Inbred brassicas, as with most other out-crossing crops, tend to lose vigour and, although a single cross will restore vigour in the Fp the seed production on the inbred parents may be uneconomically low. For this reason, whether hybrid production involves self-incompatibility or male sterility, three-way crosses are often used. If self-incompatibility is used, and if both parents are self-incompatible S^ X S2S2, then seed from both parents can be harvested. Alternatively, an open pollinated line can be used as the pollen parent and seed only saved from the self-incompatible line. Where the original SXS2 hybrid is self-incompatible, then it can be used as a male or female line together with a self-incompatible C line, and again all the seed may be harvested. That is, however, more complicated and requires deeper understanding of the levels of self-incompatibility and the relationship of each inbred line to the others involved.
With male sterility, the A or male-sterile line Smsms is maintained by back-crossing to a self-fertile B line Nmsms (S = sterile and N = normal or fertile cytoplasm). After several generations of back-crossing, the A line will resemble the B line, but there will be some inbreeding depression. For production of hybrid seed, the A line is crossed to a further unrelated line C. Only the seed on the A line can be saved. Three-way crossing could be made by crossing the A X C hybrid to a further line, D, and this may result in higher seed production levels since the female parent will now be a hybrid.
Seed production success depends on issues such as whether the two parents flower at the same time. This can easily become a problem where the parents may require different degrees of vernalization and exposure times to produce flower stalks and flowers.
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