The structure of the Brassica family is shaped by certain essential characteristics. Self-incompatibility is a special characteristic that prevents the plant from accepting its own pollen and forces it to cross-pollinate with other plants of the same species. This natural characteristic lends itself to out-crossing thereby making the brassicas natural hybrids. Some of the brassicas are not self-incompatible which means that they can breed themselves (self-pollenate) as well as interbreeding. It also means that it is easy to develop pure lines of these plants simply by ensuring that they breed only themselves.
The brassicas comprise three monogenomic (basic species with 18 chromosomes) and three amphidiploids (secondary species with 36 chromosomes);
Monogenomic (basic species)
The basic species comprises; B.oleracea, B.rapa (formerly known as B.campestris) and B.nigra.
Cabbage, kale, cauliflower, broccoli and the other cole vegetables belong to the B.oleracea family, while “Polish” rape and turnips belong to B.rapa. Varieties of mustards make up B.nigra and B.carinata, B.juncea.
Haploids (basic species with about 18 chromosomes)
B.nigra, B.oleracea and B.rapa (primary Brassica species) are self-incompatible, meaning that they will not accept their own pollen, even though they make perfect flowers (complete with male and female parts). This characteristic forces them to cross-pollinate, that is, to breed with other plants of the same species, and it is this natural necessity of out-crossing that makes these plants natural hybrids. They are haploids, meaning that the maintain the basic number of Brassica chromosomes, about 18.
Diploids (secondary species with about 18 chromosomes)
Ordinary diploids (crossbreds with about 18 chromosomes) are the result of the crossing of two lines, producing a new line with a new combination, but the same number, of chromosomes.
Amphidiploids (secondary species with about 36 chromosomes)
The secondary species comprises; B.nupus, B.carinata and B.juncea.
“Argentine” rape and rutabaga belongs to the B.napus family and varieties of mustards make up B.carinata, B.juncea.
B.napus, B.juncea and B.carinata (secondary Brassica species) have a highly unusual characteristic and are described as amphidiploids (mergers) with about 36 chromosomes. Ordinary diploids (crossbreds with about 18 chromosomes) are the result of the crossing of two lines, producing a new line with a new combination, but the same number, of chromosomes.
An amphidiploid (merger) is the joining of two lines and a doubling of the number of chromosomes, since the merger retains the original chromosomes of both lines that come together. B.juncea is a merger of B.nigra and B.rapa; B.carinata is a merger of B.nigra and B.oleracea; and B.napus is a merger of B.oleracea and B.rapa.
B.napus, B.juncea and B.carinata are not self-incompatible, which means that they can breed themselves as well as interbreeding. It also means that it is easy to develop pure lines of these plants simply by ensuring that they breed only themselves.
Nature took thousands of years to create B.napus, B.juncea and B.carinata, and certainly failed millions and millions of times along the way, but now breeders can get them to merge quite easily by means of cell biology (embryo rescue - the process of generating whole plants from a bit of plant tissue by just floating it in the proper medium).
Author - Armitage; copyright 2007