J. Dvořák

Structural evolution of wheat chromosomes 4A, 5A, and 7B and its impact on recombination

The construction of comparative genetic maps of chromosomes 4Am and 5Am of Triticum monococcum and chromosomes of homoeologous groups 4, 5 and 7 of T. aestivum has provided insight into the evolution of these chromosomes. The structures of chromosomes 4A, 5A and 7B of modern-day hexaploid bread wheat can be explained by a 4AL/5AL translocation that occurred at the diploid level and is present both in T. monococcum and T. aestivum. Three further rearrangements, a 4AL/7BS translocation, a pericentric inversion and a paracentric inversion, have taken place in the tetraploid progenitor of hexaploid wheat. These structural rearrangements and the evolution of chromosomes 4A, 5A and 7B of bread wheat are discussed. The presence of the 4AL/5AL translocation in several Triticeae genomes raises two questions — which state is the more primitive, and is the translocation of mono- or poly-phylogenetic origin?The rearrangements that have occurred in chromosome 4A resulted in segments of both arms having different positions relative to the telomere, compared to 4Am and to 4B and 4D. Comparisons of map length in these regions indicate that genetic length is a function of distance from the telomere, with the distal regions showing the highest recombination.

Mapping of the K(+)/Na (+) discrimination locus Kna1 in wheat.

In saline environments, bread wheat, Triticum aestivum L. (genomes AABBDD), accumulates less Na+ and more K+ in expanding and young leaves than durum wheat, T. turgidum L. (genomes AABB). Higher K+/Na+ ratios in leaves of bread wheat correlate with its higher salt tolerance. Chromosome 4D from bread wheat was shown in previous work to play an important role in the control of this trait and was recombined with chromosome 4B in the absence of the Ph1 locus. A population of plants disomic for 4D/4B recombined chromosomes in the genetic background of T. turgidum was developed to investigate the genetic control of K+/Na+ discrimination by chromosome 4D. Evidence was obtained that the trait is controlled by a single locus, designated Kna1, in the long arm of chromosome 4D. In the present work, K+/Na+ discrimination was determined for additional families with 4D/4B chromosomes. The concentrations of Na+ and K+/Na+ ratios in the youngest leaf blades clustered in two nonoverlapping classes, and all recombinant families could be unequivocally assigned to Kna1 and kna1 classes. The Kna1 locus scored this way was mapped on a short region in the 4DL arm and was completely linked to Xwg199, Xabc305, Xbcd.402, Xpsr567, and Xpsr375; it was also mapped as a quantitative trait. The results of the QTL analysis, based on the K+/Na+ ratios in the young leaves of greenhousegrown plants and flag leaves of field-grown plants, agreed with the position of Knal determined as a qualitative trait. Several aspects of gene introgression by manipulation of the Ph1 locus are discussed.

Enhancement of the salt tolerance of Triticum turgidum L. by the Kna1 locus transferred from the Triticum aestivum L. chromosome 4D by homoeologous recombination

Durum wheat, Triticum turgidum L. (2n= 4x=28, genome formula AABB) is inferior to bread wheat, T. aestivum L. (2n=6x=42, genome formula AABBDD), in the ability to exclude Na+ under salt strees, in the ratio of the accumulated K+ to Na+ in the leaves under salt stress, and in tolerance of salt stress. Previous work showed that chromosome 4D has a major effect on Na+ and K+ accumulation in the leaves of bread wheat. The 4D chromosome was recombined with chromosome 4B in the genetic background of durum wheat. The recombinants showed that Na+ exclusion and enhanced K+/Na+ ratio in the shoots were controlled by a single locus, Kna1, in the long arm of chromosome 4D. The recombinant families were grown in the field under non-saline conditions and two levels of salinity to determine whether Kna1 confers salt tolerance. Under salt stress, the Kna1 families had higher K+/Na+ ratios in the flag leaves and higher yields of grain and biomass than the Kna1- families and the parental cultivars. Kna1 is, therefore, one of the factors responsible for the higher salt tolerance of bread wheat relative to durum wheat. The present work provides conceptual evidence that tolerance of salt stress can be transferred between species in the tribe Triticeae.

Molecular mapping of an aluminum tolerance locus on chromosome 4D of Chinese Spring wheat

SummaryThe tolerance of aluminum (Al) of disomic substitution lines having the chromosomes of the D genome of Triticum aestivum L. cv. Chinese Spring individually substituted for their homoeologues in T. turgidum L. cv. Langdon was investigated by the hematoxylin method. The disomic substitution lines involving chromosome 4D were more Al tolerant than Langdon. The tolerance was found to be controlled by a single dominant gene, designated Alt2, that is in the proximal region of the long arm of chromosome 4D. The locus was mapped relative to molecular markers utilizing a population of recombinant chromosomes from homoeologous recombination between Chinese Spring chromosome 4D and T. turgidum chromosome 4B. Comparison of the location of Alt2 in this map with a consensus map of chromosomes 4B and 4D based on homologous recombination indicated that Alt2 is in a vicinity of a 4 cM interval delineated by markers Xpsr914 and Xpsr1051. The Alt2 locus is distal to marker Xpsr39 and proximal to XksuC2. The Altw locus is also proximal to the Knal locus on chromosome 4D that controls K+/Na+ selectivity and salt tolerance. In two lines, Alt 2 and Knal were transferred on a single 4D segment into the long arm of T. turgidum chromosome 4B.

Reconstruction of the phylogeny of the genus Triticum from variation in repeated nucleotide sequences.

