This invention relates to novel methods for breeding and propagating a male sterile plant and three novel species of male sterile plant resulting from this method. In more detail, this invention relates to novel methods for breeding and propagating a male sterile plant based on a cell carrying a nucleus of a desired pure line as its nucleus and a cytoplasm mediating male sterility as its cytoplasm.
(1) Owing to rapid development of plant biotechnology of recent years, various means for improving plant varieties have been developed.
In those means for improving plant varieties, a male sterile stock occupies a very important position. For example, the seed collection from a F.sub.1 -hybrid of a vegetable belonging to the family Cruciferae is generally carried out by utilizing a "self-incompatible" inbreeding line in which fertilization is not done normally owing to the facts that pollen does not germinate, the pollen tube cannot elongate into the style, the growing speed of the pollen tube becomes lowered or discontinued, etc. in spite of pollination, pollination does not permit seed collection because said line, though it is monoclinous and the reproductive organs of both sexes mature simultaneously, is incompatible with itself. However, it has been known that there are some inbreeding lines from which it is difficult to obtain desired F.sub.1 -hybrids efficiently because they are weak in such "self-incompatibility" though they are excellent in their practical performance per se. If the production of a F.sub.1 -hybrid using a male sterile stock as a female parent is intended in such cases, pollen is not produced from said female parent and thus the efficient production of F.sub.1 - hybrid described above can be attained.
(2) With respect to said production of a male sterile stock, however, the following disadvantages are pointed out at present.
As one of the means for producing a male sterile plant, "the nucleus substitution technique" by which a nucleus of an already established male sterile plant is replaced by a nucleus of a desired plant can be enumerated.
According to this "nucleus substitution technique", a mitochondorial DNA known as a genetic resource of male sterility is preserved in a cytoplasm, whereby the male sterility per se is preserved.
However, with respect also to a chloroplast which is known to hold its own genes exerting a serious influence upon the expression of characters of a plant, the chloroplast of said male sterile stock as a parent stock is preserved simultaneously. Accordingly, in case a male sterile stock as a parent stock is not related to a stock offering a nucleus, there are cases where an unfavorable phenomenon for producing a plant is provoked by the interaction between said chloroplast and said cell nucleus.
For example, in case that a "nucleus-substituted type (Ogura) Brassica campestris" is produced by using the Ogura cytoplasm of a Japanese radish [Ogura, H; Mem. Pac. Agri., Kagoshima Univ., vol. 6, pp. 39-78 (1968)] as a cytoplasm providing male sterility and a nucleus of a plant belonging to Brassica campestris represented by a Chinese cabbage as a nucleus, said plant suffered from chlorosis at low temperatures. In addition, the growth of a nectary is not recognized in said plant, so that it becomes difficult to attract an insect such as a honey bee or the like having a role of transporting pollen. Accordingly, it seems difficult to say that said plant is a favorable stock to be developed.
(3) Then, between the above Ogura radish cell and a plant belonging to Brassica napus (rapeseed), use as a male sterile stock of a somatic hybrid which is produced by fusing protoplasts of said plants has been studied. [Pelletier et al, Mol. Gen. Genet., vol. 191, pp. 244-250 (1983)].
In such a somatic hybrid, the male sterility held by the Ogura radish cell is preserved, while a chloroplast derived from the Ogura radish cell simultaneously falls off and only a chloroplast of a plant belonging to the above genus Brassica is preserved. As a result of these effects of somatic hybridization, said somatic hybrid has several advantages in that the above chlorosis and unsatisfactory growth of a nectary become unrecognized.
However, there is a limit to the kind of plant belonging to the genus Brassica suitable for the above protoplast fusion from the technical viewpoint. Therefore, it is difficult at the present to freely produce a male sterile somatic hybrid, which a breeder intends, directly by the above protoplast fusion techniques.