The genealogy of grape varieties

We are all familiar with the concept of a family tree (formally called a pedigree). People have been compiling them for at least a thousand years, as the first known illustration is from c.1000 CE (see my blog post on The first royal pedigree). However, these are not really tree-like, in spite of their name, unless we exclude most of the ancestors from the diagram. After all, family histories consist of males and females inter-breeding in a network of relationships, and this cannot be represented as a simple tree-like diagram without leaving most of the people out. I have written blog posts about quite a few famous people who have really quite complex and non-tree-like family histories (including Cleopatra, Tutankhamun, Charles II of Spain, Charles Darwin, Henri Toulouse-Lautrec, and Albert Einstein).

Clearly, the history of domesticated organisms is even more complex than that of humans. After all, in most cases we have gone to a great deal of trouble to make these histories complex, by deliberately cross-breeding current varieties (of plants) and breeds (of animals) to make new ones. For plants, we have also propagated genetic variants, thus making an even more complicated history (see my post Grape clones and varieties are not always what they seem).

In most cases, we have no recorded history for domesticated organisms, because most of the breeding and propagating was undocumented. So, the idea of reconstructing these histories is very different from the popular pastime of doing a bit of "family history research". Indeed, until recently it was effectively impossible. This has changed with modern access to genetic information; and there is now quite a cottage industry within biology, trying to work out how we got our current varieties of cats, dogs, cows and horses, as well as wheat, rye and grapes, etc.

Grapes

There are a number of places around the web where you can see heavily edited summaries of what is currently known about the grape pedigree. However, these simplifications defeat the purpose of this blog post, which is to emphasize the historical complexity. The only diagram that I know of that shows you the full network is one provided by Pop Chart (The Genealogy of Wine), a group that provides infographic posters for just about anything. They will sell you a full-sized poster of the pedigree (3' by 2'), but here I have provided a simple overview (which you can click on to see somewhat larger).

You can actually zoom in on the diagram on the Pop Chart web page to see all of the details. This allows you to spend a few happy hours finding your favorite varieties, and to see how they are related.

Perhaps the most interesting thing to note is just how many modern varieties have what the French call Gouais Blanc as one of the parents. This white variety is actually an indigenous Croatian variety known there as Štajerska Belina. Try to remember this the next time you drink a glass of Chardonnay or Gamay, both of which are crosses between this variety and Pinot Noir.

Why are there so many missing bits?

Reconstructing grape genealogies is often a tricky business, even with genetic data. Originally, this was done using morphological characters (the study of grape morphology is called ampelography), along with whatever historical records exist. However, these days we use DNA from whatever varieties and cultivars are available for sampling (see my post Grape clones and varieties are not always what they seem). Perhaps the biggest problem is that many of the cultivars are no longer known (there have been at least 10,000 of them recorded at some time in history), so that the pedigree is full of question marks representing unknown (unsampled) parents.

This has an odd practical consequence, which is that the time direction of the pedigree will be ambiguous whenever there is a missing parent. That is, we cannot work how which is the parent and which is the offspring among closely related varieties. Technically: estimates of what geneticists call identity-by-descent (IBD) are calculated based on the technique of linkage analysis for all pairwise comparisons of samples. However, complex inter-breeding schemes can generate IBD values such that we cannot distinguish parent-offspring relationships from sibling (brother-sister) relationships.

There are many examples of this problem in the study of the grape genealogy. As but one example, there is an online page associated with the book Wine Grapes (by Jancis Robinson, Julia Harding and José Vouillamoz). One of the web pages shows some of the pedigree diagrams from the book, and on this page there are three possible pedigrees for Syrah. The current genetic data cannot distinguish between these three possibilities (shown here).

This web page also happens to illustrate another aspect of drawing complex pedigrees. Sometimes, in order to avoid drawing intricate networks, and to make the diagram look more like a "family tree", authors resort to a standard subterfuge. The strategy is to show varieties multiple times in the diagram, to avoid drawing reticulate relationships (see my post on Reducing networks to trees, where I illustrate this using the pedigree of Zeus). This situation occurs in the pedigree for Pinot Noir (the authors note: "For the sake of clarity, Trebbiano Toscano and Folle Blanche appear twice in the diagram").