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

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Oral Communication Abstract - S4l

CAN PLANT BREEDING AND BIOTECHNOLOGY PROVIDE A FRUITFUL MARRIAGE?

RIBAUT J.-M.

CIMMYT, Int. Maize and Wheat Improvement Center, Lisboa 27, Apdo. Postal 6-641, 06600 Mexico D. F. Mexico, Phone: 52 595 95 219 00 (Ext: 1396), Fax: 52 595 95 219 83

j.ribaut@cgiar.org

The application of molecular markers to plant breeding can be divided into three main categories: (1) the characterization of germplasm, known as fingerprinting; (2) the genetic dissection of the target trait, actually the identification and characterization of genomic regions involved in the expression of the target trait; and (3) following the identification of the genomic regions of interest, crop improvement through marker-assisted selection (MAS). The first two applications have proven themselves by generating knowledge about the genetic diversity of germplasm, thereby allowing placement into heterotic groups and a better understanding of the genetic basis of agronomic traits of interest. For simply inherited traits - those that have high heritability and are regulated by only a few genes - the use of molecular markers to accelerate germplasm improvement has been well documented. Such work has proven successful by (1) tracing favorable alleles in the genomic background of genotypes of interest and (2) identifying individual plants in large segregating populations that carry the favorable alleles. In the case of line conversion - the transfer of elite allele at one or several loci from a donor to a recipient line through backcrosses - results of simulation conducted at CIMMYT demonstrate the importance of the selectable population size. It is essentially the number of genotypes heterozygous at the target loci involved into the selection and this parameter is the first one to be considered when designing a MAS experiment. For concomitant allelic introgression from a donor line into a large number of recipient lines, MAS at unselected loci conducted only once at an advanced BC cycle, while MAS at selected loci is conducted at each cycle, appears as most efficient. To illustrate the use of molecular marker in plant breeding for simple inherited traits, results for QPM (cloned gene), and MSV resistance (major QTL) will be presented as case studies of MAS experiments conducted at CIMMYT.

MAS for polygenic trait improvement is still in an exploratory phase, with only a few successful experiments. We would like to report here the effort conducted at CIMMYT to develop efficient MAS strategies for maize improvement under water-limited conditions. Because of its genetic complexity, drought tolerance is probably the most difficult trait to improve through conventional plant breeding. The challenge is even greater for developing drought tolerant plants for water-limited environments where the occurrence, timing, and severity of drought may fluctuate from year to year. The development of drought tolerant germplasm is a first priority at CIMMYT as water-limited conditions represent for maize the first cause of grain lost in developing countries. CIMMYT’s work on drought tolerant maize spans three decades, with biotechnological approaches being utilized during the last ten years. The focus of this approach has been the genetic dissection of drought tolerance at flowering, by collecting morphological data and identifying QTLs for yield components, secondary morphological traits of interest, and more recently, physiological parameters. Within the last two years we have initiated new activities based on functional genomics to screen segregating material to broadly identify significant differences in gene expression that are involved in the response of tropical maize at different periods of water stress, and in different target tissues.

To date, genetic dissection has been conducted in four different crosses, at different inbreeding levels (hybrids, F₂:F₃ families, and recombinant inbred lines (RILs)), under different water regimes (well-watered, intermediate stress, and severe stress conditions) and in several different environments (Kenya, Mexico, and Zimbabwe). As all morphological traits studied are complex and regulated by several genes, with yield being among the most polygenic and complex traits, no major QTL (more than 25% of the phenotypic variance) has been identified. The majority of single QTL expressed 3–10% of the phenotypic variance and total phenotypic variance expressed by combining all of the significant QTLs was generally 30 to 40%, and never surpassed 60 %. Due to the challenges of accurately measuring physiological parameters in field conditions, QTLs for physiological parameters are generally less significant compared to those obtained for morphological traits, and the total phenotypic variance is rarely greater than 25% for any given parameter. Even so, we consider this QTL information extremely valuable. It provides an essential bridge between the data emerging from functional genomics and morphological plant responses, which will allow us to identify and characterize the major pathways related to drought response and tolerance in tropical maize.

Functional genomics experiments can generate a large amount of data in a short period of time. The greatest challenge is how to organize and interpret these data to identify the most informative changes in gene expression, and evaluate their associated phenotypes. This requires more than simply identifying genes that change their expression under stress conditions. To provide a biological framework for the interpretation of changes in expression for genes involved in the regulation of target traits/pathways, the evaluation of a new set of morphological traits and physiological parameters have been initiated to correlate target key traits, and identify their corresponding QTLs. For all of these parameters we are generating a broad dataset. This includes phenotypic measurements (correlations and heritability), QTL characterization and changes in gene function for the key genes involved in the regulation of those pathways. By combining this information, and because the QTL location can be used as a validation tool for the candidate genes when they map at the same genomic regions, we should be able to confirm the involvement of those genes contributing to the drought tolerance process.

Other than generating information and knowledge on drought tolerance, one objective is to conduct a MAS experiment based on “universal drought genomic regions” identified on a maize consensus map. This map will incorporate (1) QTL information for yield components, morphological traits and physiological parameters; (2) candidate genes position; and (3) the corresponding information provided by gene expression studies. The underlying rationale for this approach is that genes involved in drought response are most likely located at the same position in the maize genome, independent of the germplasm performance, and that phenotypic differences across germplasm is created by the nature/quality of the alleles at those genes. To achieve this objective, a unique linkage map has been constructed using a set of anchor markers common to the different segregating populations, to identify genomic regions involved in the expression of the same trait (different crosses or environments) or different target traits (same cross and/or different crosses or environments). Following the approach described above, and combining this with data previously published by other groups, some key regions for drought tolerance in maize have already clearly been identified and will be presented. The MAS strategy for new crosses (good by good lines with different genetic background) based on those “universal” drought genomic regions, and without having to conduct a QTL study in each target cross, will be discussed.

Based on progress to date, it is clear that a multidisciplinary approach combining breeding, physiology, and biotechnology is required for an effective understanding of a plant’s response under target environment. Strategy development for polygenic trait improvement through molecular markers is a very dynamic area of investigation, because optimal strategies evolve together with the genetic information provided by ongoing and emerging technology. Considering the new type of information provided at the gene expression level, it is also time to think about new conventional breeding strategies to better complement molecular and conventional approaches. The emergence of molecular genetics and associated technologies represents a major new breeding tool; the current challenge is to integrate this tool and the information it generates into breeding schemes to further the development of efficient MAS strategies for polygenic traits.