II.3. BRASSICA CROPS (BRASSICA SPP.) – 237
production of hybrid vegetables. Some cauliflower varieties are exceptions, being natural
self-pollinators that do not exhibit the usual vigour and fertility losses.
Single seed descent
As with the pedigree method, the first step in single seed descent (SSD) breeding is
the careful choice of parents for hybridisation. However, unlike the pedigree method,
selection is not practiced until a high degree of homozygosity is reached. The object is to
advance generations as rapidly as possible and subsequently select among the randomly
derived lines. The size of the segregating population is kept at a manageable level by
planting only one randomly chosen single seed from each plant in the previous
generation.
Since the degree of homozygosity is not as critical in B. napus and B. juncea as it is in
cereals, this method has not been widely used in Brassica breeding programmes.
Backcross method
The backcross method is designed to introduce one or more specific trait(s) into an
otherwise highly desirable parent or variety. The donor parent, containing the trait(s) to
be incorporated, is crossed onto plants of an adapted, desirable, recurrent parent.
Depending on the inheritance of the trait(s) and the ease or efficiency of selection, the F 1
or selected BCF 1 plants will be backcrossed to the recurrent parent. By the fourth to sixth
backcross, the genetic makeup of the recurrent parent is expected to have been
reconstituted with the new trait incorporated. However, linkage between the desirable
trait and one or more undesirable characteristics may require selection within large
populations to identify plants or lines with an uncoupled linkage.
Frequently in the self-pollinating species, only one or two backcrosses are made
followed by pedigree selection.
Figure 3.42 illustrates the combined use of the backcross and pedigree methods.
In the self-incompatible species, backcrossing can also be effective for the
incorporation of specific traits. However, crosses in oilseed B. rapa need to be made with
sufficient numbers of recurrent parent plants to ensure that heterozygosity of the
self-incompability alleles of the recurrent parent is maintained in the backcross
generations. To overcome this potential problem, the “recurrent selection” breeding
system is widely used.
Backcrossing is also effective in the self-incompatible vegetable species. Dickson and
Wallace (1986) outline a complete backcross breeding programme for cabbage
improvement.