Rose Annual 120-122 (1969)
TRIPLOIDS IN ROSE BREEDING, AVOID OR SEEK?
Percy H. Wright
Saskatoon, Saskatchewan, Canada
BEFORE considering whether the rose breeder should seek or avoid the origination of triploids, it will be best to describe what diploids, triploids, and tetraploids are. To be sure, the word "Tetra" is now freely used to describe tetraploid snapdragons and other flowers, but using the word does not automatically mean understanding it.
A diploid living cell, whether of an animal or a plant, is a call of normal size and normal gene contents. If the wall between two such cells is never formed in the process of cell division, the gene contents of two cells will flow together within the one cell, and such a cell is called a tetraploid cell. Every gene in it is present twice over. It is as though a truck had not only a spare tire for use in emergency, but had all four (or more) tires in duplicate, not tucked away inside the body, but on the wheels and used at all times. Obviously, if a gene becomes defective through some unusual circumstance, the duplicate will carry on. This is an advantage to the seedling plant, but has its dangers too. It means that a tetraploid race is more likely to become overloaded with defective genes than a diploid one.
When a diploid is crossed with a tetraploid, the descendant seedling will have 1 1/2 times the normal gene count, or be a triploid. Triploids have very low fertility. They rarely produce egg cells, but pollen cells are produced in such prodigious numbers that it is practicable to use triploid roses as pollen parents. The viable pollen cells will be of two kinds, some of them of the count suitable for diploid plants, and some of them suitable for tetraploid plants.
The first Hybrid Teas resulted from crossing Tea roses (diploid) with native European roses, Gallicas and their relatives (tetraploids). They were, therefore, triploids. But, when the pollen of these triploids was used on the Gallicas again, enough pollen with the larger gene count was produced to result in tetraploid Hybrid Teas, and after this step was once taken, most Hybrid Teas were tetraploids from then on.
The possibility of breeding a diploid species of rose with a tetraploid and thereby getting its gene material for the improvement of rose beauty, has been amply demonstrated in the origination of the Hybrid Teas, and so the procedure is known and can be used again as often as wished. However, the rose breeder who sets out to use diploids to improve tetraploids, or tetraploids to improve diploids, is embarking on a long term project requiring much more work and patience than if he could achieve the same results by keeping within either the diploid or the tetraploid group.
Ottawa's hardy shrub rose 'Agnes' is a triploid, having been bred by putting pollen of 'Persian Yellow,' a tetraploid, on the pistil of a Rugosa flower, Rugosa being a diploid. Apparently no one has ever used pollen of 'Agnes' to fertilize a tetraploid rose, whether a Spinosissima or a Hybrid Tea, but there is, in theory, no reason why it should not be done. The result might well be to bring a few genes from Rugosa into Hybrid Teas, and thereby gain, perhaps, some of the hardiness and resistance to blackspot and mildew that the Hybrid Teas so urgently need.
The same pathway could be used to bring over the genes of any diploid rose species, barring other inhibiting circumstance, into the Hybrid Teas. 'Blanda' would be a good choice because of its freedom from thorns, its hardiness, and its resistance to blackspot, but not because it is resistant to mildew. 'Nitida' would be a good choice because of its dwarfness, its shiny foliage, its autumn coloration, and its pleasant tone of pink. 'Nitida,' one suspects, also inherits a gene for non-fading in sunlight. On the other hand, it transmits extremely weak flower stems. There are, of course, many other possibilities in the numerous diploid species.
Sometimes triploids seem not to inherit the qualities of their parental roses according to Mendelian Laws. I once bred from a valuable tetraploid yellow rose by putting its pollen on the rugosa hybrid 'Hansa,' and secured more than 300 seedlings. Both parents had double flowers, and yet, out of the whole 300 plus, only two had truly double flowers, of which only one was regarded as worth saving (my bicolor rose 'Musician'). In the same year I had 50 seedlings or more from the pollination of 'Hansa' by the tetraploid species Laxa, and not one of these was double. These two experiences in the same year made me resolve to make triploids as infrequently as possible.
However, after 15 years, I finally overcame the prejudice against triploids, and today I have a new rose, as yet unnamed, which I consider a real find; it is a triploid. I haven't the slightest clue to date as to when a triploid will obey the Mendelian Laws, and when it won't, or why. The thing to do seems to be to make triploids of a given cross in small numbers, and then, if the results are good, return to the battle and make them in larger numbers.
If there is any aim in rose breeding today which is urgent and yet overlooked, it is bringing over genes from species of roses hitherto not used in making Hybrid Teas, that will result in immunity to blackspot and mildew. Genetic control of these diseases is much more promising than chemical control by spraying.