Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 232

sources. Köhler et al. (1991) and Laver et al. (1991) suggested that a new
open reading frame, orfH522 , in the 3'-flanking region of the mitochondrial
(mt) atpA gene was associated with the CMS phenotype. It was later
demonstrated that the gene product of orfH522 , a 16-kDa protein, differed
between the male fertile and male sterile lines (Horn et al. 1991; Monéger et
al. 1994). Further diversity studies of 28 male sterility-inducing cytoplasms
and one male fertile cytoplasm of Helianthus using nine mtDNA genes and
three probes for the open reading frame clearly distinguished CMS sources
by their mtDNA organization and CMS mechanism (Horn 2002).
As mentioned above, one of the most important milestones in sunflower
breeding has been the development of hybrid cultivars, which benefit from
hybrid vigor or heterosis. Genetic distance has been widely studied as
predictor of heterosis and hybrid performance in crop plants. In sunflower,
Tersac et al. (1994) estimated genetic distance from isozyme data and found
that it was not correlated to heterosis. Cheres et al. (2000), using AFLPs and
co-ancestries, found that genetic distance alone was a weak predictor of
hybrid performance in sunflower. As reported in other crops, such as soybean
or maize, genetic distance and F 1 heterozygosity estimated from randomly
selected markers are typically not correlated with heterosis (Melchinger et
al. 1990; Gizlice et al. 1993). Conversely, approaches based on screening for
heterosis-related markers (Melchinger et al. 1990), using specific
heterozygosity (Zhang et al. 1994), and identifying favorable combinations
of allele and heterotic patterns (Liu and Wu 1998) could be exploited to
improve this prediction.
7.4 Molecular Strategies to Create and Identify Novel
Variation
Natural biodiversity is an under-exploited sustainable source that can enrich
the genetic pool of cultivated sunflower with novel alleles and molecular
approaches for exploiting this natural diversity are currently being explored
in sunflower. The use of association-based mapping techniques for
the discovery of genes and novel alleles controlling complex traits across
a broad spectrum of germplasm is being considered. Association mapping,
also known as linkage disequilibrium (LD) mapping, is a method that relies
on LD to study the relationship between phenotypic variation and genetic
polymorphism (Flint-Garcia et al. 2003). Basically it utilizes ancestral
recombination events in natural populations, evaluating whether certain
alleles are found with specific phenotypes more frequently than expected
and so does not require any special population development for
trait mapping. The extent of LD is the main factor determining resolution
of association mapping, and it is affected by natural selection,
domestication and breeding history, mating systems, mutation, migration,