TRIPLOIDY HAS PLAYED an important part in the history of garden roses. Of the principal ancestral species, five are diploid with 14 chromosomes (Rosa moschata, chinensis, gigantea, multiflora and wichuraiana) and two are tetraploid with 28 (R. gallica and foetida). It is obvious that triploids must arise from their coming together in gardens and glasshouses and sharing pollen indiscriminately.
Triploids, while often desirable plants in themselves, have the reputation for being difficult breeding material. Fruiting may take place rarely (or only parthenogenetically), and in extreme cases they may produce no viable pollen at all. It is therefore of some interest to examine triploidy in garden roses, its effect on fertility and in breeding programs.
Some 57 triploid garden roses of various classes have been under observation for up to 10 years in the National Rose Species Collection at Bayfordbury. Table I gives a survey of their reproductive ability as female parents as judged by the set of open-pollinated hips. The long hot summer of 1959 was exceptionally favorable for fruiting, and many presumed sterile roses ripened hips for the first time.
It will be seen that the fertility ranges from completely barren in all seasons up to a crop comparable to that of allied diploids and tetraploids. When grouped ancestrally, the Hybrid Musks emerge as the most fertile. The order of fertility in Class 3 may be exceedingly low. A single fertilization among 50 or more ovules is enough to swell a fruit in Rosa, and although this is sometimes lopsided or ripens only adjacent to the seed it is often quite normal to look at. One plant may have a potential hip number of many hundreds, so that five hips on a bush may represent a successful "take" of only one in several thousand.
Hip set is undoubtedly affected by environment, especially the weather at the time of anthesis. Ideal conditions may stimulate a "sterile" individual to mature seeds. Thus although Cardinal de Richelieu has never set a hip outdoors in the eight years under observation at Bayfordbury, Sam McGredy tells me it will do so occasionally under glass.
Female Fertility in Triploid Garden Roses
Key Class 1
|Abundant crop of hips|
|Moderate to good crop most years|
|Rarely a few hips in favorable years|
|GROUP||Class 1||Class 2||Class 3||Class 4|
|H. GALLICA||Cardinal de Richelieu|
|Climbing Cramoisi Superieure
|BOURBON||Gloire des Rosomanes
Souv. de St. Anne's
Souv. de la Malmaison
|NOISETTE||Mme. Alfred Carriere
|TEA||Belle Portugaise||Climbing Lady Hillingdon
|H. TEA||La France||Cheshunt Hybrid
Gruss an Aachen
|H. WICHURAIANA||American Pillar
Dr. W. van Fleet
|H. FOETIDA||Star of Persia||Agnes|
|H. RUGOSA||Fimbriata||Mrs. Anthony Waterer
Parfum de l'Hay
|FLORIBUNDA||De Ruiter's Herald
Pollen counts on 19 of these triploids (Table II) show a range from complete sterility (Parfum de l'Hay, Ruskin) up to fairly good (Papa Gontier, Lady Hillingdon). Male and female sterility are independent of one another, so that a rose may produce some good pollen but no hips or (more rarely) a few hips but no pollen.
Male Fertility in Triploid Garden Roses
|% good pollen|
|31||H. MUSK||Buff Beauty|
|30||H. CHINA||Climbing Cramoisi Superieure|
|10||H. TEA||Gruss an Aachen|
|5||H. TEA||La France|
|0||H. RUGOSA||Parfum de L'Hay|
Compared with the pollen of diploid and balanced tetraploid roses, that of triploids shows less clear-cut distinction between plump and abortive grains, so that accurate scoring on size is difficult. Intermediate as well as giant grains commonly occur. It seems very likely that chromosome numbers vary also, only balanced euploid grains being functional.
CAUSES OF STERILITY
There are many reasons why a rose may fail to reproduce its kind. Not all of them are open to ready examination. The most obvious is a lack of reproductive parts, as may be caused by flower doubling. In extreme cases all stamens and styles may be replaced by barren green or petaloid organs. A good instance of the suppression of fertility by flower doubling is seen in the two triploid Bourbon roses Souvenir de la Malmaison and Souv. de St. Anne's. The former has completely double (full) blooms and is incapable of maturing fruits. The latter is a mutation from Souv. de la Malmaison identical in appearance save for semi-double blooms. With its small quota of stamens and styles it can set an occasional fruit.
A second cause of breakdown is open to study by the cytologist, and concerns the chromosome pairing at meiosis. With three genomes in place of two, there may be a failure to find pairing partners, or competition for the same partner. Trivalents, bivalents and univalents result in varying proportions according to origin of the triploid (Rehagan 1957). A high proportion of trivalents suggests autotriploidy, as has been found in some of the miniature roses (Para Ti and Rosina) by Miss A. P. Wylie (unpub.) These seem to be of relatively pure China rose descent. An exhaustive survey of the cytology of rose triploids has been given by Rehagan (1957) and Wullf (1959).
TRIPLOID ROSES FOR BREEDING
Considerable interest attaches to the behavior of triploid roses as breeding plants, and the chromosome numbers of their progeny. Two families have been studied by Wulff (1959): one from a trispecific cross, the other a garden floribunda. He found that the chromosome level of the seedlings diverged in opposite directions. Progeny of the former, Rosa multiflora x (R. canina x R. coriifolia), dropped to the diploid level, only one triploid appearing among 28 plants of the F2, and this gave rise to nothing but diploid progeny in the F3. Schneeschirm, on the other hand, went over completely to the tetraploid level in its progeny, all 60 of which had 28 chromosomes. Apparently only diploid gametes function, although among autumn blooms Wulff reported a predominance of haploid pollen grains but no fruit set.
Thus where triploidy occurs in a rose breeding program we can expect either a deadlock owing to complete sterility or a rapid return to an even polyploid level in subsequent generations. A third outcome—the production of a hexaploid by spontaneous or induced chromosome doubling—has not yet been recorded in the genus, the alleged origin of Rosa X involuta this way (Blackburn & Harrison 1924) having been disproved by Miss Wylie (Lecture to Genetical Society 1955, unpub.). In surveying the main lines of evolution of garden roses from the species up to modern hybrid teas and floribundas, it would appear that the drift upwards from triploid to tetraploid has been selected rather than the descent to diploid. Several major groups like the bourbons, hybrid teas and floribundas (hybrid polyanthas) began as triploids but today are composed almost wholly of tetraploids with no evidence remaining of diploid members.
The absence of aneuploidy among the progeny of triploids is noteworthy considering its occurrence in hybrids of Caninae and higher polyploids (Rowley, in the press). No doubt the loss or gain of a whole chromosome has less adverse effect when buffered by several genomes than it would have at lower polyploid levels.
Triploidy has repeatedly arisen from the crossing of diploid and tetraploid roses in gardens. The triploid cultivars grown today mostly have reduced fertility, but few are so barren as to debar any chance of further hybridization. Season and environment affect the level of fertility. Flower doubling can suppress it in an otherwise fertile triploid.
The progeny of triploid roses only very rarely include triploids, the chromosome level normally falling to diploid or rising to tetraploid. Only the tetraploids have been selected as new garden roses. Aneuploid gametes formed from meiotic irregularities are not functional.