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lundi, 05 novembre 2012

THE LACK OF GENETIC DIVERSITY THREATENED THE VINE

LACK OF SEX PUTS THE CULTURE OF THE VINE IN DANGER.


Wild grapes and domesticated grapes: the differences

72 species of Vitis have been identified [i].  They grow mainly around the tropical zone in the northern hemisphere. However, only one, Vitis vinifera L, gives fruit of sufficient quantity and quality to be used for the production of wine. It was domesticated in the Near East (and possibly elsewhere) and Georgia is a likely candidate for being the cradle of viticulture and winemaking.   However, the first archaeological evidence of wine making has been found on the site of Hajji Firuz Tepe in Iran and dates back to 7400-7000 B.C.

 

Domestication of the vine has consisted in transforming a wild dioecious plant where the male organs (stamens) and the female organs (pistils) are on the same monoecious plant. It allows cross fertilisation of the plant without having to revert to insects for pollination. This leads to better flowering and ultimately results in bigger and better grapes. Domestication of the vine is very closely correlated to the development of winemaking and its industry but it is not sure today which event came first.

 

differences between vitis vinifera and vitis sylvestris.jpg

Domesticated versus wild Vitis

It is highly likely that the transformation from a uni-sexual into a hermaphrodite plant was achieved by mutation[ii] (modification of the genetic information) and not through human manipulation.

The Vine: A plant weakened by lack of sex

All forms of domestication invariably lead to a loss of genetic diversity, hence in a weakening of the plant in relation to its environment. It is highly probable that after the vine had been domesticated, numerous experiments of hybridisation (cross fertilization) were undertaken to cross the newly domesticated plant and the wild species (Vitis sylvestris) found in Europe. DNA analyses have revealed many characteristics of the wild plant, Vitis sylvestris in the genome (the entirety of the genetic information) of Vitis vinifera. However, asexual reproduction, also known as vegetative or clonal reproduction, has been the major mode of propagation of the vine over time. Sexual reproduction is a slow process and it is necessary to cross plants over several generations to obtain a cultivar with the wanted characteristics, while asexual reproduction, which consists in planting a seed or branch in the ground, is much easier and faster and leads to planting a single variety consistently throughout the vineyard.

According to DNA analyses done so far on various cultivars we can estimate the number of different grape varieties to be approximately  10,000 [iii] although commercial policies and standardisation of products and tastes limit these choices to 20-30 cultivars today. However, this mode of propagation has not resulted in a shuffling of genetic information that confers plants a greater resistance and fits better to their environment (Darwin). The various cultivars have appeared through processes of mutations (modification of genes) and genetic polymorphisms (variation in the sequence of the genes).  These modifications have resulted in some red cultivars becoming rosé or white and in having different organoleptic properties but they have failed to give the genetic mix that genes from both parents provide through sexual reproduction. This has weakened the plant over time especially when they have to face major untoward events such as the introduction of Phylloxera in Europe. The cultivars used today are extremely close to one another, for example, Pinot Noir has evolved to become Pinot Gris and eventually Pinot Blanc. Gewurztraminer and 20 other varieties come from the same parents.  Merlot is closely related to Cabernet Franc which is a parent to Cabernet Sauvignon whose other parent is Sauvignon Blanc, the daughter of Traminer, which is an ancestor of Pinot Noir which is also a parent of Chardonnay… so it goes on and on.

In fact 75% of grape varieties known today are so close that they are linked to eachother through a relation of parents-children or sons-daughters[iv].  Although the decryption of the full vine genome has not yet been completed, it would not be surprising if the 10,000 cultivars estimated to exist today are related to eachother in one way or another.

 

Genetic Structure of Grapes Fig.3.jpg

Cultivars interconnections (Myles et al.)

Odium, mildew, phylloxera, diseases of the nineteenth century that sound like a warning for today.

