Molecular Breeding - Genetics, Genomics and Breeding of Crop Plants - page 241

7.5.3 MAS in Sunflower Breeding Programs
7.5.3.1. Marker-assisted Gene Introgression
One of the most common applications of MAS is marker-assisted/accelerated
backcross breeding for gene introgression. Optimally, this is based on positive
foreground selection for a donor trait, positive background selection for the
recurrent parent genome, and negative background selection against
undesirable donor parent alleles (Frisch et al. 1999). In general, marker
assistance is expected to provide greater selection efficiency, and/or to
shorten the time taken for the backcross breeding scheme compared to
conventional methods. Different factors such as the nature of the target
locus (specific locus vs. QTL, dominant vs. recessive) and the number, nature,
and distance of the markers associated to it should be taken into account in
order to optimize the design of the marker-assisted backcross program (Frisch
2005). However, economics is usually the key determinant for the application
of molecular genetics in gene introgression programs. Morris et al. (2003)
compared the costs of marker-assisted and traditional backcrossing of a
single major gene into an elite line and found, as expected, that MAS was
faster but more costly than traditional selection. These authors concluded
that the cost-effectiveness of DNA markers is dependant on four critical
parameters: (1) the relative cost of phenotyping versus genotyping screening,
(2) the time savings achieved using MAS, (3) the size and temporal
distribution of benefits associated with accelerated release of improved
germplasm, and (4) the availability of operating capital to the breeding
program. The latter point explains to a large extent why private industry
has adopted MAS gene introgression more rapidly than public sector
programs.
In fact, marker-assisted backcross breeding in sunflower is mainly being
carried out in private companies to accelerate the introgression of target
genes into elite germplasm. As stated in their websites, traits such as downy
mildew resistance, high oleic acid content in the seed oil, and herbicide
resistance are currently the main targets, although complex traits such as
resistance to Sclerotinia , Phoma and Phomopsis are also mentioned. Despite
the absence of publications from these programs, it seems that, in general,
markers are being routinely used to select for alleles with large effects on
traits of a relatively simple inheritance. However, dissection of complex
traits into their components is contributing to the implementation of marker-
assisted backcross programs for such characters. For oil content, different
QTLs underlying its components (percentage of kernel and kernel oil
concentration) have been identified, and some of them co-located with the
phenotypic loci B , hyp and P (Leon et al. 1996, 2003; Tang et al. 2006a). This
fact was explained by Tang et al. (2006a) as a pleiotropic effect of such