Adam J. Lukaszewski

Physical distribution of recombination in B-genome chromosomes of tetraploid wheat

SummarySeveral studies have indicated a noncorrespondence between genetic and physical distances in wheat chromosomes. To study the physical distribution of recombination, polymorphism for C-banding patterns was used to monitor recombination in 67 segments in 11 B-genome chromosome arms of Triticum turgidum. Recombination was absent in proximal regions of all chromosome arms; its frequency increased exponentially with distance from the centromere. A significant difference was observed between the distribution of recombination in physically short and physically long arms. In physically short arms, recombination was almost exclusively concentrated in distal segments and only those regions were represented in their genetic maps. In physically long arms, while a majority of the genetic distance was again based upon recombination in distal chromosome segments, some interstitial recombination was observed. Consequently, these regions also contributed to the genetic maps. Such a pattern of recombination, skewed toward terminal segments of chromosomes, is probably a result of telomeric pairing initiation and strong positive chiasma interference. Interference averaged 0.81 in 35 pairs of adjacent segments and 0.57 across the entire recombining portions of chromosome arms. The total genetic map lengths of the arms corresponded closely to those expected on the basis of their metaphase-I chiasma frequencies. As a consequence of this uneven distribution of recombination there can be a 153-fold difference (or more) in the number of DNA base pairs per unit (centiMorgan) of genetic length.

The centromere structure in Robertsonian wheat-rye translocation chromosomes indicates that centric breakage-fusion can occur at different positions within the primary constriction.

Abstract. Univalent chromosomes at meiotic metaphase I have a tendency to misdivide at the centromeres. Fusion of the misdivision products may produce Robertsonian translocations. The fine structure of the centromeres in Robertsonian wheat-rye translocation chromosomes was analyzed by fluorescence in situ hybridization (FISH) using two centromere-specific DNA clones: pRCS1, derived from rice, and pAWRC1, derived from rye. Clone pRCS1 hybridizes to the centromeres of all grasses including wheat and rye, whereas clone pAWRC1 is rye specific and hybridizes only to the centromeres of rye. Four of the six wheat-rye translocations derived from a single centric misdivision event (1st generation translocations) had hybrid centromeres, with approximately half of the centromere derived from rye and half from wheat. In the two other 1st generation translocations, the entire centromere was derived from rye. Among eight reconstructed wheat and rye chromosomes that originated from two consecutive centric misdivision-fusion events (2nd generation translocations), T1BS·1BL (derived from T1BS·1RL and T1RS·1BL) and one of three T2BS·2BL (derived from T2RS·2BL and T2BS·2RL) had hybrid centromeres. T1RS·1RL (derived from T1BS·1RL and T1RS·1BL), two of three T2BS·2BL, and all three T2RS·2RL (derived from T2RS·2BL and T2BS·2RL) had rye centromeres. All three 3rd generation translocations had hybrid centromeres with approximately half of the centromere derived from rye. There were no indications that the composite structure of the centromere in these chromosomes affected their behavior in mitosis or meiosis. These observations support the notion of a compound structure of the centromere in higher organisms, and indicate that during the centric breakage-fusion event, centromere breakage may occur in different positions along the segment of the chromosome that interacts with the spindle fibers. Normal behavior of the 1st, 2nd, and 3rd generation centric translocations in mitosis and meiosis indicates that, at least in wheat and rye, centromeres are not chromosome specific.

Genetic linkage between C-bands and storage protein genes in chromosome 1B of tetraploid wheat

SummaryGenetic mapping of polymorphic C-bands allows direct comparisons between genetic and physical maps. Eleven C-bands and two seed storage protein genes on chromosome 1B, polymorphic between Langdon durum and four accessions of T. dicoccoides, were used to study the distribution of recombination along the entire length of the chromosome. Recombination in the short arm was almost completely restricted to the satellite, two-thirds of the arms length from the centromere; the Gli-B1 gene was found to be tightly linked to the telomeric C-band. In the long arm, the distal 51.4% of the arm accounted for 88% of recombination; the proximal half of the arm accounted for the remaining 12%. While the amount of crossing-over differed significantly between the four T. dicoccoides 1B chromosomes, there were no significant differences in the relative distributions of crossing-over along the chromosome. Consequently, the genetic maps obtained from the four individual T. dicoccoides chromosomes were combined to yield a consensus map of 14 markers (including the centromere) for the chromosome.

