II.3. BRASSICA CROPS (BRASSICA SPP.) – 245
Biotechnology in Brassica breeding
Introduction
Although the above breeding procedures have been very effective in combining
important agronomic and nutritional traits in superior cultivars, the process of identifying
the desired genotype in genetically stable, uniform and high-yielding varieties takes many
years. Further, the small chromosome size plus their lack of distinctive features have been
an additional limitation on the selection of superior genotypes. However, beginning in the
mid- to late 1980s, developments in tissue culture, embryo rescue, cell fusion, molecular
markers and genetic mapping have not only reduced the time from cross to market but
have given breeders powerful tools to quickly identify and assemble desirable traits in a
single genotype. In addition, these biotech tools have greatly expanded the size and
variation of the available gene pool, well beyond species boundaries.
Doubled haploid breeding
The doubled haploid (DH) breeding technique is now widely used in B. napus and
B. juncea breeding programmes (Ferrie and Keller, 2004). This breeding tool not only
eliminates the several generations needed to attain genetic stability and uniformity in
breeding lines, but also significantly reduces the size of populations needed to find a
desired genotype. For example, in B. napus, two genes code for the level of the fatty acid
erucic in the seed oil, and an additional six genes code for the content of glucosinolates in
the seed. Thus, when making a high by low cross, to produce progeny that have both low
erucic acid and low glucosinolate (double low or canola quality), large segregating
populations must be examined since the desired genotype must have all eight genes in the
recessive state.
Table 3.18 illustrates the DH technique’s increased selection efficiency, particularly
when the selected plants are completely homozygous individuals that can be used directly
as pure breeding varieties or as hybrid parents.
95% probability Table 3.18. Minimum population size required to select the least frequent homozygote at
at 95% probabilityNumber. of genes Diploid Haploid Minimum F^2 population
1 11 5
2 47 11
4 766 47
5 3 067 95
6 12 269 191
7 49 077 382
8 196 259 766
10 3 123 923 3 067
Source: Rajhathy (1976).The technique involves inducing large numbers of immature pollen grains
(microspores) from Brassica species to develop into plants with the gametic or half the
somatic chromosome number. Such plants are termed haploids and are sterile.
By applying colchicine to the developing haploid plant, cell division is temporarily
arrested, bringing about chromosome duplication. The result is a doubled haploid (DH) or