Barley for Food and Health: Science, Technology, and Products

IMPROVING THE BARLEY CROP 35
breeding is not unlike animal breeding in many respects; there is an old saying
that “the eye of the master fattens the cattle,” which in essence describes
the barley breeder’s intuition for choosing the genetic backgrounds and appropriate
breeding methods best suited to select and assemble the best genes in a
new plant.
Crossing cultivars of different genetic backgrounds is perhaps the most basic
method in barley breeding. As barley is a self-fertilizing plant, artificial crosses
are required to produce recombinant plants (Wiebe 1978). Controlled crossing
requires basic knowledge of plant morphology and the ability to recognize the
progression of events from early floral development through pollination, a degree
of physical skill necessary to dissect tender plant tissues without destroying
them, basic knowledge of maturity characteristics of the parents that are to be
hybridized, and patience. One difficulty that has to be overcome in many instances
is the difference in pollen-maturity dates (days to anthesis or pollen shedding) that
may exist between proposed parents. As barley is a self-pollinating plant, emasculation
of the female parent is required either surgically, which is time consuming,
or by using male sterile plants. A number of genes producing male sterility in
barley have been documented (Hockett and Eslick 1968). Cross-pollination can
be accomplished by a number of methods, some requiring bagging the female
spike to prevent accidental pollination by foreign pollen.
It is generally agreed by breeders that crossing barley plants that are known
to be high yielding will produce high-yielding crosses with low genetic variance
in most instances. Intercrossing plants with a restricted range of parental lines
can reduce the number of gene pairs segregated, thus preserving previous genetic
advances while providing a reasonable chance of improving specific traits (Eslick
and Hockett 1979). The disadvantage of this approach to improving various
traits is that it leads to a restricted gene pool (Anderson and Reinbergs 1985).
Despite such theoretical and demonstrated losses in genetic diversity that are the
consequence of limited parental selection, decades of selection and restriction
have nevertheless not prevented continued gains from selection (Rasmusson and
Phillips 1997; Condon et al. 2008).
Backcrossing, a technique first outlined by Harlan and Pope (1922), has been
used extensively over the years in many breeding programs. It is well suited for
transferring simply inherited characters controlled by one or two major genes
(Wiebe 1978) such as the nud (hulless) gene. Backcrossing involves repeated
backcrossing to one recurrent parent after an initial cross to another (donor) parent
containing the simply inherited (one- or two-gene) trait of interest, the aim
being to recover only the donor parent trait in a genome that becomes increasingly
similar to the recurrent parent with each successive backcross. Taken to the
extreme, backcrossing creates a nearly isogenic line, identical to the recurrent
parent in all aspects except for the desired donor-parent characteristic. In practical
terms, the number of backcrosses that can be performed is limited by time and
resources, and it is generally recognized that isogenic pairs differ by small gene
blocks rather than a single gene (Wiebe 1978) and lines are sometimes referred
to as isotypes rather than isogenic pairs. Backcrossing is widely employed for