Molecular characterization of durum and common wheat recombinant lines carrying leaf rust resistance (Lr19) and yellow pigment (Y) genes from Lophopyrum ponticum

Chromosome 7E from Lophopyrum ponticum carries a valuable leaf rust resistant gene designated Lr19. This gene has not been widely used in common wheat breeding because of linkage with the yellow pigment gene Y. This gene tints flour yellow, reducing its appeal in bread making. However, a high level of yellow pigment is desirable in durum wheat breeding. We produced 97 recombinant chromosomes between L. ponticum transfer 7D.7E#1 and its wheat homoeologues, using the ph1b mutation that promotes homoeologous pairing. We characterized a subset of 37 of these lines with 11 molecular markers and evaluated their resistance to leaf rust and the abundance of yellow pigment. The Lr19 gene was mapped between loci Xwg420 and Xmwg2062, whereas Y was mapped distal to Xpsr687, the most distal marker on the long arm of chromosome 7. A short terminal 7EL segment translocated to 7A, including Lr19 and Y (line 1-23), has been transferred to durum wheat by backcrossing. The presence of this alien segment significantly increased the abundance of yellow pigment. The Lr19 also conferred resistance to a new durum leaf rust race from California and Mexico that is virulent on most durum wheat cultivars. The new durum lines with the recombinant 7E segment will be useful parents to increase yellow pigment and leaf rust resistance in durum wheat breeding programs. For the common wheat breeding programs, we selected the recombinant line 1-96, which has an interstitial 7E segment carrying Lr19 but not Y. This recombinant line can be used to improve leaf rust resistance without affecting flour color. The 7EL/7DL 1-96 recombinant chromosome did not show the meiotic self-elimination previously reported for a 7EL/7BL translocation.

Dissection of QTL effects for root traits using a chromosome arm-specific mapping population in bread wheat

A high-resolution chromosome arm-specific mapping population was used in an attempt to locate/detect gene(s)/QTL for different root traits on the short arm of rye chromosome 1 (1RS) in bread wheat. This population consisted of induced homoeologous recombinants of 1RS with 1BS, each originating from a different crossover event and distinct from all other recombinants in the proportions of rye and wheat chromatin present. It provides a simple and powerful approach to detect even small QTL effects using fewer progeny. A promising empirical Bayes method was applied to estimate additive and epistatic effects for all possible marker pairs simultaneously in a single model. This method has an advantage for QTL analysis in minimizing the error variance and detecting interaction effects between loci with no main effect. A total of 15 QTL effects, 6 additive and 9 epistatic, were detected for different traits of root length and root weight in 1RS wheat. Epistatic interactions were further partitioned into inter-genomic (wheat and rye alleles) and intra-genomic (rye–rye or wheat–wheat alleles) interactions affecting various root traits. Four common regions were identified involving all the QTL for root traits. Two regions carried QTL for almost all the root traits and were responsible for all the epistatic interactions. Evidence for inter-genomic interactions is provided. Comparison of mean values supported the QTL detection.

Terminal Regions of Wheat Chromosomes Select Their Pairing Partners in Meiosis

Many plant species, including important crops like wheat, are polyploids that carry more than two sets of genetically related chromosomes capable of meiotic pairing. To safeguard a diploid-like behavior at meiosis, many polyploids evolved genetic loci that suppress incorrect pairing and recombination of homeologues. The Ph1 locus in wheat was proposed to ensure homologous pairing by controlling the specificity of centromere associations that precede chromosome pairing. Using wheat chromosomes that carry rye centromeres, we show that the centromere associations in early meiosis are not based on homology and that the Ph1 locus has no effect on such associations. Although centromeres indeed undergo a switch from nonhomologous to homologous associations in meiosis, this process is driven by the terminally initiated synapsis. The centromere has no effect on metaphase I chiasmate chromosome associations: homologs with identical or different centromeres, in the presence and absence of Ph1, pair the same. A FISH analysis of the behavior of centromeres and distal chromomeres in telocentric and bi-armed chromosomes demonstrates that it is not the centromeric, but rather the subtelomeric, regions that are involved in the correct partner recognition and selection.

