Agriculture Reference
In-Depth Information
distribution in the F
2
. In this case, single-seed
descent is done strictly without selection.
As mentioned earlier, most wheat breeders
would like to know the mean and standard devia-
tion of the lines derived by their breeding popula-
tions, so they can concentrate on working in the
best populations and quickly discard those with
less potential. Breeders also want to know which
segregating lines within a population have the
greater potential to develop superior inbred lines.
Thus the mean and standard deviation both
among populations and among segregating lines
within a population are important. In single-seed
descent breeding, as well as the doubled haploid
breeding method described next, wheat breeders
can easily estimate both the population mean and
standard deviation among derived lines. However,
in practice, this is rarely done because once the
homozygous lines are developed, new crosses
have been made, and it is diffi cult to return to the
better cross to generate additional homozygous
lines. Rather plant breeders view their lines as a
fi nished product of the cross or population and
move on to other crosses and populations.
Wróbelska and Szarejko 2003)—it is important
to know if the lines developed by both
doubled haploid breeding systems are equivalent.
In their work, Guzy-Wróbelska and Szarejko
(2003) compared in single rows the yields of
anther-culture-derived doubled haploids, maize-
pollination-derived doubled haploids, and single-
seed descent lines and found the lines developed
by each method to be similar or equivalent.
In larger plots they compared the anther-culture-
derived doubled haploids to maize-pollination-
derived doubled haploids and again found them
to be equivalent. Additional testing in more envi-
ronments may identify small differences, but at
this time, both methods create lines with similar
agronomic value. Considering the top 10% of the
created single-seed descent and doubled haploid
lines (a method to remove any lines that might be
poor performers due to gametoclonal variation),
again no differences were detected, indicating all
three methods develop lines useful for plant
breeders.
However, in a recent study, Guzy-Wróbelska
et al. (2007) found that the recombination fre-
quency in anther-culture-derived doubled hap-
loids was much higher than in the maize-derived
doubled haploids, as determined by the genetic
map developed from the anther-culture doubled
haploids being 41% larger than the map devel-
oped from the maize-derived doubled haploids.
They did not compare the recombination fre-
quency in the two doubled haploid methods to
the single-seed descent lines, so it is unknown
which recombination frequency is closer to the
single-seed descent process used to create mapping
populations. While the recombination frequency
was different, the gene order was identical with
one exception.
Double haploidy is an expensive method but
requires the least amount of time to develop
inbred lines, especially when breeding winter
wheat, where the vernalization requirement slows
single-seed descent breeding. Doubled haploidy
and single-seed descent have the same end point:
production of a random set of inbred lines for
subsequent assessment. Both doubled haploid
and single-seed descent methods have a lower
desirable genotype quotient compared with lines
Doubled haploid breeding
Doubled haploid technology generates homozy-
gous lines from haploid tissue. The method
involves production of plants from haploid tissue
and doubling the chromosomes (Guzy-Wróbelska
and Szarejko 2003). The resultant plant will
be completely homozygous and homogeneous.
Haploid tissue may be produced by chromosomal
elimination in wide crosses or by the direct use of
pollen grains, microspores, or ovules as haploid
tissue (Schaeffer et al., 1979; Laurie and Bennett
1988; Laurie and Reymondie 1991). The chromo-
some numbers of the haploid plant are doubled
by application of the alkaloid colchicine, which
interferes with microtubular activity. Embryo
rescue methods may be used to propagate haploid
tissue, especially in the case of chromosome elimi-
nation in wide crosses where the endosperm does
not form. As two predominant methods are avail-
able to create doubled haploids—the wheat-by-
maize system (Laurie and Bennett 1988) and the
anther culture or microspore system (Guzy-
