American Rose Annual, 1987
A PROBABLE AMPHIDIPLOID OF ROSA ABYSSINICA AND ROSA RUGOSA
Dr. Robert E. Basye
In the spring of 1955, I made the cross, R. abyssinica x R. rugosa. This is the story of how the union of these two diploids, each with 14 somatic chromosomes, yielded in the next generation a probable amphidiploid; that is, a tetraploid, each of whose cells contain 14 abyssinica chromosomes and 14 rugosa chromosomes.
|R. abyssinica growing on the campus of the University of Santa Clara, Santa Clara, CA. Photo by Rev. Theodore Mackin, S.J.|
R. abyssinica is a musk rose which, by a report that I cannot verify. Rev. George Schoener received as a personal gift from Emperor Haile Selassie. He had it growing in Santa Barbara, then took it with him when he moved to Santa Clara. I first saw it there in 1952, growing just a short distance from the quadrangle of the old Spanish mission on the grounds of the University of Santa Clara. I saw it again in 1979. A large supporting trellis had been built around the giant bush. In full bloom it is indeed a thing of beauty.
|Rosa rugosa. Photo from ARS files|
The rugosa parent is, more precisely, R. rugosa rubra, which I imported in 1950 from B. R. Cant & Sons in England.
I still remember the exuberance with which the young seedling grew. It became a huge bush whose appearance was intermediate between the two parents. As is often the case with parents from different sections of the genus, the hybrid was almost sterile. Yet, in most years, I was able to find one or two or three small hips. I grew the seeds carefully, hoping to be able to duplicate the luck which led Wilhelm Kordes to his amphidiploid of R. wichuraiana and R. rugosa. (See H. D. Wulff, "Max Graf and Its Progeny, with Special Reference to Rose Kordesii," American Rose Annual, 38:111-122, 1953).
During the years 1964, 1965, and 1966, 1 grew ten seedlings of the diploid cross. To determine somatic chromosome counts I used leaf tips, which are more convenient to collect than root tips. I found that three were diploids, three were triploids (21 chromosomes), and four were tetraploids. I discarded the diploids and triploids and eventually saved only one of the tetraploids, bearing the number 67-305, as being the most likely candidate to be a true amphidiploid.
The criterion for an amphidiploid is that an unreduced male gamete of the diploid cross unites with an unreduced female gamete of this cross. The resulting tetraploid will be a true amphidiploid. The occasional production of unreduced gametes at meiosis is one of Nature's secrets for working herself out of a tight corner of sterility.
But tetraploids can also result in another way. The bees can carry pollen from it nearby tetraploid species to the diploid cross. Any resulting seed will give rise to a tetraploid if the companion female gamete of the diploid cross is unreduced. The resulting rose will not be an amphidiploid but a trispecies hybrid.
An amphidiploid should behave very much like a true species since the chromosomes occur in the same homologous pairs as in the diploids of the original cross. Self-fertility may be lessened, however, if homologies also exist between chromosomes of one of these diploids and those of the other. As a general rule, therefore, the more sterile the diploid cross, the more self-fertile will be the amphidiploid.
Coming now to the main question, is 67-305 an amphidiploid or a trispecies hybrid? This is not an easy question to answer, since my garden contained many wild tetraploid roses. One clue can come from growing a population of selfed seedlings, easily done since 67-305 is highly self-fertile. If a third species is involved, then the population should exhibit a noticeable segregation of characters. To my untutored eyes this was not apparent in a small population of 30 selfs which I grew. (I should have grown a larger population and called in a trained taxonomist). Instead, the modest variation which did occur struck me as being that which is normal within many species.
Nevertheless, I must be content with calling 67-305 a probable amplidiploid.
The bush is vigorous, up to six or eight feet, and appears to be intermediate between the ancestral parents. The flowers are single, an attractive bright pink, close to three inches in diameter, and occur in clusters of one to ten. They have little or no fragrance. Blackspot resistance is very good. As might be expected from the parentage, the armature of thorns is downright formidable.
I have used the bush enough in my breeding work to know that it is quite compatible with garden roses generally, both as male and female parent. The same is true of Commander Gillette, a thornless descendant of R. carolina which I described in the 1985 American Rose Annual. These two roses are tetraploids and together involve the three species, rugosa, abyssinica and carolina, all of which have high resistance to blackspot. Might these two roses be useful in the problem of ridding our garden roses of both thorns and blackspot?
|R. carolina. ARS file photo|
I believe that the governing genes for thorns are quite few in number and that thornless roses should soon begin to appear in our gardens.
