Agriculture Reference
In-Depth Information
DNA fi ngerprinting, private enterprise has also
evolved to contribute to wheat improvement in
these same countries. More recently the concept
of participatory involvement in cultivar improve-
ment has been promoted in some countries which
lack strong national agricultural research service
(Ceccarelli et al., 2000). Segregating populations
from either simple or complex crosses are made
available to farmers who then select within the
population for strains that best fi t their environ-
ment and combination of biotic and abiotic
stresses.
Wheat cultivars released in the later half of the
20th century and in the 21st century are primarily
purelines, with relatively few hybrids and multi-
line cultivars. At this time there are neither apo-
mictic-derived cultivars nor clonally derived
cultivars. The importance of apomictic or clonally
derived cultivars will increase if hybrid wheat
becomes important, as both methods are used to
preserve or perpetuate genetically unstable lines
(e.g., lines that cannot be maintained by selfi ng,
which is the normal form of reproduction in
wheat; Flament et al., 2001). Apomictic and clonal
propagation of a pureline results in the same
progeny as would selfi ng, so apomictic or clonal
propagation of a pureline cultivar offers no
advantage. However, if apomictic propagation
were successful, it would certainly spur research
in hybrid wheat.
The two most important aspects of a segregat-
ing population are its mean and standard devia-
tion—in this case the standard deviation among
lines derived from the population. By knowing
these two aspects, the breeder can use statistical
estimations to predict which populations have
highest probability of producing the best lines.
For example, if a breeder had to choose
between two populations, both with a mean of
4,000 kg ha −1 for grain yield and a standard devia-
tion among the lines of 200 kg ha −1 (population I)
or 400 kg ha −1 (population II), the breeder would
be more interested in population II because the
chance of developing a higher yielding line (e.g.,
4,400 kg ha −1 ) would be greater. Of course, most
breeders are confronted with populations having
different mean grain yield and standard deviation
(e.g., 4,000 kg ha −1 with a standard deviation of
200 kg ha −1 vs. a population with a mean grain
yield of 3,850 kg ha −1 and a standard deviation of
425 kg ha −1 ) or with many breeding methods
having very poor estimates of grain yield and no
estimate of variation.
As virtually every breeding program has
improving grain yield as its goal, this trait will be
used as an example throughout the rest of the
chapter. The other common goal is maintaining
or improving end-use quality, though feed wheat
and wheat forage systems also exist. Other exam-
ples of breeding objectives (e.g., disease or pest
resistance, tolerance to abiotic stresses) will be
used to highlight different aspects of wheat breed-
ing as appropriate.
THE CONTEXT OF APPLIED
WHEAT BREEDING
ACCESSING GENETIC RESOURCES
In the following sections, we will describe various
currently used and projected breeding methods to
develop improved wheat cultivars. In comparing
strengths and weaknesses of each method, it
should be understood that the fi rst consideration
is whether the breeding method effi ciently meets
the breeding objective. In fact it is impossible to
compare breeding methods without fi rst describ-
ing the breeding objective. The second consider-
ation is that every breeder is looking for the rare
exception that becomes a cultivar. If developing
cultivars were a common occurrence, there would
be far more of them.
Food production and security depend on the wise
use and conservation of agricultural biodiversity
and genetic resources (Esquinas - Alcázar 2005).
Crops and their wild relatives comprise the plant
genetic resources for food and agriculture
(PGRFA), or the genetic variability that provides
the raw material for breeding new crop cultivars
through classical breeding and biotechnological
techniques. The center of origin of wheat is the
Middle East. Today wheat is grown from low
latitude to high latitude and from low altitude to
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