Proceedings of the XLV Italian Society of Agricultural Genetics - SIGA Annual Congress

Salsomaggiore Terme, Italy - 26/29 September, 2001

ISBN 88-900622-1-5

Poster Abstract

GENETIC AND PHYSICAL MAPPING OF DURUM WHEAT RECOMBINANT PRODUCTS BETWEEN WHEAT 7A AND A THINOPYRUM PONTICUM 7AG CHROMOSOME CARRYING THE LR19+YP GENES

GENNARO A.*, MICALI S.*, FORTE P.*, BITTI A.*, DE VITA P.**, CEOLONI C.*

* Dept. of Agrobiology and Agrochemistry, Univ. of Tuscia, 01100 Viterbo

ceoloni@unitus.it

** Cereal Research Institute, section of Foggia

wheat-alien transfers, group 7 chromosomes, resistance genes, in situ hybridization

A wide spectrum of durum wheat-Thinopyrum ponticum (syn. Agropyron elongatum, a wild wheat relative) transfers was obtained by ph1-induced homoeologous recombinantion between wheat 7A and a primary 7A/7Ag recombinant chromosome previously transferred from common into durum wheat. The practical target consisted of the incorporation into the latter species of Th. ponticum 7Ag segments of reduced size with respect to the primary type, still containing the Lr19 (leaf rust resistance) and Yp (yellow endosperm pigmentation) genes. Such genes, of great potential value for durum wheat improvement, were known to be closely linked on the long arm of the alien 7Ag chromosome (7AgL), with Yp being distally located with respect to Lr19.

By use of in situ hybridization (ISH) as a selection tool throughout the transfer procedure, several secondary and even tertiary recombinants were isolated, which not only include lines with considerable breeding potential, but also represent highly valuable materials for research purposes. First of all, correlating the GISH (Genomic ISH)-based physical maps of the recombinant chromosomes and the Lr19 and Yp phenotypes of the corresponding lines, location of the two genes could be rather precisely determined. Lr19, in particular, turned out to be positioned within the 1% chromatin fraction (as calculated on contracted somatic metaphase chromosomes) differentiating two secondary recombinant chromosomes with terminal 7AgL segments, one present in a leaf-rust resistant line with 7AgL representing a 23% of the total arm length (FL=fraction length, i.e. distance of the wheat-alien breakpoint from the centromere=0.77), and the other with a 22% 7AgL (FL=0.78) carried by a susceptible line. Both Lr19 and Yp, then, appeared to be included in an interstitial 7AgL segment, comprised between FL 0.77 and 0.90. Such a segment is carried by a tertiary recombinant line which was obtained as a result of homologous recombination in the 7AgL region shared by two secondary recombinants. Recombination frequency within the interstitial segment indicates a genetic length of over 60 cM.

Using anonymous and known-function RFLP and PCR clones already mapped on the long arm of wheat 7A, the durum wheat-Th. ponticum recombinants, representing a wide array of well scattered physical breakpoints along the 7AL arm, are being currently employed in a comparative wheat-Th. ponticum genetic mapping. This can turn useful to identify markers closely linked to the target genes, to verify the extent of interspecific gene sinteny between the corresponding critical regions, and eventually to isolate Lr19 and Yp. The gene-rich, distal portion of wheat group 7 chromosomes appears of particular interest, harboring genes/markers which suggest possible analogy with the corresponding region of 7AgL. They include: a) resistance gene analogs (RGAs), containing conserved sequence motifs found within the nucleotide binding site-leucin-rich repeat (NBS-LRR) class of genes involved in specific recognition events of plant resistance reaction to pathogens; b) a cluster of defense response (DR) genes, coding for enzymes/proteins involved in the host defence response toward a range of offensive stimuli, including pathogen challenge; and c) a 7A marker linked to the major locus controlling fluor color in common wheat.

Genetic mapping in fact indicated this last marker to identify a corresponding locus within the 7AgL interstitial segment of the tertiary recombinant, proved to possess the Lr19+Yp genes. On the other hand, the Lr19 gene did not result to be related with any of the probes for either the 7AL DR genes or RGAs tested so far. In particular, the whole DR gene cluster turned out to lie beyond the distal border of the same 7AgL segment. Similarly, the RFLP pattern of the available RGA clone mapping on wheat 7L, while showing polymorphic bands onto the DNA of the primary 7A/7Ag recombinant line (possessing a nearly entire 7Ag chromosome), failed to reveal any such bands to be associated with the Lr19 gene. To this respect, absence of interspecific cross-hybridization signals for several NBS-LRR probes, or nonsyntenic map locations for NBS-LRR loci seem to represent frequently occurring phenomena in cereal genomes. In addition to further mapping analyses, differential analyses on mRNAs of wheat-Th. ponticum recombinants carrying or lacking the Lr19 gene are also planned to elucidate its functional characteristics.