Chronica Horticulturae volume 49 number 2 ... - Acta Horticulturae

Figure 4. Rosehips from Rosa sherardii. Photo: M. Uggla.
Figure 5. Meiosis in tetraploid dogrose (2n=4x=28) is peculiar with uneven transmission of
chromosomes from the seed and pollen parent. In megasporogenesis (left) two of the
genomes line up at the equatorial plate in metaphase 1 to form bivalents. The other 14 chromosomes
remain unpaired as univalents at one of the poles. Separation at anaphase 1 will
result in one cell with 21 chromosomes (7 chromosomes from the bivalents plus all 14 univalents)
and one cell with 7 chromosomes. At the next division, the univalents divide with the
chromosomes from the bivalents resulting in two cells with 21 chromosomes that survive
and two cells with 7 chromosomes that will be lost. One of the cells with 21 chromosomes
will develop into the embryo sac. In microsporogenesis (right) there is bivalent formation in
metaphase 1 but the 14 univalents remain at the equatorial plate, separate as chromatids,
and move to the poles at anaphase 1. At the second division, the chromosomes from the
bivalent chromosomes divide normally, but the univalents lag behind resulting in a tetrad
consisting of four cells with 7 chromosomes together with micronuclei from the univalents.
At fertilization, only pollen with 7 chromosomes and no micronuclei unite with the 21 chromosome
egg restoring the tetraploid number.
M1
A1
M2
A2
plant. Studies have shown differences in germination
rates depending on species and the temperature
during rosehip maturation (Fig. 4).
Genetics
In contrast to most other rose species, the
dogroses are self-fertile, but cross pollination
and hybridization occurs. All species within
sect. Caninae are polyploid with pentaploidy
(2n=35, x=7) as the most common, but hexaploidy
and tetraploidy also occur. In most other
plants, the male and the female parent contribute
equally to the offspring, i.e. half of the
genetic material from each parent. But a plant
with 35 chromosomes must solve its meiosis in
another way and the dogroses have come up
with a unique solution to this problem. This
solution was not discovered until 1920, and is
regarded as an oddity in the plant world ever
since. At the female meiosis of a pentaploid
species, only 2 of the 5 genomes form the bivalents.
The remaining 3 genomes (21 chromosomes)
occur as univalents and are not involved
in any of the recombination. After the separation
of the bivalents and the second division,
the result is two cells with 28 chromosomes
each of which one will become the embryo sac,
and two cells with 7 chromosomes each, which
will subsequently be lost. At the male meiosis,
again 2 genomes form the bivalents, but this
time it is the univalents that are subsequently
lost after the second division, and the resulting
tetrad consists of four pollen cells with 7 chromosomes
each. However, not all of the univalents
are lost. They can occur as chromatids or
as parts of chromosomes in the pollen cells and
thus disturb the viability of the pollen. Actually,
the pollen viability in Caninae species is only 20-
35%. The fertilization will then restore the original
35 chromosomes (Fig. 5).
This strange meiosis has of course a strong influence
on dogrose plant breeding since four-fifths
of all genetic material comes from the female
parent and only one-fifth from the male parent
(three-quarters and one-quarter, and five-sixths
and one-sixth, respectively in a tetraploid and
hexaploid species). There is a very strong morphological
resemblance between the offspring
plant and the female parent, and this led to the
conclusion that the dogroses could propagate
by apomixis, the production of seeds without
fertilization of the egg cell. Recent studies have
shown this to be true, but to what extent it happens
in nature is difficult to say. The meiosis in
itself is a combination between apomixis
(through the univalents that are passed on
through the maternal line without recombination)
and self-pollination as it has been shown
that the bivalent formation usually takes place
between the most similar-looking genomes.
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USES
Rootstocks
During the first half of the 19th century, budding
and grafting of ornamental roses became
ISHS • 10