Why Do We Have White Grape Varieties?

Is your favorite wine a Chardonnay, Riesling, or one of our favorites, Chenin Blanc? I did not know this until I took the U.C. Davis online course, that if it was not for a certain genetic phenomenon, the world would be full of only red grape varieties. I looked into this a little further and learned some interesting factoids.
First of all, the red color in grapes is attributable to the anthocyanins located in the grape berry skin.¹
Just as I did when blogging about red wine color, I'm going to break up this blog about white grapes into little pieces otherwise the blog will become too long and difficult to follow.
So, to recap: The production of anthocyanin, which is responsible for red color in wine is after the enzyme leucocyanidin dioxygenase or LDOX. In the figure below, the expression of mRNAs homologous to each of the anthocyanin biosynthesis genes was determined in the white varieties Riesling, Muscat Gordo, Semillon, Chardonnay, and Sultana as well as the red varieties Cabernet Sauvignon, Pinot Noir and Shiraz using northern blot analysis.¹

The northern blot experiment clearly showed that none of the white varieties expressed anthocyanins as shown by the lack of expression of UFGT (UDP glucose-flavonoid 3-o-glucosyl transferase. Furthermore, there was a difference in expression of genes in the white varieties between Riesling, Semillon and Chardonnay versus Muscat Gordo and Sultana.
When this paper was written in 1996, the authors speculated that:

• It is quite likely that the white grape has arisen independently a number of times from different red varieties
• The mutations could be different in each case so it is not surprising that patterns of gene expression were different in the various white varieties
• Loss of anthocyanin synthesis can arise by mutations in individual structural genes in the flavonoid pathway
• Loss of anthocyanin synthesis can arise by alterations of regulatory genes controlling expression of a number of the structural genes

In the world of scientific investigation, this is ending the experimental work in a mystery. Stay tuned for updated experiments that give more insight into why white grape varieties exist in our world today.
References:
1. Boss P, Davies C, Robinson S (1996b), "Expression of anthocyanin biosynthesis pathway genes in red and white grapes", Plant Mol Biol, 32:565 - 569.
2. Lijavetzky et al., 2006, Mol. Genet. Genomics, 2006: 427-435.
3. José Tomás Matus, Felipe Aquea and Patricio Arce-Johnson, BMC Plant Biology, 2008, 8:83.
4. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.