RHA Newsletter 3(3): 4-5
ROSA ACICULARIS GENES FOR NEW ROSES
by Percy H. Wright
The Spring 1972 Newsletter of the Rose Hybridizer's Association arrived yesterday, and was so full of interesting topics that it is difficult to see what topics are left for future issues. Congratulations to the Editor and all those who contributed.
When I re-read my own contribution, "Getting Roses on Their Own Roots," it occurred to me that further information on some of the topics I touched upon may be of interest, and hence the present little item. Before I go further, however, I apologize for an error which appeared in the bottom line of Page 6, where, of course, the word "hybrids" should have read "species".
The possibilities for the rose hybridizer in Rosa acicularis, the "Circumpolar Rose", do not appear to have been properly recognized. In this neglect of the wealth of genetic material in the hardiest of all rose species, I have been as remiss as others, and with less excuse, for the hardiness that is inherent in the species is probably more important here in the prairie provinces of Canada than anywhere else in the world, except Alaska and Siberia.
Rosa acicularis occurs in at least two forms (probably more), a hexaploid form and an octoploid form, which is a phenomenon hard to explain in terms of the story of evolution. It has been assumed that the hexaploid form is North American and the octoploid form Asian, but the octoploid form also occurs in Alaska, and may descend along the Rocky Mountain chain as far south as southern Alberta. It would be an interesting project to make a map of the two continents, showing where each form grows, and where they overlap.
Apparently the area where they overlap is quite extensive. A cooperative correspondent in Alaska, at Fairbanks, sent me two plants of the form native there. One of these was tested for chromosome number by the cytologist at the Canada Agriculture Research Station here at Saskatoon, and found to be a septaploid, undoubtedly the result of a chance cross between the two forms in the area where they overlap.
When the hexaploid form is crossed with Hybrid Teas (mostly tetraploid), the result must be a pentaploid, which would have as little fertility as the septaploid described above. However, if the octoploid form were crossed with Hybrid Teas, the result would be hexaploids, which could be expected to be fertile. It is a wonder that some rose breeder in northern Europe has not made this cross long ago. Any such hybrids would probably become the first hexaploid domestic roses in history.
When the hexaploid form is crossed with Rosa rugosa, which is a diploid, the result is a fertile hybrid, undoubtedly a tetraploid. The rule is that when two plants of differing chromosome number are crossed, the hybrid is intermediate between the two in chromosome number, and that if this number happens to be odd, near sterility results, whereas if this number is even, the chance of fertility is good.
Mr. Robert Erskine of Rocky Mountain House, Alberta, has reported to me that he once made a cross between the form of Rosa acicularis native in his area and a tetraploid rose, and achieved fertility, which result is hard to explain except on the supposition that the Asian form of this unusual species has penetrated as far south as the mountains of southern Alberta.
Presumably a traveller could make a good guess as to where the two forms of the species overlap, by noting where plants without heps occur, frequently.