Sauvignon Gris, or why are some grapes pink? - Maipo Valley, Chile

A few days ago I wrote about the Tempranillo Blanco grape and the genetic underpinnings of white berry color in grapes.  In that post, we discovered that nearly all grapes are black in the wild and that while there are a series of genes that are responsible for creating and regulating the pigments (anthocyanins) in dark-berried grapes, a disruption in two of those genes (VvmybA1 and VvmybA2) is responsible for nearly all white-berried grapes that we enjoy today.  In short, a mobile genetic element (a retrotransposon called Gret1) jumped in front of the VvmybA1 gene hundreds of years ago and disrupted its function.  There was another mutation at some other point that also damaged the VvmybA2 gene, which created a recessive allele set with the potential to create white-berried vines.  A vine carrying this recessive allele was probably self-pollinated, and one or several of the seedlings carried both copies of the allele and a white berry was born.  It is estimated that 95% of white grapes today possess this mutation, which may indicate a common origin for all of them.  The story is more complicated (and interesting, I would argue) than the brief summary above, so I'd encourage you to read my prior article before continuing.

As you can see from the picture above, grapes don't just come in black/blue/purple and white/green.  There are also pink grapes, like Pinot Gris, Grenache Gris, Roditis and Gewürztraminer, to name a few.  Most of the pink-berried vines that we are familiar with are berry color mutations of other vine, meaning basically that the vines that have a pink-berried mutation are also often found in both white and dark-berried form as well.  Knowing how we can get from dark-berried vines to white-berries ones is interesting, but it doesn't really give us a lot of information on how pink-berried vines may come to be.  We know that if a vine has at least one working copy of the VvmybA1 gene, meaning even if it has one copy of the white-berried mutation, then the berries are dark but if it has two copies of the Gret1 insertion which deactivates VvmybA1, then the berries are white.  There doesn't seem to be any way to get to pink with this mechanism.

While this particular set of mutations isn't able to tell us why some berries are pink, it does at least give us a clue as to where we might look.  There are several VvmybA genes in sequence, and it is thought that these genes together are responsible for a lot of the anthocyanin regulation in grapes.  The Gret1 mutation effectively blocks all anthocyanin production not because VvmybA1 is the only gene responsible for grape skin coloration, but because it is the first gene in the sequence, and when you knock out the first gene in the sequence, the sequence never starts and anthocyanins are not produced at all.  Variations in VvmybA1 that do not completely shut the gene down and variations in the downstream genes are the most likely candidates to be responsible for berry color variation.  A study done in 2009 (citation 1) seems to indicate that this is true.  The statistical modeling in the study found that just a few differences in the genes VvmybA1, VvmybA2 and VvmybA3 accounted for 84% of the total variation in berry color.

Some of the mutations in this gene cluster are caused by the Gret1 mutation jumping back out of its spot in front of VvmybA1.  Remember that Gret1 is a mobile DNA element that is capable of jumping around in a genome, meaning it can not only jump into a spot, as it did to create this mutation, but it can jump out again as well.  Just as white-berried vines can suddenly appear in a vineyard planted to dark-berried vines, so can dark-berried vines show up in vineyards planted to white berried vines.  When the Gret1 jumps back out of its position in front of VvmybA1, it leaves a little bit of itself behind and while this little bit isn't enough to deactivate the VvmybA1 gene, it does sometimes inhibit it a little bit.

In 2009, a Japanese research team examined two red-berry sports of a normally white-berried grape called Italia (citation 2).  One of the grapes, Benitaka, was darker  and had more anthocyanins in its skin than the other, Ruby Okuyama.  The team found that in the lighter grape, Ruby Okuyama, the Gret1 transposon had hopped out and left a little bit of itself behind.  VvmybA1 functionality was restored, but in a limited capacity and Ruby Okuyama grapes are a light pink color as a result.  In the darker grape, Benitaka, the Gret1 transposon was actually still present, but it was farther away from the gene than in white-berried grapes.  What had happened was something called Double Strand Break Repair, which is a complicated process that I don't want to spend too much time on, but in brief, what happens is that a piece of DNA breaks off and is repaired by a another piece of DNA from the matching second chromosome set.  In this case, what happened was that part of the sequence between Gret1 and the VvmybA1 gene broke away on one chromosome and was repaired by a bit of DNA from between an area in front of VvmybA3 and into that gene from the other chromosome.  This "repair" essentially restored the gene's functionality and pushed Gret1 far enough away that it couldn't interfere in the gene stream anymore.

