240 – II.3. BRASSICA CROPS (BRASSICA SPP.)
However, within a small population of potential elite parents, diallel crossing,
i.e. hand crossing each parent with all other potential parents, followed by progeny
assessment, can eliminate the heterozygous parent(s). Although labour intensive, this
technique is suitable for most vegetable Brassicas because individual plants (parents) can
be vegetatively maintained over many generations. Vegetative propagation also makes
possible the use of the polycross breeding method used to identify desirable parents with
good general combining ability. In this method, parental clones are space planted in a
field design that assures each parent is equally exposed to pollen from all the other
parents in the nursery. Progeny evaluation then identifies the best parents for inter-
pollination to produce seed of a new variety.
Hybrid varieties
The vigour, yield and uniformity advantages associated with hybrids in both oilseed
and vegetable Brassica crops have been demonstrated by many breeders. The main
constraint to their commercial exploitation has been an effective pollen control-fertility
restoration system. Vegetable breeders have utilised the variations in SI alleles, which
control the self-incompatible system, to produce single and double cross hybrids.
Kuckuck (1979) illustrates how lines, selected for general combining ability and specific
S alleles, are programmed to produce double-cross cabbage hybrids (Figure 3.44).
The self-incompatible parent can be maintained through bud pollination, micro
propagation or by overcoming the SI barrier by exposing flowering plants to high CO 2
concentrations. Nuclear male sterility in oilseed rape has also been used commercially in
China but the segregating male fertile progeny have to be removed by hand (Fu et al.,
1997), thus making the system expensive in many regions.
Figure 3.44. Self-incompatability scheme for breeding cabbage hybrid seed production
Source: Kuckuck (1979).
The most practical and efficient system is that of cytoplasmic male sterility (CMS).
More than 17 different male sterile forms have been investigated in Brassica species
(Stiewe et al., 1995; Prakash et al., 1995). Only a few have been developed to the
commercial stage, but varietal development programmes worldwide are rapidly moving
to the use of CMS-restorer systems for hybrid seed production. The CMS systems are
based on genetic miscommunication between cytoplasmic mitochondria and nuclear
genes, resulting in the disruption of normal anther and/or pollen development. There are
three components to the system: the A line, carrying the cytoplasmic mitochondrial
genome that results in male sterility, the B line that is fully fertile and maintains the
S 1 S 1 S 2 S 2
S 1 S 2
x S 3 S 3 S 4 S 4
S 3 S 4
x
x
Certified seed
Single cross
Double cross
