A team of researchers from Wageningen University & Research, which includes breeding researchers and geneticists, has succeeded in creating a highly detailed genetic map of roses.
This was a complicated assignment: roses are tetraploid, which means that they have four sets of chromosomes instead of two, like humans do. In roses it is also unclear how the four sets of chromosomes pair in cell division (meiosis), as the modern varieties of cut and garden roses have been bred using several different wild roses species. It is that very pairing of chromosomes that is vital in determining the way in which genes are passed on to the next generation after the crossing of two roses. The researchers had to develop new software to be able to create a good genetic map. More than 25,000 genetic markers can be found on the new genetic map of roses which are evenly divided across the 4 x 7 chromosomes.
Selection based on genetic maps
Genetic maps are an important tool for plant breeders, as they allow them to develop new varieties more quickly, more easily and at a lower cost. If you know exactly where and on which chromosome a specific genetic flag is located and can connect the characteristics of plants to these flags, it becomes much easier to select for multiple traits at the same time. These maps also help plant breeders to make a conscious choice in terms of specific combinations of parent plants for crossing. Until recently, there were only good genetic maps for diploid varieties and diploid relatives. Yet, there are also many different tetraploid, hexaploid (six sets of chromosomes) and even octoploid (eight sets of chromosomes) crops such as potatoes, leeks, roses, chrysanthemums, alstroemeria, begonia and strawberries. The methods and software that have been developed can be used for crossing at the same ploidy-level as the crops.
The researchers at Wageningen University & Research are working together with twelve breeding companies that are not only making a financial contribution to the development of methods and software, but also providing their input regarding populations and data to improve the methods.
Software used by participating companies
The rose's genetic map is the first success resulting from this collaboration. Meanwhile, participating companies have been able to work
with the software during the development phase using the genetic markers of their own crops. Researchers at the companies were trained to use the software by researchers from Wageningen University & Research. The software will be further improved and expanded, with the next goal being a hexaploid crop such as the chrysanthemum.
Thanks to this new software, researchers discovered that the way in which rose chromosomes pair during meiosis is not straightforward. This pairing is mostly random, but at certain locations on the chromosomes the same two chromosomes nearly always or always pair.
Cultured roses are the result of crossings between a number of wild rose varieties. Many cultured roses don't just have two sets of chromosomes: they have four. During the formation of pollen in the egg cells, in a process of cell division called meiosis, similar chromosomes look for one another in the centre of the cell. This allows the one half of the chromosomes to go to one direction, and the other half to the other direction. Four sets can split into two sets in different ways. In roses, the question is whether the same two chromosomes will always look for one another (left in the figure) in these four sets of chromosomes, or whether it is a random combination. If it is a random combination, the four equal chromosomes (one from each set) could create one large group of four chromosomes (right) during meiosis. Having a knowledge and understanding of this process is vital, as the chromosomes are connected so closely during meiosis that genes are exchanged between the chromosomes. In the figure on the left (A always pairs with A and B with B) the genes of A will never end up in the B chromosomes or the other way around. This is possible in the figure on the right, meaning that
genes are exchanged between the four chromosomes on a much larger scale.