J. Jahier

Molecular-genetic characterisation of a new nematode resistance gene in wheat

Abstract Bread wheat lines introgressed with Aegilopsventricosa chromosomes were evaluated for their resistance to the Australian cereal cyst nematode (CCN, Heteroderaavenae) pathotype Ha13. Higher levels of resistance relative to the phenotype of the Cre1 CCN resistance gene in wheat were found in the donor Ae. ventricosa parental lines and chromosome-5Nv substitution or addition lines. The newly identified resistance to pathotype Ha13 on chromosome 5Nv, designated, Cre6, was shown to be independent of the Ae. ventricosa-derived Cre2 gene, effective against several European pathotypes. Another Ae. ventricosa derived gene, Cre5, showed partial resistance to pathotype Ha13. Inhibition of Ha13 female nematode reproduction was ranked in the order Cre6 >Cre1 >CreF≥Cre5. Cre6 was inherited as a single dominant locus. Gene sequences encoding nucleotide-binding sites and leucine-rich repeats (NBS-LRR) from the Cre3 CCN-pathotype Ha13 resistance locus were used as probes to isolate related sequences from one of the donor Ae. ventricosa parents. Related sequences from Ae. ventricosa (71–73% similarity at the amino-acid level to the Cre3-derived sequences) of chromosome 5Nv origin were identified and served as diagnostic molecular markers for the presence of 5Nv. CCN-susceptible plants, found as variants in some of the purported chromosome 5Nv lines, were also found to be missing the diagnostic 5Nv RFLP markers assayed by the NBS-LRR probe. An alloplasmic chromosome-5Nv addition line with Ae. ventricosa cytoplasm in the wheat cultivar, Moisson, background was particularly variable, with 43% CCN-susceptible plants and a corresponding loss of the diagnostic chromosome-5 molecular markers.

RFLP analysis of an Aegilops ventricosa chromosome that carries a gene conferring resistance to leaf rust (Puccinia recondita) when transferred to hexaploid wheat

RFLP analysis has been used to characterise XMv, a chromosome of Aegilops ventricosa present in a disomic addition line of wheat. This chromosome is known to carry a major gene conferring resistance to leaf rust (Lr). The analysis demonstrated that XMv is translocated with respect to the standard wheat genome, and consists of a segment of the short arm of homoeologous group 2 attached to a group 6 chromosome lacking a distal part of the short arm. Lr was located to the region of XMv with homoeology to 2S by analysis of a leaf rust-susceptible deletion line that was found to lack the entire 2S segment. Confirmation and refinement of the location of Lr was obtained by analysis of a spontaneous resistant translocation in which a small part of XMv had been transferred to wheat chromosome 2A.

Production and cytogenetic analysis of BC1, BC2, and BC3 progenies of an intergeneric hybrid between Triticum aestivum (L.) Thell. and tetraploid Agropyron cristatum (L.) Gaertn.

SummaryIntergeneric hybrids between Triticum aestivum cv ‘Chinese Spring’ and Agropyron cristatum 4x (2n= 5x=35, ABDPP genomes) with a high level of homoeologous meiotic pairing between the wheat chromosomes were backcrossed 3 times to wheat. Pollination of the F1 hybrid with ‘Chinese Spring’ resulted in 22 BC1 seeds with an average seed set of 1.52%. Five BC1 plants with 39–41 chromosomes were raised using embryo rescue techniques. Chromosome pairing in the BC1 was characterized by a high frequency of multivalent associations, but in spite of this there was no evidence of homoeologous pairing between chromosomes of wheat and those of Agropyron. All of the plants were self sterile. The embryo rescue technique was again essential to produce 39 BC2 plants with chromosome numbers ranging from 37 to 67. The phenomenon of meiotic non-reduction was also observed in the BC3 progenies. In this generation male and female fertility greatly increased, and meiotic pairing was fairly regular. Some monosomic (2n=43) and double monosomic (2n=44) lines were produced. Analysis of these progenies should permit the extraction of the seven possible wheat-Agropyron disomic addition lines including those with the added chromosomes carrying the genes involved in meiotic non-reduction and in suppression of Ph activity.

Comparison of TAF46 and Zhong 5 resistances to barley yellow dwarf virus from Thinopyrum intermedium in wheat

AbstractBarley yellow dwarf virus (BYDV) is one of the most important plant viruses in the world. Two sources of resistance to BYDV derived from Thinopyrum intermedium were compared in wheat backgrounds. A source of resistance was confirmed in the partial amphiploid TAF46, the group 7 addition line L1, and translocation TC14. The other source of resistance derives from the partial amphiploid Zhong 5 and is present in the group 2 addition line Z6. Six ditelosomic addition lines have been derived from Z6. The resistance of genotypes derived from Zhong 5 is more effective at reducing virus multiplication throughout plant growth than that of genotypes derived from TAF46. The translocation line TC14, derived from TAF46 showed 30% plants escaping virus infection whereas all plants derived from Zhong 5 were infected. This suggests that the two sources of resistance are associated with differing mechanisms of resistance. Methods to better understand the genetic control and the mechanisms of these two resistances are suggested. The pyramiding of different sources of resistance to construct durable resistance is discussed.

