Grapevine DNA – The genetics of wine

At first glance Genetics may seem a strange topic for a Wine Blog, but look closer and you’ll discover that humans have been genetically modifying the grapevine ever since it was first farmed over 6,000 years ago by selecting the best growths and characteristics (results of genetic variation and mutation) and then locking them in by vegetative propagation (aka cloning). As we progress into the 21st Century advances in molecular genetics allow us to look deep into the DNA of this intoxicating plant to uncover its history and potentially allow further manipulation of its future. 

First some background Biology. There are over 60 distinct species of fruit producing vines of the Genus Vitis, in the Family Vitaceae, but, with a few rare exceptions (such as the Norton grape) there is only one that attracts the attention of you and I, the Eurasian vine Vitis vinifera L. used in the production of table grapes, raisins and wine which has spread around the world with human agriculture (the L. shows that this was one of the original plant species named by the founding father of biological classification – Carl Linnaeus in his Species plantarum in 1753).

Genetic research by scientists in Australia show that originally all grape varieties were red, but several thousand years ago two independent genes involved in skin colour mutated at about the same time to produce the first pale fruit, the ancestor of today’s white varieties. The earliest known white wine has been confirmed from the time of Tutankhamun, more than 3300 years ago.

Most members of the Vitis species, and all V. vinifera, have 19 pairs of chromosomes; 38 units of hereditary that carry the DNA within each cell and on into the next generation. Research has shown that V. vinifera has approximately 30,000 genes spread over 500Mb of DNA (Mb = Megabase pairs, a unit of DNA length). Compare this to humans, who have 23 pairs of chromosomes containing 3300 Mb of DNA carrying..… 30,000 genes. Yes, as a species we have no more genes that a grapevine, just a lot more junk DNA in between them!

The majority of Vitis are dioecious, separate plants are either male or female and cannot pollinate themselves, while V. vinifera are hermaphrodes and can self-fertilise (the Norton variety mentioned above is referenced as V. aestivalis, but the fact that it can self-fertilise suggests more than a touch of V. vinifera in its parentage). Rather than let nature take its course modern viticulture is based on using cuttings (cultivars) to produce clones of the original plant to preserve their desirable characteristics, such as fruit quality. There could be up to 10,000 cultivars of V. vinifera in existence (about 7000 red and 3000 white varieties) and these are the names we see on the labels of our favourite wines; Cabernet, Syrah, Riesling, Verdejo, Assyrtiko etc. In trying to visualise the varieties genetically I liked the description found on Professional friends of wine”  likening them to human populations “each variety should be considered a “surname” which can have its own close family and extended relations stretching back in time, but which may have different names or marriages into other families”.  

In their 2006 paper Vouillamoz & Grando of the Istituto Agrario di San Michele all’Adige discussed a key aspect of cloning; namely that it was difficult, often impossible, to know the family history of a single cultivar – genetically it may be tens, hundreds or even thousands of years old.

While leaf morphology was used to guess relationships in the past now molecular genetic techniques allow for more precise identification of related varieties of V. vinifera. Similar to the 1997 finding at UCDavis that Cabernet Sauvignon is the offspring of Cabernet Franc and Sauvignon Blanc they showed that Pinot Noir, “one of the most ancient western European cultivars still in cultivation today”  is related to Syrah (either a “great-grandfather, great-uncle or cousin”). Unfortunately they also pointed out that getting a complete family tree of the major varieties is not realistic as many of the contributory family members will likely be extinct, something just avoided with Gouais Blanc, the almost defunct white variety which, along with Pinot, was involved in the parentage of grapes such as Chardonnay and Gamay.
Nevertheless a glimpse of this family tree was presented in January 2011 in New Scientist magazine from a study by Sean Myles of Stanford University.

Cloning of our favourite varieties is not the only genetic dabbling done in the name of viticulture, how about hybrids and chimeras?  The devastation of European vineyards by Phylloxera in the 19th century led to the widespread grafting of old world V. vinifera onto the rootstock of native American species such as V. aestivalis,  V. riparia, V. rupestris, V. champinii, V. candicans etc. (or on crossings of these with V. vinifera).
Let’s just be clear here, the grafting of components of one distinct species onto another separate species or a hybrid cross of mixed species – that is genetic modification of the highest level, and something done in agriculture for hundreds of years, not just with grapevines. Of course all of this is done primarily for disease resistance and maintaining plant and fruit characteristics.