SummaryThe potential of variation in repeated nucleotide sequences as a tool for phylogenetic studies was examined by investigating the phylogeny of 13 diploid species of the genus Triticum L. sensu Bowden. Low intraspecific variation in repeated nucleotide sequence families in Triticum indicated that restriction fragment profiles of repeated nucleotide sequences in Southern blots are reliable and uniform characteristics of each species. Cloned repeated nucleotide sequences were hybridized with Southern blots of DNAs of the Triticum species and the outgroup, Lophopyrum elongatum (Host) Á. Löve. The presence or absence of bands in the Southern blot autoradiograms was considered to be a character for phylogenetic analysis. A most parsimonious tree was resolved with the PAUP version 3.0L computer package. The tree was consistent with cytotaxonomic and evolutionary data available on the species.

Comparison of the genetic organization of the early salt-stress-response gene system in salt-tolerant Lophopyrum elongatum and salt-sensitive wheat

Lophopyrum elongatum is a facultative halophyte related to wheat. Eleven unique clones corresponding to genes showing enhanced mRNA accumulation in the early stages of salt stress were previously isolated from a L. elongatum salt-stressed-root cDNA library. The chromosomal distribution of genes complementary to these clones in several genomes of the tribe Triticeae and their copy number in the L. elongatum and wheat genomes are reported. Genes complementary to clones pESI4, pESI14, pESI15, pESI28, and pESI32 were found in homoeologous group 5, those complementary to pESI18 and pESI35 in homoeologous group 6, and those complementary to pESI47, pESI48, pESI3, and pESI2 in homoeologous groups 1, 3, 4, and 7, respectively. The genes are present in a single copy per genome in L. elongatum with the exception of those complementary to pESI2 and pESI18 which are present in at least two and five copies, respectively. Since similar copy numbers per genome were found in wheat (except for pESI2), the ability of L. elongatum to accumulate higher mRNA levels than wheat in response to salt shock apears to have evolved by changes in the regulation of these genes.

Chromosome and nucleotide sequence differentiation in genomes of polyploid Triticum species.

SummaryThe nature of genome change during polyploid evolution was studied by analysing selected species within the tribe Triticeae. The levels of genome changes examined included structural alterations (translocations, inversions), heterochromatinization, and nucleotide sequence change in the rDNA regions. These analyses provided data for evaluating models of genome evolution in polyploids in the genus Triticum, postulated on the basis of chromosome pairing at metaphase I in interspecies hybrids.The significance of structural chromosome alterations with respect to reduced MI chromosome pairing in interspecific hybrids was assayed by determining the incidence of heterozygosity for translocations and paracentric inversions in the A and B genomes of T. timopheevii ssp. araraticum (referred to as T. araraticum) represented by two lines, 1760 and 2541, and T. aestivum cv. Chinese Spring. Line 1760 differed from Chinese Spring by translocations in chromosomes 1A, 3A, 4A, 6A, 7A, 3B, 4B, 7B and possibly 2B. Line 2541 differed from Chinese Spring by translocations in chromosomes 3A, 6A, 6B and possibly 2B. Line 1760 also differed from Chinese Spring by paracentric inversions in arms 1AL and 4AL whereas line 2541 differed by inversions in 1BL and 4AL (not all chromosomes arms were assayed). The incidence of structural changes in the A and B genomes did not coincide with the more extensive differentiation of the B genomes relative to the A genomes as reflected by chromosome pairing studies.To assay changing degrees of heterochromatinization among species of the genus Triticum, all the diploid and polyploid species were C-banded. No general agreement was observed between the amount of heterochromatin and the ability of the respective chromosomes to pair with chromosomes of the ancestral species. Marked changes in the amount of heterochromatin were found to have occurred during the evolution of some of the polyploids.The analysis of the rDNA region provided evidence for rapid “fixation” of new repeated sequences at two levels, namely, among the 130 bp repeated sequences of the spacer and at the level of the repeated arrays of the 9 kb rDNA units. These occurred both within a given rDNA region and between rDNA regions on nonhomologous chromosomes. The levels of change in the rDNA regions provided good precedent for expecting extensive nucleotide sequence changes associated with differentiation of Triticum genomes and these processes are argued to be the principal cause of genome differentiation as revealed by chromosome pairing studies.

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.

Linkage relationships among stress-induced genes in wheat

Linkage relationships among genes responding to water-deficit, salt stress, and heat shock were investigated in diploid wheat, Triticum monococcum L. The position of these gene loci relative to closely linked markers and the centromeres is reported. It is proposed to continue to use the present T. monococcum mapping population and the genetic maps based thereon as a framework for future determination of relationships among other genes related to environmental stress in the tribe Triticeae.

The Q locus of Iranian and European spelt wheat

Abstractu2002A dominant allele at the Q locus on chromosome 5A is believed to be the principal factor responsible for free-threshing, square-head spikes with a non-fragile rachis in bread wheat, Triticum aestivum ssp. aestivum. The spelt syndrome, resulting in pyramidal spikes with a brittle rachis and hulled grain in T. aestivum, is believed to be principally caused by the q allele. Chromosome 5A of European and Iranian spelt was substituted for 5A of bread wheat and the lines were characterized with molecular markers. The substitution of bread wheat chromosome 5A by 5A of European spelt resulted in weakly hulled, pyramidal spikes with a non-brittle rachis, whereas and the substitution of 5A by 5A of Iranian spelt did not alter spike morphology at all. It is concluded that the expression of the spelt syndrome depends, to a large extent, on the interactions of q with genes controlling glume tenacity and rachis fragility on other chromosomes. The genetic basis for the spelt syndrome and the apparent presence of the Q allele in Iranian spelt are discussed.