Seventy years after cultivation in South Sweden: did the rubber dandelion leave genetic traces behind?

Work package 4 of DRIVE4EU aims to study the interactions between the rubber dandelion, Taraxacum koksaghyz (TKS), and wild dandelions. The emphasis is on the intercrossing and the exchange of genes between dandelion species. In Task 1, the hybridization of TKS in natural habitats in Kazakhstan is studied, in Task 2, we want to see what happens if TKS is cultivated outside its natural range. Specific questions are: can TKS maintain itself and does hybridization occur between TKS and wild dandelions? To get insight into this two of the sites where TKS seeds have been produced in the past were investigated . In South Sweden between 1945 and 1950 TKS was grown on a relatively large scale and also seed production has taken place. The locations in South Sweden provide an unique opportunity to investigate potential exchange of genes after many generations.

This article is about Task 2. In the end of May 2016 two KeyGene researchers Rolf Mank and Peter van Dijk together with IBOT researcher Jan Kirschner visited areas in southern Sweden, where nearly 70 years ago TKS seeds were produced at a large scale.

In Sweden, briefly after the war, TKS was grown and bred. The Swedish breeding program was pretty successful. In six generations the rubber percentage in the roots improved from 5 to 15%. That is a faster increase than that of the sugar content of sugar beet in the 19th century. The Swedish research program also devoted attention to the agronomy and seed production. The Swedish reports indicate that on three locations in South Sweden seeds were produced: Hammenhög, Tomelilla and Lidhult. From the report of Joseffson we can calculate that on the first location between 1947 and 1950 almost 25 kg TKS seed was produced. At a weight of ~0.5 mg per seed that is ~50 million seeds. If 20% of these seeds was blown away prior /during harvesting this would mean that 10 million seeds were spread into the environment. These seeds could have developed into TKS plants, and may have settled or they can have been pollinated by local common dandelion plants (Taraxacum officinale, TO) . TO plants in Sweden are all apomictic, meaning that they produce clonal seeds. Apomicts sometimes develop viable pollen and can thus, in principle, pollinate ‘escaped’ TKS plants. Subsequently their descendants can maybe multiply further. However the expectation is that this is not so easy, because TKS and TO are hard to cross and the apomicts are usually triploids, and these make little good pollen.

Besides the seed production fields also fields for rubber production will have been present in the vicinity of the research locations. As most TKS does not need vernalization (flowering induction after a cold period) these will flower during the first year. This also might have led to further seed dispersal and possible gene flow.

During our visit in South Sweden a large number of locations have been inspected visually and have been described botanically by expert Jan Kirschner. The sample locations focused around two of the seed production locations: Hammenhög & Tomelilla. Both these locations could still be found back and sufficient amounts of dandelions were observed. The region still has a large focus on agronomy and is not strongly urbanized. On basis of the expert opinion only TO clones could be observed at the different locations. Nowhere TKS as such or any look-a-likes were observed.

Leaf samples from over 20 locations have been collected by Peter van Dijk and Rolf Mank for further research at KeyGene. These 600 samples will be screened using both TO and TKS DNA markers in order to quantify possible historical gene flow between both species although the plants found in the field look 100% TO. This research is still in progress.