Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 245

each using a different LG nomenclature system (Berry et al. 1995; Gentzbittel
et al. 1995; Jan et al. 1998). Fortunately, for both the sake of public research
and ease of use, these early maps have been superseded by one based on
SSR markers. The vast majority of these microsatellites are in the public
domain thanks to the efforts of Prof. Steven Knapp (The University of Georgia,
Athens, USA) and co-workers, but this may not be the situation for the next
generation of SNP-based markers. However, in the short to medium term
SSRs will remain the marker system of choice in commercial MAS programs.
In most breeding companies, sunflower is regarded as relatively
unimportant when compared to crops such as maize, but unlike maize
there are many important traits in sunflower that are controlled by single,
major genes. Most breeding companies are able to deploy any published
marker-trait linkages extremely rapidly in their own breeding programs. As
discussed earlier in this chapter, many disease resistance genes have been
mapped and since sunflower is very prone to fungal attack, the manipulation
of these resistances via MAS is seen as essential in providing hybrid yield
stability. For example, the marker-assisted introgression of Verticillium dahliae
resistance allowed the yield testing of the iso-hybrids with and without the
resistance QTL. The resistant iso-hybrid out-yielded the susceptible control
by 54% for seed yield and 65% for oil yield under Verticillium attack (Alberto
Leon pers. comm.). In some environments certain resistance genes, such as
those controlling resistance to the parasitic weed Orobanche cumana in Spain
and Turkey or resistance to sunflower rust ( Puccinia helianthi ) in Australia,
are a prerequisite for commercialization. Unfortunately, pathogen
populations can rapidly evolve to overcome new sources of single gene
resistance and so gene pyramiding using markers is essential to provide
some durability. We are beginning to understand the linkage arrangement
of resistance gene clusters in the sunflower genome; however, they are often
located in regions of high recombination (e.g., near the telomeres or in gaps
in the genetic map), which makes the identification of tightly-linked flanking
markers more problematic. A further difficulty is that with the current marker
resolution it is impossible to select for recombinants within these resistance
gene clusters.
Wild, diploid annual species have mainly been used as a source of
novel resistance genes for cultivated sunflower, but often little is known
about their genome organization and so gene transfers may be unstable due
to differences in chromosome structure (i.e., translocations). A key
consideration when screening accessions for novel resistance genes is
whether the resistance is dominant, additive or recessive. Recessive or
additive resistances will require the marker-assisted introgression of the
gene or genes into both the male and female parent of a given hybrid, thus
doubling the amount of marker work. That said it is now possible, with the
use of flanking markers, to manipulate recessive genes very efficiently in