Location: Cereal Crops ResearchTitle: A genomic comparison of homoeologous recombinants of the Lr19 (T4) translocation in wheat) Author
Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/24/2013
Publication Date: 3/1/2014
Publication URL: http://handle.nal.usda.gov/10113/59064
Citation: Somo, M., Chao, S., Acevedo, M., Zurn, J., Cai, X., Marais, F. 2014. A genomic comparison of homoeologous recombinants of the Lr19 (T4) translocation in wheat. Crop Science. 54:565-575. Interpretive Summary: The use of resistance genes has been the most effective way to combat the cereal rusts around the world. However, plant pathogens have the ability to evolve into new variants able to challenge resistant commercial varieties. It is becoming increasingly difficult to identify new resistance genes in germplasm collections of common wheat, yet its wild relatives and progenitors remain an abundant reservoir of such genes. During the process of transferring genes from the wild relatives to cultivated wheat, deleterious genes and genes conferring poor agronomic traits can also be brought in, due to a phenomenon known as linkage drag. Therefore, efforts have to be made to separate unwanted genes from the resistance genes before it can be used in breeding. The objective of this study was to characterize five wheat lines carrying different sized (alien) chromosome fragments from a wild relative. Each alien fragment contains a copy of the Lr19 leaf rust resistance gene. Chromosome painting methodology to visualize the alien fragment under a fluorescence microscope was used in combination with DNA marker genotyping to compare the sizes of the alien chromosome segment in each line. The line that retained Lr19 in association with the least alien chromatin was identified. Twenty-two DNA markers mapped within its introgressed region and will facilitate further efforts to manipulate the alien segment in breeding.
Technical Abstract: The Lr19 translocation continues to provide broad resistance to Puccinia triticina in many parts of the world and can be particularly useful when employed in resistance gene pyramids. Previously, an associated gene for yellow endosperm pigmentation precluded the use of the translocation in many countries, and as a result recombinants and mutants lacking the pigmentation genes have been developed. Such a primary white endosperm recombinant (Lr19-149), four secondary recombinants (Lr19-149-252, Lr19-149-299, Lr19-149-462, and Lr19-149-478), and a putative tertiary recombinant (Lr19-149-299.478) were obtained in earlier studies. The secondary recombinants were produced through the induction of allosyndetic pairing between the primary Lr19-149 recombined translocation and 7BL of Chinese Spring. The present study aimed to characterize the parental and recombined translocation chromosomes by making use of fluorescence genomic in situ hybridization (FGISH), simple sequence repeat (SSR), sequence tagged site (STS) and single nucleotide polymorphism (SNP) markers. The FGISH images showed size differences among the recombinants and the presence of a small wheat telomeric region in all of the recombinants thus confirming their intercalary nature. The molecular markers aided in differentiating the recombinants according to their alien fragment sizes and produced six patterns (chromosomal recombination bins) of absence/presence (wheat chromatin) polymorphisms demarcated by the allosyndetic crossover positions. The primary recombinant, Lr19-149, clearly had the largest Thinopyrum fragment. Secondary recombinants Lr19-149-299, Lr19-149-252 and Lr19-149-462 resulted from single, proximal crossovers whereas Lr19-149-478 apparently resulted from a double crossover. Secondary recombinant Lr19-149-299 retained the least alien chromatin while Lr19-149-299.478 turned out to be the same translocation as Lr19-149-299. Microsatellite marker loci Xbarc32 and Xgwm577, the STS marker locus XGbFGbR, and nineteen SNP marker loci mapped to the same recombination bin (E) as Lr19. All of the bin E markers can detect all of the Lr19 recombinants; however, XGbFGbR is the most specific for marker-assisted selection of the Lr19 translocation. The alien chromatin in all the recombinants overlaps with the location on 7BL of the slow rusting gene Lr68 which may preclude the development of pure lines carrying both resistances.