Sciency Answers: Variegation Part 1

Henry has a question about variegation:

I would like to ask you a question about variegated Hippeastrum reticulatum. Usually variegated gene is recessive but why the variegated mid-rib of
hippeastrum reticulatum is dominant in all its F1 hybrids, and
successive breeding dilutes the character from white mid-rib to yellow
mid-rib?

Looking forward to your sciency answers !!!

Well! I'm going to have all sorts of fun with this. I hope you are all ready, because the world of variegated leaves is cool and crazy.

First, some pictures to illustrate what he's asking about. This is the beautiful foliage of Hippeastrum reticulatum with distinctive white stripes down the center of each leaf:

And here is an example of a more typical sort of variegation, as seen on a Clivia leaf:

The word variegation covers both these types of leaf patterns. In fact, the most basic defintion of the word courtesy of Miriam-Webster is “Having discreet markings of different colors.” That could be applied to bicolored flowers, spotted cows, or striped curtains. But when someone says a plant is variegated, they almost always mean that it has some kind of white or yellow (or rarely other colors like pink) pattern on the leaves. So though the dictionary would tell you you are correct if you called the blotch in the center of a pansy flower variegated, say that to another gardener, and they'll be looking for white on the leaves.

But even narrowing down the term to white (or yellow or maybe pink) patches on leaves there are still a lot of different sorts of things being called variegation. 

Most variegations in your garden are what I'd call non-adaptive variegations. Some random mutation that would never survive in the wild and only persists because we think it is pretty. A classic example would be hostas. Visit wild hostas in the forests of Japan, and you'll see solid green leaves. The various white and yellow patterns so familiar to us in our shade gardens are man-made (or at least, man-preserved) and if humans went extinct, would die out almost at once. My second picture, with the white and green steaked leaves would be another example of this type of variegation.

There are, however, some wild plants that have what I'd call adaptive variagation, white or other colored patches on their leaves that help them make a living in the world. I've talked before about the example of wild Caladiums which apparently use their white variegation to trick insects into not eating them.

The difference between adaptive and non-adapative variegation is like the difference between a very pale white person from Norway, and an albino from Africa. 

A blond Scandinavian

An albino african

Pale skin in Northern Europe is the result of adaptation over time to low UV light levels, allowing people to get sufficient vitamen D during nearly sunless winters, and is caused by many different genes interacting to lower skin pigmentation. Albinoism, on the other hand, is a simple mutation in a single gene that knocks out the production of Melanin, and isn't at all beneficial in a region with intense sunlight.

The two types act differently when it comes to the next generation as well. The children and grandchildren of an albino will either be completely albino, if they ended up with two copies of that gene, or dark if they don't. It is a recessive gene, like the recessive gene for most non-adaptive variegations described in the question.

The descendants of a marriage between a Norwegian and an African, on the other hand, will have many different skin colors. Dark skin tones tend to be dominate in the first generation (just as Henry sees the white midrib as dominate in the first generation of crosses with Hippeastrum reticulatum) but in subsequent generations, we don't see white or black, but a whole range of beautiful colors (just as Henry sees the yellow mibribs in subsequent generations) as the many different genes controlling skin tone get arranged into new combinations each generation.

But... this just scratches the surface of the fascinating world of variegation. After all, an albino human has a mutation that makes it impossible for their body to produce melanin, they aren't variegated with blotches of different colors. A plant with the equivalent mutation would produce no chlorophylle at all, which would quickly make it dead. So how do plants get white leaf edges and green centers, or an mix of green and white? The adaptive variagates do it through having a bunch of genes that carefully contol where to express pigment and where not to. For the non-adaptive variegation, it gets very strange and very cool... So tune in next week for Variegation Sciency Answers Part 2, featuring mythical beasts and jumping genes!