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Marker-assisted
selection for maize improvement: practical examples and strategies
J.M. Ribaut,
M. Banziger, K. Pixley & D. Hoisington
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 their value 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. Given the results already produced in maize at the molecular level,
including QTL analysis, gene cloning, expressed sequence tag (EST)
identification, and the development of new molecular applications, MAS
for maize improvement is becoming more and more efficient. 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 alleles at one or several loci from a donor to a
recipient line through backcrosses)- results of simulations conducted at
CIMMYT demonstrate the importance of the selectable population size.
Population size is essentially dependent on 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 is conducted
only once at an advanced BC cycle, while MAS at selected loci is
conducted at each cycle. MAS
for polygenic trait improvement is still in an exploratory phase, with
only a few successful experiments reported. After crossing elite
material for a target environment (good by good), innovative experiments
considering MAS at the early stage of recombination and for one or a few
cycles are ongoing at CIMMYT. 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.
To illustrate our ideas on MAS strategies, results for QPM (cloned
gene), MSV resistance (major QTL) and drought tolerance (polygenic
trait) will be presented in details as case studies of MAS experiments
conducted at CIMMYT.
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