Maize Starch for Industrial Applications - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 181

elimination of trait-negative plants at the seedling stage, and less contamination at
the grain production stage would be expected.
Neither transgenic waxy nor amylose have yet been commercialized in corn.
Waxy has been shown to be an improved source material for ethanol production
[ 27 ]. It is ethanol or other large-scale industrial markets that would most likely
provide the demand for a genetically modified waxy or amylose. Aside from
approaches to use genetic transformation, and with attention to the dual food/
industrial use required of specialty hybrids, several interesting breeding strategies
have been and continue to be applied.

Advantages of Marker-Assisted Selection

Marker-assisted selection (MAS) is a very valuable breeding tool to accelerate
backcrossing (Table 9.3 ). The most rapid conversions to the elite are captured in the
BC1-BC3 generations. Use of the marker-assisted backcross is somewhat compli-
cated by recessive genetics, as the recessive gene, if not fixed prior to marker
analysis, will reduce the number of useful selections by half, and two recessive
genes will reduce selections to 25 %.
Greenhouse or continuous nursery production, especially in combination with
MAS, can fully convert an inbred in less than 2 years. Seed increase, yield testing,
and functionality qualification however add significant time to specialty inbred
release schedules.
The major advantage for the use of MAS for waxy is that it can be used to reduce
breeding generations and field pollinations. The waxy trait is a phenotype that is
easy to distinguish visually and is a binary phenotype, producing either 100 %
amylopectin or wild type. Traditional backcrossing is an effective process to
convert elite regular corn to the waxy specialty, and using MAS the process can
be accelerated.
The use of MAS for the development of amylose inbred lines is less straightfor-
ward. There is at least one major modifier gene required to produce inbreds with
greater than 70 % amylose [ 66 ]. Additional genetics require additional costs when
either traditional or MAS breeding methods are employed. The breeding dynamics

Table 9.3 Comparison of backcrossing efficiency using marker-assisted selection
Breeding
generation

Average elite contribution
(%)

Potential elite contribution using markers
[ 65 ]

F1 50 Markers not useful

BC1 75 83.8 %

BC2 87.5 91.2 %

BC3 93.75 97.3 %

BC4 96.88 Markers effective but small differences

BC5 98.44