Dogrose oddities

I have been admiring the late hips on the dogroses that I grow, Rosa rubiginosa and Rosa canina. They form beautiful arching shrubs. Although the flowers are not as spectacular as some species roses - generally a very soft, rather washed-out pink - and the scent is not very obvious, they are still magnificent in wilder parts of the garden. Left to themselves they are tough, undemanding and disease-resistant. They reward handsomely with their freely produced flowers in June followed by good crops of attractive hips.

Despite the fact that they are implicated in the parentage of the alba roses they have not been much used in breeding. This is partly due to a genetical oddity. They are all polyploid (with four to eight sets of chromosomes but they only ever contribute one set to the pollen, the rest being contributed to the egg. This phenomenon, unique in the plant kingdom is called "unbalanced meiosis" or simply "canina meiosis".

The majority of the canina group species are pentaploid with five sets of chromosomes (or in the tradition notation of cytology: 2n=5x=35). The pollen is 1n=1x=7 and the egg is 1n=4x=28. When the sperm from the pollen combines with the egg, the full complement of 35 is again achieved. Dogroses may also be tetraploid (2n=4x=28), hexaploid (2n=6x=42) or even octoploid (2n=8x=56). But the unequal meiosis is the same, for instance 7 to the pollen and 21 or 35 to the egg in tetraploids and hexaploids respectively.

The fact that more chromosomes are transmitted in the maternal line than in the paternal line means that progeny resemble their mothers. The Canina roses are therefore unique. As a group they are nearly completely confined to Europe, and ever since the Swedish botanist Täckholm (Täckholm G. 1920. On the cytology of the genus Rosa. Svensk Bot. Tidskrift 14: 300–311) discovered "Canina meiosis" many botanists in Europe have studied the group, but the mystery of the origin of this oddity remains unsolved.

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List of some dogroses (Section Caninae DC.) with chromosome numbers

• Rosa agrestis (2n=35, 42)
• Rosa caesia [incl. R. vosagiaca] (2n=35, 42)
• Rosa canina (2n=35)
• Rosa ferruginea (2n=28)
• Rosa micrantha (2n=35, 42)
• Rosa mollis (2n=26, 56)
• Rosa obtusifolia (2n=35)
• Rosa sherardii (2n=26, 35, 42)
• Rosa stylosa (2n=35, 42)
• Rosa tomentosa (2n=35)
• Rosa rubiginosa (2n=35)