II.3. BRASSICA CROPS (BRASSICA SPP.) – 239
Synthetic varieties
Allard (1960) defines a synthetic variety as one “that is maintained from open
pollinated seed following its synthesis by hybridisation in all combinations among a
number of selected genotypes”. This method, which is widely used in breeding forage
crops, is also effective for the breeding of oilseed B. rapa (Falk et al., 1994).
Equal amounts of seed from varieties or recurrent lines that arise from widely different
gene pools are mixed and sown in Syn.-0 isolation plots. Seed harvested from the
Syn.-0 plot constitutes the Syn.-1 generation. Syn.-1 seed from a two component
synthetic will consist of 25% from each parental genotype and 50% hybrid seed.
Thus, if the parental lines are good combiners, a significant amount of heterosis can be
captured.
The method (Figure 3.43) has also been explored in B. napus (Becker, Löptien and
Röbbelen, 1999) but breeding programmes in this species are now directed to F 1 hybrid
varieties.
Normally, despite the high multiplication rate (1 000: 1), there is insufficient Syn.-1
seed for commercialisation so that Syn.-1 seed is sown to provide commercial Syn.-2
seed. This procedure has been used in Canada to produce the first commercial B. rapa
synthetic varieties, Hysyn 100 and Hysyn 110. Because of the large number of genotypes
within the parental lines, there is very little loss in heterosis between the Syn.-1 and
Syn.-2 generations (Falk and Woods, 2003). If the market is very large a Syn.-3
generation could be added.Figure 3.43. Breeding scheme for development of commercial synthetic varieties of oilseed
Brassica cropsSource: Becker, Löptien and Röbbelen (1999).
Diallel and polycross methods
In vegetable crops, uniform maturity, head size and appearance are critical to the
success of a variety and seed yield is of secondary importance. Further, the numbers of
parents that make up a variety are few and the market price of seed is substantially greater
than the commodity oilseed crops. Thus, breeding methods used for vegetables can be
more intensive than the large population breeding methods used in oilseed improvement
programmes. For example, if a deleterious trait is controlled by a recessive gene,
it is difficult to completely eliminate it from a self-incompatible plant population.