitivity in Wheat

A number of diploid and polyploid species of wheat have been analysed for the sensitivity of their chromosomes to conditions of high temperature. A comparative estimate of deoxyribonucleic acid in the root-tip cells of different species has also been made by the autoradiographic technique. The diploid and hexaploid species differed greatly in temperature sensitivity of their chromosomes. The diploid, tetraploid and hexaploid species contained an increased amount of DNA in that order but the increase was not proportional to the chromosome number in these species. No simple correlation was found to exist between the amount of DNA in the cells of different species and the temperature sensitivity of their chromosomes.

Temperature Induced Condition of Univalence in Wheat

A number of diploid and polyploid species of wheat have been subjected to the condition of high temperature. The various temperature treatments have been found to induced formation of univalents in a number of Pollen mother cells of different varieties. The distribution of cells showing such univalents has been studied. It has been found that in almost all the plants, the distribution is of a poisson type. The significance of this type of distribution has been discussed.

Repeated and non-repeated nucleotide sequences in diploid and polyploid wheat species

Reassociation rate studies of dissociated DNAs from diploid, tetraploid and hexaploid wheat species revealed that all the species have a large proportion of their DNA which shows characteristics of repeated DNA, under the conditions used in the reassociation experiments (fragment size about 450 nucleotide pairs, incubation at 60° C. in 0·12 M phosphate buffer, C0t = 100). The relative proportion of this fraction increased with ploidy in tetraploid and hexaploid wheat. Genome size (amount of non-repeated DNA) was larger in hexaploid and tetraploid wheat in comparison with diploid wheat. Heterologous reassociation of repeated and non-repeated fractions of DNAs showed considerably more divergence in the repeated sequences at both diploid and at polyploid levels. Non-repeated sequences of wheat species showed greater homologies, and appear to be more conservative in composition.

TEMPERATURE SENSITIVITY OF POLYPLOID WHEATS AND THEIR DIPLOID RELATIVES

A comparative assessment of temperature sensitivity, based on stability of chromosomal mechanisms and fertility, of some polyploid wheat species and their diploid progenitors was made by subjecting plants to a temperature of 40±1°C (R.H. 70) for durations ranging from 12 to 72 hours. All the diploids were observed to be considerably more sensitive than the polyploids. The differential response of varieties within a ploidy group was very marked; winter types being strikingly more resistant than the spring forms. These observatiens have been discussed with regard to the contributions of polyploidy in relation to adaptability of the bread wheats.

CHROMOSOME TRANSLOCATIONS IDENTIFIED IN VARIETIES OF COMMON WHEAT

Introduction Chromosome structural changes in polyploid wheat species appear to have played roles of varying significance in evolution. Pai et nl. (1961) compared total lengths and D N A contents in both diploid and hexaploid species with the summation of corresponding values for the prototypes of their presumed diploid progenitors and concluded that a considerable degree of chromatin elimination had talten place in polyploid wheats subsequent to their origin. Tolerance of loss of chromosome material may be expected to be associated with polyploidy. Kimber and Riley (1963), on the other hand, critically analyzed chromosome pairing under different situations in the absence of the gene(s) on chromosome 5B (V) which, when present, prevents associations between homoeologous chromosomes in common wheat (Triticu7~z aestiwz~m ssp. wlgare (Vill., Host) Mac Key) and concluded that speciation at ?he polyploid level had occurred by gene mutation and sujbstitution and that chromosomal rearrangements had played an insignificant evolutionary role. They also pointed out that identification of the diploid progenitors of hexaploid wheat by genome analysis would not have been possible in the presence of gross chromosonlal alterations. Sears (1953) reported that three varieties of hexaploid wheat differed by single reciprocal translocations from Chinese Spring. O n the basis of nullisomic-tetrasomic compensations which enabled chromosomes to be allocated to seven homoeologous groups, Sears (1954) suggested that Chinese Spring probalbly had the primitive chromosonle structure within which translocations lhad occurred. H e pointed out the lack of critical sensitivity associated with features of the con~pensation test but concluded that homoeologous groups were essentially intact in Chinese Spring since homologies within groups were of a different order from homologies between groups. Sears (1953) indicated that the variety Thatcher had a translocation relative to Chinese Spring, which involved chromosomes 4A ( IV) and 6B (X) ; in the case of Poso (a variety of T . aestiwz~m ssp. compactzun (Host) Mac Key) the chromosomes implicated were 5B and 7B (VII), whilst in the variety Indian chromosomes 3B (111) and 7B had undergone translocation. In addition to the varietal translocations described by Sears, Holdfast and Chinese Spring differ by a reciprocal translocation involving chromosomes 3B and 3D (XVI) (Riley and Kimber, 1961). T h e fact that Chinese Spring appears to possess the primitive chromosonle structure is fortunate in view of the wide use of this variety in studies concerned with monosomic analysis. Meiotic analyses to select monosomic and disomic F1 sibs in each of ?he twenty-one crosses will reveal the existence of structural differences due to translocations in other varieties and enalble the chromosomes involved in such interchanges to be identified. These are thus documented relative to the same primitive base.

Temperature Sensitivity of Chromosomes in Diploid and Polyploid Species of Wheat

THE two genera, Triticum and Gossypium, provide classical examples of evolution of agronomically superior plants through allopolyploidy. While the role of genome addition in this respect has been generally recognized, not much attention has been paid to the interaction between polyploidy and such components of the natural environment as temperature, which determines the adaptability of the newly arising species. The work on thermal sensitivity of chromosomes and of the meiotic cell division in Triticum monococcum, T. durum, T. dicoccum and T. aestivum recorded here was undertaken to obtain a measure of the adaptability of A, AB and ABD genome and genome combinations in a temperature-conditioned environment. Stability of the nuclear material constitutes a primary requirement of adaptation and it is known that plants of the temperate region often show a breakdown of their chromosomal mechanisms under conditions of high temperature1.