Ξ March 3rd, 2008 | → 2 Comments | ∇ A Day at a Time, Technology, Wine History |
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 – Vitis vinifera L. This is the Eurasian vine used in the production of grapes, raisins and wine which has spread around the world with human agriculture. (The L. often used at the end 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.)
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!
Most members 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 suggest more than a touch of V. vinifera in its parentage. However 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 it’s 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 UC Davis 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 extinct white variety which, along with Pinot, was involved in the parentage of grapes such as Chardonnay and Gamay.
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.
Velasco’s paper is the grape equivalent to the Human Genome Project and is a fascinating read, if somewhat technical, 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? In comparison wild grape varieties typically exhibit significantly more disease resistance, and this is because they 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! However the Pinots, Cabernets etc, because of long-term cloning, have not been allowed to update to counter the new pathogens the vine is exposed to now. Of course you could cross wild and cultivated varieties to breed in the new resistance, but this would also affect the good 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.
Elsewhere 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 white grapevine, 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.
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 by mentioning Dennis Gray of the University of Florida who has been working with Muscadine grapes for many years. According to The Economist last year, in their review of the Pinot Noir sequencing story, he’s already started field trials of genetically engineered grapes against Pierce’s disease (a condition which has been touched upon in an earlier Reign of Terroir post).
As a trained geneticist researching this topic allowed 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 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?
Science daily, Pinot Noir Grape Sequenced
Science daily, Ripening Secrets Of The Vine Revealed