285markers that have been developed and are being used in plants include restriction
fragment length polymorphisms (RFLPs), amplified fragment length polymor-
phisms (AFLPs), randomly amplified polymorphic DNAs (RAPDs), sequence-
tagged sites (STS), expressed sequence tags (ESTs), simple sequence repeats
(SSRs) or microsatellites, sequence-characterized amplified regions (SCARs), and
single nucleotide polymorphisms (SNPs) (Paterson et al. 1991 ; Hoisington et al.
1998 ; Joshi et al. 1999 ). Molecular markers have also proved to be useful for gene
pyramiding (Castro et al. 2003 ). The increased use of crop landraces and wild rela-
tives in modern breeding programs cannot be dissociated from the advances in
molecular genetics.
Restriction fragment length polymorphisms (RFLP) and Amplified Fragment
Length Polymorphism (AFLP) markers have been the basis for most work in crop
plants a decade ago. But with the development of Polymerase Chain Reaction
(PCR) based markers a major step forward in the study of the wheat genome has
been seen, by providing the tools needed to generate essential knowledge and data
in the areas of fingerprinting and genetic conservation (Bennett and Smith 1976 ).
Valuable markers have been generated from the Simple Sequence Repeats (SSR)
or microsatellite markers have been developed in the near past for most of the major
crop plants and it is still widely implemented in many breeding programs. The new
platforms for genomic sequencing of have greatly reduced the price and time and
increased the resolution as compared to the previous AFLP/RFLP based systems.
Thus DArT array technologies, SNP platforms or genotype by sequencing (GBS)
allows breeders to more easily and cheaply identify new markers and assess wild
genome introgressions fastening/accelerating the introduction of new alleles from
wild relatives and decreasing the non-desired drag linkage effect. While expecting
the results of the wheat genome sequencing, several consensus maps have been
developed. In 2004, the union of 4 genetic maps developed out of SSR markers
resulted in a more than 1200 loci consensus map (Somers et al. 2004 ). Plant breed-
ers are now taking advantages of the molecular technologies, which are somehow
speeding up the breeding program.
The use of molecular markers in plant breeding programs is relatively recent and
its full potential is still being studied and developed. In previous Marker Assisted
breeding approaches, the markers used were usually linked to or targeted genes of
qualitative or major effect over traits of agronomical or end-use interest. However,
these techniques often neglect epistatic effects and small effect genes (Lee et al.
2002). Instead, high-throughput genotyping or genome-wide molecular markers
may allow breeders to select based on full genome information, leading to a true
genomic selection. Genomic selection will improve breeder’s predictions on pheno-
typic behavior of new breeding lines according to the genetic information obtained
from genome-wide molecular markers (Meuwissen et al. 2001 ). Efforts are being
made to exploit traits that are expected to play an important role in drought avoid-
ance under receding soil moisture conditions by improving water availability to the
plant through more efficient extraction of available soil moisture.
Drought tolerance is a complex phenomenon involving many known and
unknown pathways. To improve drought tolerance, QTLs have been identified for
Breeding and Genetic Enhancement of Dryland Crops