Physical distribution of translocation breakpoints in homoeologous recombinants induced by the absence of the Ph1 gene in wheat and triticale

The physical distribution of translocation breakpoints was analyzed in homoeologous recombinants involving chromosomes 1A, 1B, 1D of wheat and 1R of rye, and the long arms of chromosome 7S of Aegilops speltoides and 7A of wheat. Recombination between homoeologues was induced by removal of the Ph1 gene. In all instances, translocation breakpoints were concentrated in the distal ends of the chromosome arms and were absent in the proximal halves of the arms. The relationship between the relative distance from the centromere and the relative homoeologous recombination frequency was best explained by the function f(x)=0.0091e0.0592x. The pattern of recombination in homoeologous chromosomes was essentially the same as in homologues except that there were practically no double exchanges. Among 313 recombinant chromosomes, only one resulted from a double crossing-over. The distribution of translocation breakpoints in translocated arms indicated that positive chiasma interference operated in homoeologous recombination. This implies that the reduction of the length of alien chromosome segments present in translocations with wheat chromosomes may be more difficult than the production of the original recombinants.

Further manipulation by centric misdivision of the 1RS.1BL translocation in wheat

Complete chromosomes 1R and 1B were reconstructed in wheat from the centric wheat-rye translocation 1RS.1BL. Three substitutions: 1R(1A), 1R(1B), 1R(1D), and three new centric translocations: 1RS.1AL, 1RS.1BL, 1RS.1DL were produced from the reconstructed chromosome 1R. Each one of these has the same rye chromosome arm 1RS which was present in the original translocation 1RS.1BL of ‘Kavkaz’ wheat. Reconstructed chromosome 1B and a normal chromosome 1R were used to produce a new 1RS.1BL translocation. This translocation has the long arm from the original 1RS.1BL translocation of ‘Kavkaz’, but a different 1RS arm. The third generation centric translocations were mitotically stable and were normally transmitted to progeny. Misdivision frequency of the reconstructed chromosomes 1R did not change relative to normal 1R, whereas the misdivision frequency of the two reconstructed chromosomes 1B tested was significantly higher relative to normal 1B. These experiments demonstrate that repeated cycles of centric breakage and fusion do not impair the function of centromeres in wheat and rye but may change chromosomes susceptibility to misdivision.

Development and mapping of DArT markers within the Festuca - Lolium complex.

BackgroundGrasses are among the most important and widely cultivated plants on Earth. They provide high quality fodder for livestock, are used for turf and amenity purposes, and play a fundamental role in environment protection. Among cultivated grasses, species within the Festuca-Lolium complex predominate, especially in temperate regions. To facilitate high-throughput genome profiling and genetic mapping within the complex, we have developed a Diversity Arrays Technology (DArT) array for five grass species: F. pratensis, F. arundinacea, F. glaucescens, L. perenne and L. multiflorum.ResultsThe DArTFest array contains 7680 probes derived from methyl-filtered genomic representations. In a first marker discovery experiment performed on 40 genotypes from each species (with the exception of F. glaucescens for which only 7 genotypes were used), we identified 3884 polymorphic markers. The number of DArT markers identified in every single genotype varied from 821 to 1852. To test the usefulness of DArTFest array for physical mapping, DArT markers were assigned to each of the seven chromosomes of F. pratensis using single chromosome substitution lines while recombinants of F. pratensis chromosome 3 were used to allocate the markers to seven chromosome bins.ConclusionThe resources developed in this project will facilitate the development of genetic maps in Festuca and Lolium, the analysis on genetic diversity, and the monitoring of the genomic constitution of the Festuca × Lolium hybrids. They will also enable marker-assisted selection for multiple traits or for specific genome regions.

A comparison of physical distribution of recombination in chromosome 1R in diploid rye and in hexaploid triticale

SummaryPolymorphism for six C-bands on chromosome 1R was used to study the frequency and distribution of recombination along the chromosome in a diploid rye (Secale cereale L.) and in a hexaploid triticale (X Triticosecale Wittmack) derived from it. In rye, the total recombination frequency in five segments of chromosome 1R was 93.7%. Recombination was concentrated in the distal regions of both chromosome arms and was infrequent in the proximal regions. In hexaploid triticale the total recombination frequency in the same chromosome was reduced to 51.7%. In both backgrounds the distal half of the long arm showed similar recombination frequencies, 51.4% and 45.7% for rye and triticale, respectively. The remaining about two-thirds of the chromosome length showed 42.3% recombination in rye but only 6% recombination in triticale. The results demonstrate that the genetic background in which mapping is performed not only affects the total amount of recombination, but also its distribution along the chromosome length.