Collector's Edition WINES&VINES 143
WINE EAST GRAPEGROWING
pensive but is now very affordable at a total
cost of $3,000, or less than a penny per marker.
The map of each chromosome is very much
like a roadmap with mile markers that show
the distance between locations. Once the map-
ping population and genetic mileposts are in
place, researchers then measure the desired
trait or phenotype (in this case, powdery mil-
dew or downy mildew resistance) somehow
and associate the trait with specific markers
on the genetic map.
From then on, it's a matter of statistics.
Researchers use statistical methods, most com-
monly a technique called quantitative trait loci
(QTL) analysis, to associate phenotypes with
DNA markers. These markers — once validated
— can be used by grape breeders for marker-
assisted selection of the best vines.
Phenotyping
Obtaining the DNA markers from the 200 prog-
eny — once inconceivably difficult — is now
easy and routine. The real bottleneck in the
process is measuring the desired traits and how
they vary (i.e., phenotyping).
Consider powdery mildew, for example. The
challenge in phenotyping powdery mildew
resistance is that field populations present no-
toriously variable environmental conditions,
even within a small planting. To address this
AMERICAN
PHYTOPATHOLOGICAL
SOCIETY,
2016
DOI:
10.1094/PHYTO-02-16-0080-FI
A leaf disk sample susceptible to powdery mildew (A,C) compared to a resistant sample (B,D)
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Susceptible Resistant (but many trichomes)
1a
2a
c
A B
C D
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FIGURE 2:
