STRATEGIES, TOOLS AND PROGRESS TO DATE IN BREEDING PEPPER AND TOMATO FOR BROAD SPECTRUM DISEASE RESISTANCE
M KYLE JAHN1, I PARAN2, KD LIVINGSTONE1 and J PELEMAN3
1Dept. of Plant Breeding, Cornell University, Ithaca, NY 14953, USA; 2The Volcani Institute, PO Box 6, Bet-Dagan, Israel 50250; 3Keygenes, Agro Business Park, 6708 PW Wageningen, The Netherlands
Background and objectives
The objective of disease resistance in crop breeding has been a dominant focus for several decades, however to date, we have not achieved the full spectrum of resistance in any crop desired by crop producers and consumers. Nevertheless, the addition of disease resistance to crop varieties has resulted in enormous gains in crop yield and quality, various environmental benefits, and defines the second major revolution in vegetable breeding after the widespread use of hybrids. The genetic resources that have contributed to this revolution are primarily wild accessions that are sexually compatible, but often very far removed from crop types. Conventional breeding methods, especially backcross, pedigree, and permutations thereof, have provided very powerful and efficient means to transfer and combine many of these resistances. Their success depends upon the availability of necessary resistances within the gene pool that are reasonably heritable, and the availability of accurate, reliable phenotypic screens that are simple, cheap, non-destructive and preferably yield results before pollination decisions must be made. Typically, the maximum benefit from disease resistance breeding efforts occurs only after all or almost all the diseases of a specific type, e.g., leafspots, viruses, etc., are controlled. Thus, realization of the full benefit of disease resistance breeding requires both the transfer of disease resistances from their sources to cultivated types and their subsequent combination in elite genetic backgrounds. One objective of our work is to augment conventional selection strategies and traditional genetic resources for resistance in solanaceous crop species by developing well-saturated comparative genetic maps showing all disease resistance loci located for each species. This effort should allow improved detection of resistance alleles, and provide tools to expedite their transfer and selection, especially when several are segregating in one population.
Materials and methods
Due to the development of high-throughput marker systems, highly saturated molecular marker maps have been generated for several solanaceous species. Among these, a large comparative map of pepper has been generated (Livingstone et al., in preparation) using pepper and tomato libraries, AFLP and RAPD markers and cloned genes of known function, emphasizing disease resistance. A number of mono- and polygenic disease resistances have also been mapped, emphasizing pathogens that widely infect solanaceous species, allowing the development of integrated marker databases. These databases allow the selection of a set of markers that represent the complete genome to expedite the introgression and combination of genes in elite genetic backgrounds. PCR-based markers for indirect selection have been/are being developed and implemented in variety breeding programs.
Results and conclusions
We will present examples of how molecular markers and transgenic approaches have augmented the array of disease resistances available with emphasis on pepper and tomato. We will discuss the status of the use of indirect selection for polygenic resistances that are difficult to select phenotypically, such as resistance to cucumber mosaic virus in pepper. Similar to results in other species, e.g., lettuce, we observe that disease resistance loci tend to cluster within a species, although the pathogens involved may be very different. These clusters tend to occur at corresponding positions across host species, however, no evidence of conserved specificities has been obtained to date. Nevertheless, this observation suggests the existence of conservative forces and perhaps conserved mechanisms in the evolution of disease resistance genes and possibly that similar marker sets may be broadly useful across diverse crop species. The implications of these trends and tools will be discussed for breeding strategies and management.