Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 77

The first interest of a molecular map, with a quite complete cover of
linkage groups, was that the major genes could be located on it. Thus relations
between quite different characters could easily be studied, and breeding
programs were planned according to the position of genes on the same or
different linkage groups. The first sunflower molecular maps became
available in 1995 and in the 12 years since then, many characters have been
mapped and breeders have got used to thinking at least in terms of “beads
on strings”, for example Pl2 and Pl6 downy mildew resistance genes are
known to be in the same cluster (Mestries 1998) and so not easy to recombine,
whereas broomrape resistance genes are clustered on a different linkage
group (Gagne 2000) and so it is no problem to combine the two resistances.
Probably the greatest use to the breeder has been the definition of
quantitative trait loci (QTL) for quantitative characters, since this makes it
possible to transform statistics into “genes” that can be positioned along
linkage groups almost as if they were major genes. It is then possible to infer
that different lines have the same or different genes controlling a particular
character and whether it will be possible to combine them and thus obtain
a higher level of the character concerned, be it yield, earliness or quantitative
disease resistance.
Even more important, positioning of QTLs on linkage maps makes it
possible to compare the positions of genes (or QTLs) for different characters.
It has become possible to distinguish between real pleiotropy and linkage
between characters due to effects of origin. For example, the recessive
branching gene b1 (Putt 1964) has a pleotropic effect on seed size and oil
content, whereas the linkage between Sclerotinia resistance and oil content
observed in the first studies on this disease resulted from the genetic origin
of resistance. Favorable linkages have also become evident: a Sclerotinia
resistance QTL linked to a protene-kinase gene (Gentzbittel et al. 1998) has
been shown to be involved also in resistance to other diseases such as Phoma ,
so the lines carrying the favorable allele at this QTL are particularly
interesting to use in breeding (Bert et al. 2004). Of course, the efficiency of
conclusions drawn from QTL positions depends on the closeness of
molecular markers, but with fine mapping it will eventually be possible to
determine the genes truly controlling facets of important characters.
Comparison with genes whose effects are known in other species should
help breeding for characters such as drought resistance, where so far little
progress has been made because the overall phenotypic effect is difficult to
measure. Perhaps, one day, breeding will be for a genotype that will have a
predictable mean phenotype rather than depending on a large number of
phenotypic observations to estimate this value.