In the UK at this time of year, you may well see pink or white flowers that belong to a prickly shrub called the dog rose (Rosa canina) peeking out of a hedgerow. Later in the summer these flowers will be replaced with orange-red fruits called rosehips, which can be used to make a variety of food products including syrup, tea and marmalade. Rosehips are a rich source of vitamin C, and during WWII, British schoolchildren were actively encouraged to forage for them (1).
The dog rose—which is native to Europe, North Africa and western Asia—also has medicinal properties and was used in many traditional remedies. The seeds contain a diuretic, its leaves contain a laxative, and the plant was even used to treat rabid dog bites. Roses also feature in other aspects of human culture. For example, the flowers of the dog rose and other wild roses inspired the stylized roses in medieval heraldry (e.g. the Tudor rose). The pleasant scent of rose flowers means they have been popular ingredients in perfumes for centuries, and cultivated “ornamental” roses are found in many gardens.
Cuttings of ornamental roses are often grafted onto established dog rose root systems to generate healthy and uniform-looking plants for gardens, therefore there is a lot of interest in understanding dog rose genetics. Like our genetic information, the DNA of the dog rose is arranged into structures called chromosomes. We have two sets of 23 chromosomes (one set inherited from each parent when an egg and sperm cell fuse). Dog roses, on the other hand, have five sets of seven chromosomes (35 in total) and only one set comes from the male parent (Figure 1). This is possible because dog roses and their close relatives make egg and sperm cells in a slightly different way to most other organisms.
Egg and sperm cells are made in a type of cell division called meiosis. During meiosis in humans and most other organisms, each chromosome pairs up with the matching (but not necessarily identical) chromosome from the other set. These chromosome pairs are separated before the cell divides to make daughter cells with half the number of chromosomes.
However, since dog roses have 5 sets of chromosomes, it is not possible for all of them to pair up. Instead, only two sets of chromosomes (14 in total) form pairs (Figure 1; reference 2). These “bivalent” chromosomes are then divided equally between the daughter cells. In the female part of the flower, the remaining 21 “univalent” chromosomes join one of the sets of bivalent chromosomes to make an egg cell with 28 chromosomes. However, in the pollen, the 21 chromosomes are left out of the daughter cells to make sperm that only have 7 bivalent chromosomes.
Research suggests that it is usually the same two sets of chromosomes that pair up during meiosis (3). This might be because the other chromosomes are not similar enough to be able to form pairs. It is also possible that these chromosomes are capable of pairing up, but are prevented from doing so by a control system within the cell. Therefore, over many generations, the genes contained within the univalent chromosomes are likely to become more different to their gene copies on the bivalent chromosomes—which are mixed and rearranged when they pair up during meiosis.
The chromosomes in dog roses appear to have originated from different ancestral species of rose. This suggests that the extra sets of chromosomes are the result of the production of hybrids (when members of two different species reproduce). Hybrids are often sterile (i.e. mules) because the genetic material they inherit from the two parent species is too different to allow the different sets of chromosomes to pair up properly in meiosis. However, this problem can be avoided by duplicating all of the chromosomes so those from each species can pair up separately. For example, durum wheat—which has 28 chromosomes—originated from two grass species that normally have two sets of seven chromosomes each.
Although many plant species have more than two sets of chromosomes, the dog rose and its close relatives are some of the only plants known to inherit unequal numbers of chromosomes from their male and female parents. The next challenge for researchers is to understand how this process works at a molecular level. In the meantime, it gives me another reason to admire the dog rose as I pass by…
- Laws (2010) Fifty plants that changed the course of history. David and Charles.
- Ritz, CM, Köhhnen, I, Groth, M, Theiβen, and Wissemann, V (2011) To be or not to be the odd one out – Allele-specific transcription in pentaploid dogroses (Rosa L sect. Caninae (DC.) Ser) BMC Plant Biology http://www.biomedcentral.com/1471-2229/11/37
- Lim, KY, Werlemark, G, Matyasek, R, Bringloe, JB, Sieber, V, El Mokadem, H, Meynet, J, Hemming, J, Leithc, AR and Roberts, AV (2005) Evolutionary implciations of permanent odd polyploidy in the stable sexual, pentaploid of Rosa canina L. Heridity http://www.nature.com/hdy/journal/v94/n5/full/6800648a.html