several techniques by which this phenomenon can be induced. The most common tech-
nique is through artificial culture of the male gamete (microspore) or the tissue containing
those gametes (anthers). By culturing those tissues on a growth medium, we cause the
haploid cells to undergo mitotic divisions. At some stage, a natural doubling may take
place when mitotic nuclei fail to divide into separate cells. This can also be induced by
the addition of a chemical calledcolchicinethat interferes with normal cell division. The
culture can then be forced to regenerate into a normal diploid plant (see Chapter 5).
Techniques to produce doubled haploids have been developed in many crop species, and
the technique is used routinely in species such as wheat and barley. While the cost of pro-
ducing doubled haploids is often greater than that of producing SSD lines, the ability to
accelerate the breeding program and get to market faster with a new variety is often
worth the added cost.
3.5.2 Marker-Assisted Selection
Earlier it was mentioned that individual genes contributing to complex plant traits can
sometimes be discovered through their association with genetic markers. This procedure,
calledquantitative trait locus(QTL)analysis, provides the foundation for a more efficient
type of genetic selection called marker-assisted selection (MAS) (Fig. 3.16). Rather than
selecting traits, which are the outcome of many genes, MAS is based on selecting specific
alleles at marker loci that are known to be linked to the genes that cause the desired trait.
The theoretical advantages of MAS are that it (1) avoids errors caused by environmental
variance; (2) can be applied at a juvenile stage before a trait is expressed; (3) can be
applied on a single plant, whereas phenotypic selection of some traits might require seed
Figure 3.16.A simplified strategy for marker-assisted selection (MAS). Here, a significant associ-
ation between a QTL (Q) and a molecular marker allele (M) is identified in an experimental popu-
lation. This information is applied in future populations in order to select Q indirectly through its
linkage to M.
3.5. BREEDING ENHANCEMENTS 75