Supine, Resupine, and Reresupine

One of the many lines of evidence supporting the ancient origin of life and its evolution by modification and natural selection is the pervasiveness of workaround solutions in biology. For examples: land vertebrates breath and swallow using the same tube (the 3,000 US citizens who die each year from choking on food unanimously agree with me that this is a terrible design); the blood supply and connective tissue in the vertebrate retina is in front of the light detecting cells; wisdom teeth; the panda's thumb; and a zillion less familiar examples.

Orchids also provide examples, perhaps the best of which is the twisting of flowers during bud development to situate the lip of the flower either above or (more commonly) below the rest of the flower. The lip is a highly modified petal that ancestrally was in the dorsal position (at the "top" of the flower). A great deal of the diversity of orchid pollination depends on elaborations of the amazing lip structure, but that is another story. Some orchids position their lip at the top of the flower (e.g. Prosthechea cochleata, below), but most position their lip at the bottom of the flower to act as a gravity-assisted landing pad for their pollinators. 

Prosthechea cochleata, from Wikimedia Commons

This "lip at the bottom" form is called resupinate (meaning literally: again brought to lie on the back with face up). If you look closely at the stem holding your favorite orchid flower you may be able to make out how this happens: the lip starts out dorsally (when the bud has just begun to form) and the flower stem twists around 180° before opening. Many orchid flower stems have ridges that make this apparent in the mature flower, obvious in the flower facing left in the photo below. The twist is usually most obvious near the base of the stem so you may have trim away the bract covering the stem base to see the twist. (Unfortunately the common Phalaenopsis that I have inspected have nearly perfectly cylindrical flower stems so the twist is very hard to see.) This is The Exorcist head-twisting approach to lip positioning.

Anacamptis picta var. alba from wildnaturespain.blogspot

Okay, so far this is fairly strange - the lip forms on the top of the flower and then the flower twists around to get the lip on the bottom where it has to be to facilitate pollination. But here is the kicker: some orchid flowers (e.g. Angraecum superbum, also called A. eburneum) that are non-resupinate (lip at the top) start with the lip at the top, then twist it around 360° as the flower grows! Let me restate this bizarre fact: the lip starts where it should be, then twists around to where it shouldn't be, then keeps right on twisting around until it is back exactly where it started. How did this happen? Nobody is sure, but most likely these species have ancestors that are resupinate (lip at the bottom) but something changed that made it advantageous for the lip to be at the top. There are two ways to adapt to this change: eliminate the twist altogether, or continue the twist for twice as far. Apparently the second possibility is what actually happened, at least in some cases - what Steven Jay Gould called contingency or historical contingency. Charles Darwin noted the 360° twist in Angraecum superbum and used it as an argument for evolution (not natural selection necessarily, but an historical process evident in current life).

It would be interesting to find a study of what fraction of non-resupinate flowers have no twist vs. the double twist. Anybody known of cases? I have not heard of a flower that is resupinate with a 1.5 twist (540°) - it would presumably have evolved from an ancestor that had 1 twist, which had an ancestor with 0.5 twists, which had an ancestor with no twists. Vizzini of The Princess Bride would have loved that one if he had survived the poison duel.