THE terms diploid, triploid and tetraploid are becoming more and more familiar to rose growers, especially since Modern Roses began adding the figures 14, 21, 28 or some other multiple of seven in brackets after the botanical description. But mere familiarity does not necessarily result in understanding and, unless someone attempts to explain these terms to the non-genetic minded, they are likely to seem little more than a system of classification that somehow divides rose varieties from other rose varieties. Haploid means having one set of chromosomes (rod-like bodies contained in the nucleus of a cell, bearers of the hereditary material or gene). Diploid—an organism with two sets of chromosomes; triploid—three sets, etc., polyploid an organism with more than two sets of chromosomes.
The truth is that, for the rose breeder and hence for anyone interested even though not active, in the improvement of the rose, the chromosome number is of enormous significance. The various "ploidy" groups are not completely cut off from one another. It is possible to cross a diploid with a tetraploid and eventually bring some desired quality from the diploid over into the tetraploid race and vice versa. But the rose breeder who confines his hybridization to one ploidy group of varieties will find his path smoothed and his aims more quickly achieved.
We may suppose that all forms of life which developed a sexual method of reproduction once included a diploid phase in their life cycle and that diploidy is the foundation for the further edifice of chromosome increase. A diploid inherits one set of chromosomes from its male parent and another similar, but rarely identical set, from its female parent.
When self-fertilization occurs, the same individual is both male and female parent and the two sets of chromosomes are more likely to be identical. If in outbreeding species, inbreeding or self-fertilization is continued long enough, the plants tend to become identical, and the more nearly identical they become the less vigor they retain. This general, but not invariable, rule is the basis for the practice of inbreeding, selection and outbreeding, in that order, as a means of purifying a given strain of plant or animal of its defective chromosomes.
But the processes of reproduction are not regular enough to ensure that every diploid parent produces a diploid offspring. If a pollen cell, or an egg cell, happens to be unreduced, that is, if the complete diploid cell becomes a pollen cell or egg cell instead of the usual haploid cell, the resultant offspring plant will have too many chromosomes and all its cells will be that much larger to accommodate the extra material.
It is usually a pollen cell which is produced in the doubled form because pollen cells are thrown out in so much greater numbers than egg cells that any accident which occurs is more likely to occur in a pollen cell. But the situation is similar if it is the egg which is unreduced. Suppose a pollen cell, because accidentally unreduced, has a chromosome number of fourteen in place of the usual seven. Then suppose that, in fertilization, it meets an egg cell of the usual constitution. Since seven plus fourteen is twenty-one, you get a triploid, that is, a plant with its cells 1 1/2 times the basic, or haploid number, seven.
Now, when this triploid comes to form pollen and egg cells in its turn, it is in difficulty. Half of twenty-one is not any multiple of seven. The extra seven chromosomes do not know what to do with themselves. They go at random to either half of the dividing cell. If about half of the seven go to one cell and half to the other, an infertile pollen or egg cell is produced and that line ends then or there. Chance, however, will occasionally determine that all the extra chromosomes go to one of the two cells, which leaves none to go to the other. In this case, two fertile reproductive cells are made, one doubled, and the other the usual haploid. If two of these doubled cells happen to meet each other, one a pollen cell and the other an egg cell, in fertilization, at once you have a tetraploid race, produced from a diploid.
Take as an example the shrub rose 'Betty Bland,' originated by Dr. F. L. Skinner of Dropmore, Manitoba, by putting pollen of a garden rose on pistils of Rosa blanda, a native wild rose of Manitoba. R. blanda is diploid and so is 'Betty Bland.' But the pollen parent of 'Betty Bland' was probably a triploid hybrid tea. If the pollen cell which produced 'Betty Bland' had happened to be double-number, 'Betty Bland' would have been triploid. But we can only suppose that it was the usual haploid pollen cell, produced by a chance migration of all the seven extra chromosomes to the other pollen cell, wasted in this case. This is the only way we can account for a diploid hybrid between a diploid species and a hybrid tea.
When the first hybrid teas were produced, they were all triploids. They resulted from the union of a haploid reproductive cell from the diploid race of the teas, with a doubled reproductive cell from the tetraploid race of the gallicas or one of the other European tetraploid roses. These original hybrid teas would produce the occasional pollen cell with the diploid number and such pollen cells meeting egg cells of the European tetraploids would result in tetraploids with some of the chromosomes of the teas. The hybrid teas, for the first time, were placed on a permanent basis as a new race of roses.
An interesting question comes out of this explanation of the hybrid tea's origin. Should we develop hybrid teas of the opposite or diploid constitution? In other words, we now have gallica-type roses with the gene in the teas that makes for everblooming, but we do not have teas with genes of the gallica-type roses that make for hardiness. A whole race of hybrid teas of different make-up, which was (and is) a possibility, has never been developed. How much have we lost by our failure to make diploid hybrid teas?
The rules of heredity as worked out by Gregor Mendel were for diploid species. The picture is somewhat simpler than in the case of tetraploids. When we are working with simple rules, we are more likely to reach our goals quickly. In addition, it should be easier to attain the maximum of a given quality, say yellow color in the petals, if there is no second chromosome which fails to carry the same gene for color in the second set of chromosomes of tetraploids.
Why does someone not undertake to put pollen of 'Persian Yellow' (R. foetida persiana) on one of the nearest-to-yellow teas? The hybrid so produced should make a proportion of haploid pollen cells. These, fertilizing the teas again, should create a new race of roses in which it should be somewhat easier to intensify the yellow color than in our existing hybrid teas. Also, it should be easier to avoid inheriting the gene or genes for susceptibility to blackspot which come down from 'Persian Yellow.'