Like many plants V. vinifera is highly heterozygous, meaning that for its 19 pairs of chromosomes the 2 members of each pair (the homologues) show a large degree of DNA variation when compared to each other. This was highlighted in the 2007 mapping of the Pinot Noir genome by Riccardo Velasco & colleagues, also of the Istituto Agrario di of San Michele all’Adige. Their findings showed that, on average, there was an 11.2% variation between each of the 19 sets of homologues, which is an enormous amount. As both of Pinot Noir’s parents provided one of every homologue this shows how different, genetically, those parent varieties were, and by assumption all V. vinifera varieties – that level of variation is greater than across all the members of the great ape families; Orangutans, Chimpanzees, Bonobos, Gorillas and, of course, Humans. This also explains why vegetative propagation of grapevines is a necessity, since V. vinifera does not seem to tolerate any degree of inbreeding and in normal sexual reproduction actively mixes up the DNA it passes onto the next generation, which would create chaos for viticulturists trying to maintain favourite features. I wish the adventurous Randall Grahm luck with his recently announced plan to grow a vineyard entirely from seedlings, given the recombination data above each new plant is likely to be radically different to its parents!

Velasco’s paper is the grape equivalent to the Human Genome project and is a fascinating, if somewhat technical, read showing how genes for disease resistance make up a large proportion of the genome. But if this is so why are commercial varieties so vulnerable to disease while wild grape varieties typically exhibit significantly more disease resistance?
Wild vines reproduce sexually and resistance evolves competitively with the diseases and vectors they’re exposed to – think of it as allowing their genes to download the latest Operating System updates and Anti-Virus software. Unfortunately, because of long-term cloning, the Pinots, Cabernets etc have had their automatic updates disabled and are no longer able to counter any new pathogens the vine is exposed to. Of course you could cross wild and cultivated varieties to breed in the new resistance, but this would also affect the grape characteristics you want to keep.

Velasco suggests the research could lead to new “molecular breeding” programs, where clusters of resistance genes from wild strain vines could be selectively crossed into the domesticated  varieties without losing the genes involved in grape or wine quality.This was the premise of the planting of a GMO Vineyard in France (Steve Savage summarised the plan concisely in his 2010 blog post), unfortunately in a Luddite incident barely a month later the vines were destroyed by protestors.

It’s not just the vine itself that is now open to genetic changes, there’s another species involved in winemaking – Yeast. There are already new strains of these single cell organisms that can tolerate higher alcohol levels than ever before, perfect for the “No Wimpy Wines” generation and others designed to finish at lower levels to counter these Frankenwines, but that would be the start of a whole new controversy, so I’ll bring this article to an end with Dennis Gray of the University of Florida, who has been working with Muscadine grapes for many years.
In The Economist review of the Pinot Noir sequencing Gray was mentioned as having started field trials of genetically engineered grapes against Pierce’s Disease, a bacterial infection which has been touched upon in an earlier Reign of Terroir post. More recently he’s been working on fungal disease resistance with Thompson Seedless table grapes the hope of producing disease-resistant varieties of Chenin Blanc able to thrive in Florida’s climate.

Researching this topic allowed me an intriguing glimpse into aspects of viticulture and winemaking I had not truly appreciated and begs the question, should we be afraid of future genetic tinkering with our favourite beverage? For me, as a trained geneticist, the answer is no; partly because the fermented grape juice that makes it into our glasses doesn’t contain active DNA anyway, but mostly because we’ve been genetically modifying the vine for thousands of years – so why should we stop now?

Greybeard

(Based on, and updated from, an original article published March 3rd, 2008, on Reign of Terroir)

Additional reading from Science Daily:
Bringing Better Grapes a Step Closer to Reality
Pinot Noir Grape Sequenced
Ripening Secrets Of The Vine Revealed

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One Response to Grapevine DNA – The genetics of wine

  1. Karl Laczko says:

    With the publishing of the mighty tome “Wine Grapes” additional pedigree charts are available, especially detailed is the Pinot Family Tree;

    http://winegrapes.org/wg/wp-content/uploads/2012/09/Gatefold_pinot_FINAL.pdf