The B chromosome system of Inner Mongolian Agropyron Gaertn. 1. Distribution, morphology and cytological behaviour

SUMMARYB chromosomes were found in varying frequencies in the diploid (56%) and tetraploid (5%) species in natural populations of Inner Mongolian Agropyron Gaertn., but there was no relationship between the presence of Bs and the ecological and climatic conditions in which the populations were found. Bs were usually smaller than As and showed polymorphism with three kinds of morphology: telocentric, metacentric and acrocentric with this latter being by far the most frequent. The Bs showed great mitotic instability (from 0 to 6 Bs per cell) and they were present in pollen mother cells, in primary roots and in stem tissues, but not in adventitious roots. It was impossible to analyse their transmission through gametes. However, the meiotic behaviour of the Bs was regular and smilar to that of As. They paired between themselves as bivalents, multivalents up to pentavalents. Usually, they segregated normally and were rarely lost et AI and AII The heterochromatic nature of Bs in Agropyron is discussed although ...

Genetics of two mechanisms of resistance to Meloidogyne naasi (FRANKLIN) in an Aegilops variabilis Eig. accession

SummaryAccession No. 1 of Aegilops variabilis has complete resistance to the root knot nematode, Meloidogyne naasi. F2 segregation in a cross between this and two susceptible accessions showed that one dominant gene named Rkn-mn1 prevented development of galls on the roots and consequently of female nematodes. The study of the numbers of females in galls on the F2 plants allowed detection of a recessive gene, Rkn-mnAv, suppressing development of J2 larvae into females. The presence of Rkn-mnAv also resulted in a decrease of the level of galling. Rkn-mn1 has already been introduced into wheat. The interest in transferring also Rkn-mnAv is discussed in relation to extending durability of the nematode resistance.

The B chromosome system of Inner Mongolian Agropyron Gaertn. 2. Effects of Bs on homologous and homoeologous meiotic chromosome pairing

SUMMARYB chromosomes in diploid Agropyron species alter the level of meiotic homologous pairing. The decrease of pairing was gradual and appeared significant when the number of Bs was higher than 2. A significant drop in homoeologous chromosome associations was also observed in B-containing intergeneric hybrids between wheat and diploid or tetraploid Agropyron. B chromosomes of Agropyron in which A chromosomes can interact with the pairing regulatory gene Ph1 of wheat have comparable effects on homologous and homoeologous pairing as those found in other genera in Triticeae: Secale and Aegilops. Evidence of similarities with the activity of the Ph system of wheat is presented.

The eyespot resistance genes Pch1 and Pch2 of wheat are not homoeoloci

Key messagePhenotyping and mapping data reveal that chromosome intervals containing eyespot resistance genesPch1 andPch2 on 7D and 7A, respectively, do not overlap, and thus, these genes are not homoeloci.AbstractEyespot is a stem-base fungal disease of cereals growing in temperate regions. Two main resistances are currently available for use in wheat. Pch1 is a potent single major gene transferred to wheat from Aegilops ventricosa and located on the distal end of chromosome 7D. Pch2, a moderate resistance deriving from Cappelle Desprez, is located at the end of 7AL. The relative positions of Pch1 and Pch2 on 7D and 7A, respectively, suggest that they are homoeoloci. A single seed decent recombinant F7 population was used to refine the position of Pch2 on 7A. New markers designed to 7D also allowed the position of Pch1 to be further defined. We exploited the syntenic relationship between Brachypodium distachyon and wheat to develop 7A and 7D specific KASP markers tagging inter-varietal and interspecific SNPs and allow the comparison of the relative positions of Pch1 and Pch2 on 7D and 7A. Together, phenotyping and mapping data reveal that the intervals containing Pch1 and Pch2 do not overlap, and thus, they cannot be considered homoeloci. Using this information, we analysed two durum wheat lines carrying Pch1 on 7A to determine whether the Ae.ventricosa introgression extended into the region associated with Pch2. This identified that the introgression is distal to Pch2 on 7A, providing further evidence that the genes are not homoeoloci. However, it is feasible to use this material to pyramid Pch1 and Pch2 on 7A in a tetraploid background and also to increase the copy number of Pch1 in combination with Pch2 in a hexaploid background.

Revisiting meiotic pairing in wheat synthetics in relation to the evolution of the meiotic system in wheat

A large panel of hexaploid wheat synthetics was developed. Their tetraploid parents consisted of either four extracted wheat tetraploids (ETWs) or four natural present-day tetraploids, and their diploid parents consisted of twenty accessions of Aegilops tauschii. Analysis of meiotic behaviour of the synthetics showed that chromosome pairing is highly variable and depends on the progenitor. The meiotic behaviour in the four ETWs was compared to that of the natural tetraploid wheats. It appears there was no evolution at the hexaploid level of the meiotic genes carried by the A and B genomes. We also reach the conclusion that the neo-allohexaploids at the origin of present-day wheat had a meiotic behaviour close to that of the present-day hexaploid wheat. It is likely that other neo-hexaploids with an impaired meiosis were formed, but they had no future due to their more or less rapid disappearance due to increasing aneuploidy level and structural changes, mainly Robertsonian translocations.

Agronomic evaluation of AABB wheat tetraploids extracted from wheat neo-allohexaploids

In order to understand if the mechanisms leading to stabilization of hexaploid wheats occurred in early generations after hybridization or accumulated slowly over the evolutionary course, we extracted tetraploids from newly synthesized allohexaploid wheats and characterized the effects of changes in polyploidy levels. Extracted wheat tetraploids (ETW) were developed from two neo-allohexaploids and their five phenotypic traits were compared to the parents. The ETWs displayed few small amplitude differences relative to the tetraploid parent of the synthetics. This suggest that genomic changes that might have occurred during the first three generations of the newly-synthesized wheat allopolyploids had very weak effects on the phenotypes.