Chardonnay grapevines exhibit small differences in their properties and the quality of grapes they produce. Scientists now show that small mutations in the DNA of these plants can be linked to these properties. In addition, they show that the Chardonnay family tree is a lot more complicated than expected.
In the twentieth century, the production of grapevines largely took place by mass selection. Only the vines with favourable properties were propagated. Nevertheless, there was a great genetic variation between these different grapevines on the vineyard. This caused large differences between the grapevines in yield, quality and susceptibility to disease. The emergence of clonal selection – i.e. propagating grapevine cuttings derived from only one plant – enabled a more precise selection of characteristics of the grapevine and limited the variation in the vineyard. New vineyards subsequently consisted of vines derived from only one plant, clones with all exactly the same DNA and the same beneficial properties. This resulted in more uniformity in yield, quality and susceptibility to disease, but also narrowed the genetic pool drastically. The worldwide arsenal of Chardonnay vineyards currently consists of a limited number of these clones. Each clone has a few very small differences in its DNA and therefore also slightly different properties. The clones differ, for example, in the amount of grapes they produce, the shape and size of the bunches, and the sensitivity to fungi. Knowing which parts of the DNA provide these different properties may be useful when cultivating new grape varieties.
Scientists of the Australian Wine Research Institute investigated the DNA of 15 popular Chardonnay clones. They found that they could find unique mutations in the DNA for each clone, and that they could also recognize the clones based on these mutations in their DNA. Figure 1 shows a phylogenetic tree that shows how closely the different clones are related to each other. The closer the clones are to each other in the figure, the stronger they are related to each other. Burgundy clones 118 and 124 are very closely related to each other and show only 23 differences. Also the group of clones “CR red”, “I10V1” and “Waite Star” have only 40 mutations that are different, and are therefore closely related.
A small part of these mutations could already be directly linked to specific characteristics of the Chardonnay clone. For example, Chardonnay clone 809 is a grape with muscat aromas and is the only clone that has a mutation that causes the increased production of monoterpenes, the molecules that are known to give grapes their aromatic muscat characteristics.
In the distant past when the grapes were freely allowed to reproduce sexually, the Chardonnay grape originated from a cross between the Pinot noir and Gouais blanc grapes. Pinot noir and Gouais blanc are both grape varieties belonging to the Vitis Vinifera species and have a diploid genome. This means that all DNA is present twice in each of their cells. In the case of sexual reproduction, each parent plant passes on one of the two copies of the DNA to the offspring. Half of the Chardonnay grape genome should therefore consist of Pinot noir DNA and half of Gouais blanc DNA. However, this is not the case at all!
With a new DNA screening technique the scientists were able to view both copies of the DNA independently of each other. In this way they could determine more precisely which part of the Chardonnay DNA came from Pinot noir and which part from Gouais blanc. They showed that 49% of the Chardonnay DNA is derived from Pinot noir, 34% from Gouais blanc, and the other 17% is also derived from Pinot noir! Namely, in 17% of the Chardonnay genome, both copies of the DNA are Pinot noir-DNA. Because one of the DNA copies comes from Pinot noir and the other from Gouais blanc, this means that Gouis blanc already contained Pinot noir DNA. This can only be caused by an earlier cross between an ancestor of Gouais blanc and Pinot noir. Subsequently, when Gouais blanc is crossed with Pinot noir, it passes a part of this Pinot noir DNA to its offspring resulting in the duplicated parts of Pinot noir DNA. This is shown schematically in Figure 2. As such, the Chardonnay grape originated from inbreeding between its ancestors Gouais blanc and Pinot noir!
Complex Chardonnay family tree
Pinot noir and Gouais blanc show a high degree of kinship, which has resulted in the Chardonnay genome with inbred characteristics (i.e. the duplicated Pinot noir DNA). With only one crossing between Pinot noir and a Gouais blanc ancestor, one would expect that 75% of the Chardonnay genome comes from Pinot noir and 25% from Gouais blanc. However, the ratio is with 66%-34% slightly different. This may be caused by spontaneous mutations and/or recombination of the DNA, but above all shows that the Chardonnay family tree is a lot more complicated than previously thought.
Roach MJ, Johnson DL, Bohlmann J, van Vuuren HJJ, Jones SJM, Pretorius IS, et al. (2018) Population sequence reveals clonal diversity and ancestral inbreeding in Chardonnay grape growing. PLoS Genet 14 (11): e1007807. https://doi.org/10.1371/journal.pgen.1007807