For blackspot I hold a different view. For 30 years I have shattered many a lance on this worst enemy of the rose, with very little to show for it except hard experience. I believe the problem is difficult because the governing genes are probably many and give no visible evidence of their individual contributions, thus preventing us from dealing with them one by one. Then there is the powerful temptation by the breeders to pursue the lovely blooms, neglecting all else. These are the two main reasons why we continue to bear the burden of cruel thorns and debilitating blackspot. There is a clear need for some dedicated young rose breeder out there to take on the problem, someone who believes that the glory lies as much in the bush as in the flower.
May I outline just one plan of attack which I would consider if I were that young rose breeder? I would consider starting with a nucleus of three tetraploids: Commander Gillette and the two amphidiploids, 67-305 and R. kordesii. These three stud roses carry genes of the four species, carolina, rugosa, abyssinica and wichuraiana, all of which are highly resistant to blackspot. And Commander Gillette has the potential of removing the thorns. I use the phrase "highly resistant" because I'm not sure that any rose is completely immune.
We would not begin by charging into the hybrid tea china shop with our four wild bulls. We would begin by crossing the two amphidiploids and growing a population of F1 Seedlings. We would expect no great variation here in blackspot resistance, but if there should be, let us select the best ones for selfing. In each of the resulting F2 generations of selfs we have a segregation of characters and thus a better chance of variation in blackspot resistance. Again we select from each F2 the plants with the highest resistance. Let A designate this final group of plants of highest resistance. We would hope that their resistance equals or excels that of the two amphidiploids. In any case, we now have plants that carry genes of rugosa, abyssinica and wichuraiana.
1 have never crossed the two amphidiploids. Herr Kordes sent me kordesii in 1954, but I lost it when I had to move my garden in 1968, just after 67-305 was born. They should he highly compatible, however, since rugosa is a common ancestor, and the other two ancestors both lie in the same Synstylae section of the genus.
It remains to introduce the fourth species, R. carolina, and take the first step in the thorn problem, Commander Gillette is ideally equipped for this. I mentioned in the 1985 article that the cross 67-305 x Commander Gillette produced a rose, 77-361, which was free of thorns and bristles and had perfectly smooth midribs of the leaves. Recently, I repeated this cross and confirmed this possibility. But before making the crosses A1 x Commander Gillette, where A1 denotes a member of the group A, we first make a cosmetic change in Commander Gillette.
Commander Gillette itself is free of thorns and bristles and has smooth midribs. Among the selfs, however, the bristles will often appear; also a rare thorn or a slight roughness on the midribs. Those recessives are easily bred out by several successive selfings. The criterion for success in such a self is that one further selfing produces a population completely free of the undesirables. One reason I have not done this before in my other breeding work is that it can lead to the loss of other recessives that are desirable. For example, Commander Gillette contains a latent gene for recurrency which might be lost. I nevertheless recommend the cosmetic change for the labor-saving dividends it will pay down the road — not a small item.
We return now to the crosses of the type A1 x Commander Gillette, where A1 denotes a member of the group A, and Commander Gillette has been subjected to the cosmetic change described. A small percentage of the seedlings of this cross should be free of thorns, bristles and roughness on the midribs. Several successive selfings of each of these should produce one or more plants homozygous with respect to each of the three traits. We repeat this routine for each member of group A. All the roses so obtained form a group B. Our final group G comes by selecting from B the plants with outstanding resistance to blackspot.
To further reduce the labor of the operation just described, it might be best to use the reverse crosses, Commander Gillette x A1, and mix the pollens of the A1.
Of the group G we can say that each rose in it has high resistance to blackspot, is homozygous with respect to freedom from thorns, bristles and roughness of the leaf midribs, and, last but not least, carries genes of four of Nature's noblest roses. With this army of gentle bulls we are ready to enter the hybrid tea china shop.
This is just one approach to the problem of thorns and blackspot. Whichever approach we choose, the road that lies yet ahead will be long and arduous. But if the rose is truly our Queen of Flowers — and now our National Flower — do we not owe it to her to follow through?
Commander Gillette, 67-305 and 77-361 are growing in the Huntington Botanical Gardens, 1151 Oxord Road. San Marino, California 91108. Budsticks will be sent to rosarians having understocks in return for any modest donation to the Huntington Rose Research Fund. Letters may be addressed to the Curator of the gardens.
In December of 1986, since writing the above, I have received two rooted cuttings of R. kordesii from Dr. Felicitas Svejda. Agriculture Canada, Research Station, Ottawa, Ontario; I shall be glad to furnish budsticks of this rose to interested rosarians.