 

So it was a rather fragile plant that grew in Europe in the nineteenth century, a period that witnessed a huge development of commercial trade with the invention of the steam engine. It is in this way that the American wild Vitis plants carrying these diseases were introduced to Europe.  It is Odium that struck first in the mid-1850’s, then mildew, and in 1870’s phylloxera hit the Languedoc and the Rhone before spreading through the major wine growing areas of France, Portugal, Spain, Australia and other areas of the world with the exception of Chile and a few other places. Champagne was only hit in 1890. The only vineyards not to be affected were those planted on sand and in high altitude, the aphid does not survive in these conditions. The country responsible for this disaster was to provide the solution: grafting Vitis vinifera on American rootstocks naturally resistant to phylloxera or having acquired resistance over time. It was noticed fairly rapidly that the American rootstocks did not impart the ‘foxy’ taste found in wines made with American Vitis and that the grapes were roughly equivalent in quantities and qualities to those obtained before grafting.he viticulture was saved, but uprooting the entire vineyards to replant Vitis cultivars on American rootstocks undoubtedly resulted in a further weakening of the genetic material of the plant as it had lost all the benefits of hybridisation that had been carried out before.

 

mildew.jpg

Mildiew

One would have thought that such a warning would have pushed the authorities to try to reintroduce some genetic diversity in the plant through hybridisation. In fact, it is the opposite that occurred, as replanting took place against the background of the development of the productivist agriculture based on the use of chemical inputs which was expected to provide a response to all viticulture problems.    The lengthy and costly hybridisation work was largely confined to the research institutes such as the French INRA, and even the hybrids were  forbidden and uprooted from French vineyards as they did not meet the standards set by the viticulture authorities.

 

Phylloxera_Vigne7.jpg

Phylloxera

Additional factors of weakening the vine.

 

Monoculture 

Contrary to most plants, vines prefer poor soil to give their best results. This feature naturally led to vineyards being established on poorer soil and monoculture developed. This was further facilitated by the fact that monoculture for all plants became the norm with the development of  agro-chemical business industries, but monoculture leads to an imbalance in the ecosystem, which encourages the development of all kinds of pests as they have no predators to compete with.

This is why, today, growers use plants as ‘enticers’ or  ‘refrainers’ to try to establish a more natural environment to control diseases but the system has its limits.

Chemical treatments 

The  Development of the agro-chemical industry initiated with the industrial revolution developed considerably after the two wars in Europe and  chemical treatment became the norm for all cultures, and vines got more than their fair share of chemicals on the whole. Today it is becoming more and more difficult to treat chemically as many infectious agents have become resistant to treatments. Environmental and consumer pressures are also forcing authorities to start rethinking the conventional model of agriculture.

The return towards sustainable viticulture, albeit by a minority of growers at the moment, is a step in the right direction but in no way will it improve the genetic diversity that would make vines more resistant.

More so than in the nineteenth century the vine is a fragile plant, weaknessed exacerbated by the climatic chnages whether anthropic or not and that could be suceptible to pest infestations on the magnitude equal to that of the Phylloxera.

 

NEXT WEEK:   HOW TO REINTRODUCE GENETIC DIVERSITY IN VINE AND MAKE IT MORE RESISTANT.

 



[i] Wan YZ, Schwaninger H., Dan Li, CJ Simon, Wang YJ, Zhang CH, « A review of taxonomic research on Chinese wild grapes », dans Vitis, vol. 47, no 2, 2008, p. 81-88

[ii] Douzan, J.P. Rives M. Sur le déterminisme génétique du sexe dans le genre Vitis. Ann. Amélior, Plantes 17. 105-11 (1967)

[iii] Alleweldt G (1997) Genetics of grapevine breeding. Prog Bot 58:441–454.

[iv] Sean Myles, Adam Boyko, Christopher Owens, Patrick Brown, Fabrizio  Grassi, Mallikarjuna 

Aradhya, Bernard Prins, Andy Reynolds, Jer-Ming Chia, Doreen Ware, Carlos D. Bustamante,

and Edward Buckler. Genetic structure and domestication history of the grape. Proceedings of the National Academy of Sciences (2011)

 


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