Like other triploid plants, roses which possess three sets of 7 chromosomes each are generally sterile. As early as 1920, Tackholm 4, 5 demonstrated how this sterility arises from irregularities of their meiotic divisions.
In 1929 and 1933, Erlanson1, 2 gave further details of the cytology of triploid roses, drawing special attention to the fact that some of them are able to set seed. The triploid hybrid Rosa blanda x R. carolina, for instance, proved to produce "a crop of good hips and achenes."
Dr. Erlanson asked (and so may we) whether the progeny of such a fertile triploid would also be triploid, or diploid or tetraploid. In the last cases, propagation of diploid or tetraploid offspring involves certain irregularities in the reduction division, but normal sexual processes still occur. If the progeny is triploid like the mother plant, an alternative arises; the mother plant might be apomictic (and thus represent asexual propagation) or it might behave like Rosa canina and produce egg cells and pollen grains with different chromosome numbers (14 and 7, respectively).7
In recent years, two other triploid roses of hybrid origin and of rather high fertility were found among the creations of the late nurseryman, Mathias Tantau, Sr. (seedling number 83 and Schneeschirm). Dr H. W. Rehagen and I investigated their cytology at the Botanical Institute of the University of Kiel (Germany), and made the first successful attempt to raise seedlings in order to study their cytology as well.3
Seedling number 83 [Rosa multifora x (R. canina x R. coriifolia froebelli)] produced a new fertile rose and 27 diploid sister plants. Morphologically, this triploid plant was highly similar to the female parent. So, we concluded that it contained two chromosome sets of R. multiflora (probably from an unreduced egg with 14 chromosomes) and one set of 7 chromosomes from the male parent (R. canina x R. coriifolia froebelii). I then published a description of its reduction division.6 Here let me point out that: 1) meiotic chromosomes pairing predominantly leads to the formation of 3-4 trivalents, besides 4-3 bivalents and 4-3 univalents; 2) the further course of the reduction division is highly irregular. A pronounced elimination of chromosomes occurs, and the viable pollen grain and eggs cells of this triploid rose evidently received only 7 chromosomes, for its offspring (six plants) were diploid. Five of these six plants very closely resembled R. multiflora.
The ancestry of Schneeschirm, an ornamental rose, is not quite clear. It blooms twice a year. The first flowering period lasts from June to August. The second begins after a short interval, and ends with the first frosts.
It is a very remarkable fact that the flowers of both periods differ in their meiotic behavior; only those produced during the first period are able to produce hips and viable achenes. Their reduction division is characterized by the occurrence of only 0-3 trivalents, with pairing to 7 bivalents and 7 univalents or 1 trivalent, 6 bivalents and 6 univalents. This reduction division proceeds rather regularly. The univalents, splitting twice, are mostly taken up into the young tetrad nuclei. Chromosome elimination is low, and so viable pollen grains and egg cells will contain 14 chromosomes.
The offspring of about 60 plants were tetraploid, each seedling having 28 chromosomes in the root tips.
We also studied the reduction division of these tetraploid seedlings. In most pollen mother cells (63%), 14 bivalents were formed. The remaining pollen mother cells showed 10-13 bivalents and the corresponding numbers of uni-, tri- or quadri-valents.
Growth habit and leaf shape are highly similar in the tetraploid seedling plants and their triploid mother. Differences chiefly exist in some floral characters. The diameter of the tetraploid flower is smaller (4-9 cm) than in the triploid (8-12 cm), but the former have a tendency to produce double flowers. The number of anthers and ovaries is lower in the tetraploids, and their pollen grains are larger and show a higher degree of fertility. The production of hips and achenes is far better in the tetraploids than in the triploid mother plant.
Reduction division in the second period Schneeschirm flowers shows a remarkable increase of trivalent formation. Here only 5% of the pollen mother cells have 0-1 trivalents. 3-5 trivalents are found in 76% of the pollen mother cells. The general course of meiosis is very irregular, chromosome elimination is high and there is a clear tendency to form pollen grains with 7-9 chromosomes (instead of 14 as in the flowers of the first period).
What we are learning from the differences in the meiotic behavior of the flowers of both periods is that the reduction division of the triploid Schneeschirm can be rather easily affected by environmental conditions. It seems reasonable to suppose that the warm temperature from June to the end of August causes the formation of no or few trivalents. This prevents chromosome elimination, and favors the production of hips and achenes. On the other hand, the colder temperatures of September and later months seem to be responsible for the high meiotic irregularities and the failure of fruit-setting.
Answering the above questions, we may now say that fertile triploid roses have a restricted sexual reproduction which depends on the chance-formation of viable egg cells and pollen grains. We can preclude the possibilities of apomixis, or the existence of a Rosa canina-like sexuality, unless further of other triploid roses would reveal them.