This is one explanation for pink-berried grapes, but it isn't the only one.  One of the more interesting explanations comes from a study done on Pinot Gris grapes (citation 3).  That study found that several Pinot Gris clones were actually chimeras.  Longtime readers of this site may remember a prior discussion of chimeras in the context of Pinot Meunier.  Essentially, a chimera is an organism which has cells with different DNA growing within it.  In nearly all organisms, every cell that the organism possesses has the same DNA.  Different genes are active in different cells, but the overall genotype is identical in every cell.  Chimeras have cells with different genotypes coexisting in the organism.

Here's how it happens.  The growth regions of plants, like buds and shoots, are called meristems and the meristem is divided into 3 cell layers.  The cells in the meristem are something like stem cells in human beings, in that they do not have any specific function while they are dividing, and become differentiated cells at a later point.  In a chimera, a cell in one of these layers undergoes some kind of mutation that makes its DNA different than the cells around it.  This mutated cell divides and becomes a part of the plant.  Chimeric mutations do not always lead to visible changes in the plant, but those that don't are typically not discovered. Chimeric mutations are not passed along to offspring, but they can be vegetatively propagated, or cloned, so chimeric mutations in grapes are passed along as cuttings from the vines are planted.  Those interested in reading more should check out this site, which is informative and fairly easy to understand.

In the study referenced above, the authors analyzed a few different clones of Pinot Gris and found that several (but not all) of them were chimeras.  They decided to self-pollinate two of these chimeric vines in order to see how the plant's DNA  was passed along to its offspring.  What they found was that the plants that grew from every single one of the seedlings had white berries instead of pink.  They also noticed that on some of the berry clusters from the original vines, there were berries that were half white and half pink!  It seems like what is happening is that Pinot Gris started out as Pinot Blanc and at some point, a chimeric mutation happened on the Pinot Blanc vine and the mutation had the allele to create red pigment.  Both cell types are in the berries with some of them creating unpigmented cells and others creating pigmented cells.  Together, these two cells create the appearance of pink or grey berries.  Chimerism doesn't explain all pink-berried vines, but it does seem to explain at least one mechanism for pink-berried vines that does not directly rely on the VvmybA gene group (though this group is indirectly involved via the unpigmented cells).

All of which, finally, brings us to Sauvignon Gris.  Sauvignon Gris is a pink-berried mutation of Sauvignon Blanc, though which mechanism discussed above is responsible for the pink skins is something I've not been able to figure out.  It is currently grown in Chile, Australia, New Zealand and France.  It is becoming more popular in Bordeaux in recent years and current plantings there stand at just over 330 hectares.  There is a story that the grape was virtually extinct until it was rediscovered in the 1980's by Jacky Prey in the Loire Valley, who calls it Fié Gris.  Fié is an old synonym for a particular clone of Sauvignon Blanc that was notoriously low-yielding and was thought to have been wiped out by phylloxera.  It's a nice story, but I'm not sure if I buy that Prey's discovery is the source for all of the Sauvignon Gris grown in the far corners of the world today, as it looks to me like the grape was sent to Chile and Australia before the 1980's.

I was able to pick up a bottle of the 2009 Cousino-Macul Sauvignon Gris from the Maipo Valley in Chile for around $12 from my friends at Bin Ends.  In the glass, the wine was a medium silvery lemon color.  The nose was fairly lackluster with some subtle white pear aromas and a faint whiff of peach of and grapefruit citrus.  On the palate the wine was medium bodied with fairly low acidity.  There were round white peach flavors along with some white grapefruit, grapefruit peel, and white pear.  The character of the wine was broad and fat, which surprised me given the acid levels in most Sauvignon Blanc based wines.  The finish was bitter, chalky and pithy and not really all that enjoyable. There were some similarities to Sauvignon Blanc in this wine, but not enough of them.  It's certainly possible that this wine may have been a little past peak, but I really expected it to still be showing decently since I don't think I've ever had a Sauvignon Blanc wine shut down after only 3 years in bottle.  I'm not ready to write the grape off after this experience, though, and if I come across a younger example or one from Bordeaux, I'll definitely post a note below.

Citations

1)  Fournier-Level, A, Le Cunff, L, Gomez, C, Doligez, A, Ageorges A, et al.  2009.  Quantitative genetic bases of anthocyanin variation in grape (Vitis vinifera L. ssp sativa) berry: A QTL to QTN integrated study. Genetics.  183, 1127-1139.

2) Azuma, A, Kobayashi, S, Goto-Yamamoto, N, Shiraishi, M, Mitani, N, Yakushiji, H, & Koshita, Y.  2009.  Color recovery in berries of grape (Vitis vinifera L.) 'Benitaka,' a bud sport of 'Italia,' is caused by a novel allele at the VvmybA1 locus.  Plant Science.  176, pp 470-478.

3) Hocquigny, S, Pelsy, F, Dumas, V, Kindt, S, Heloir, M-C, & Merdinoglu, D.  2004.  Diversification within grapevine cultivars goes through chimeric states.  Genome.  47, pp 579-589.