Peloric Flowers and Orchid Evolution

If you browse for novel orchid flower types for sale or for fun, you will occasionally run across "peloric" flowers. Peloric in botany means abnormally regular or symmetrical, and that's what these flowers tend toward. To explain how and why, I need to talk about orchid evolution and the genetics of flower parts. Though the genetics of orchid flower parts has not been studied in detail, we can make reasonable inferences from studies of other flowers that have been studied.

Orchids originated from a group with flowers with 3 identical sepals and 3 identical petals. The sepals and petals were 3-fold symmetric and each formed a "whorl" in the developing flower bud, the sepals forming the outer whorl and the petals the inner whorl. (Some plants have additional symmetric inner whorls that form the stamens and styles, which contain the pollen and female receptive organs respectively. In orchids these parts are transformed almost beyond recognition and form the single column.)  One of several evolutionary innovations in the orchids was the breaking of the 3-fold petal symmetry, such that one of the petals became the labellum (lip) of the flower. In many orchids, the 3 sepals remain nearly identical, though this symmetry is also broken in some specific groups. Typically, in currently extant orchids the lip is dramatically changed in shape and color, but usually the lip plus the two petals retain 3-fold positional symmetry.

Phalaenopsis Fantasy Musick, comparing normal flower form (left) with strong petal to lip peloric form (right)

Peloric flowers most often arise when the two petals acquire some or all of the character of the labellum, resulting in a flower that is more symmetric than normal. In extreme cases, the two petals look nearly identical to the lip, giving rise to a true 3-fold radially symmetric flower with respect to both petals and sepals (photo above). More often, the two petals acquire bumps, deformations, or colors that suggest some characters of the labellum, but the transformation is incomplete. Sometimes the "peloric" changes are so slight that it is seems to me to be a guess that they have become lip-like and the flowers certainly are not radially symmetric. In rare cases, the peloric transformation is the reverse - the lip becomes more like the petals (photo below). These are peloric in exactly the sense as the more common type - the flower approaches 3-fold radial symmetry - though the transformation is reversed and the visual effect is very different. All of these changes are probably due to mutations in the genome because they are stable in a given plant, though the best test is to make crosses and flower the progeny and I am not aware of any such data.

Dendrobium Kuranda Classic x Classic Gem individual showing strong lip to petal peloric form

Often flowers in which the petals look odd in any way are called "peloric", but this isn't strictly correct unless some element of additional floral symmetry results. The genetic basis of such changes might be partial petal to lip transformations, or they could simply be changes in genes that control petal form (and have nothing to do with the lip). Nevertheless, a lot of these odd petals do look reminiscent of the lip on the same plant and I think extending the meaning of peloric to these is sensible. Examples of all types are easily observed by a web search for "peloric orchid". 

What does this tell us about orchid flower development? Now I am only guessing, but I think these are educated guesses. First, the genes that determine lip shape have been added on top of an originally three-fold symmetric petal whorl. Second, since the most common peloric type causes petals to look more like lips, the lip-shape genetic program has the potential to be expressed at an early stage in petal development, but it is normally shut off in petals. Mutations that partially or fully prevent this shut off give rise to the common peloric type. Third, it is possible, though rarer, to acquire mutations in the genetic program that specifies the labellum shape, so that the lip becomes more petal like. Why these are rarer I don't know, but it suggests that the number of possible mutations of the latter type is smaller than the former (in genetics jargon, the "target size" is smaller).

I don't find peloric flowers to be particularly attractive, but they are interesting and someday perhaps we will understand in detail which genes are affected and how. The genes involved in controlling flower parts are well understood in model plants such as Arabidopsis thaliana, and the same sorts of genes are probably responsible for flower parts in orchids. Orchids are a poor model system for geneticists because they grow so slowly, but they have one huge advantage - thousands of people all over the world are growing millions of orchids and keeping an eye out for strange flower forms. These might form the basis of effective inference about the nature of these genetic changes at some time in the future, probably through a lot of genome sequencing combined with a few genetic crosses.