Mistletoe-a parasite?! The beautiful holiday spirit of romantic kisses-a parasite?! I couldn't believe it when my wife Julie pointed out an ugly tangle in a treetop. I'd assumed it was a crow's nest. Somehow, my mental model (completely based on a total lack of real information), was of a robust, yet beautiful climbing ivy full of European romance and charm. Of course, Julie is right. So I thought I'd like to know more about it and plant parasitism, in general.
What we think of as mistletoe from myth and fable, is European Mistletoe or Viscum album. In fact the "Golden Bough" of Greek Myth was probably Viscum. It is a member of the Santalales order and specifically the family Loranthaceae. Originally, all mistletoes were contained in the family Loranthaceae, but they have now been expanded into three additional families in the same order (Misodendraceae, Eremolepidaceae, Santalaceae). The largest family of mistletoe, Loranthaceae¸ contains over 900 species, many in South America, Australia and New Zealand.
Mistletoes can live in a wide range of host trees or shrubs. Normally they are able to moderate their growth, but under some conditions kill their hosts by heavy infestation. They are spread from tree to tree by birds carrying their seeds; the seeds germinate in the bird poop. Most species of mistletoes are pollinated by birds, but a few are pollinated by insects. When one of the sticky seeds of the comes into contact with the bark of a tree, it sends out a thread-like root, flattened at the end. This pierces the bark and roots itself in the wood, from which it has the power of selecting and appropriating all the juices it needs for its own sustenance - all its nourishment is obtained from its host. The root becomes woody and thick. Traditionally, mistletoes are thought of as pests, but a more recent view recognized the ecological niche that they fill as a food source for many species of birds and insects.
The berries of mistletoe are poisonous. However, as with many herbal toxins, they have found uses in traditional medicines for controlling blood pressure, epilepsy and internal hemorrhaging. There is a commercial "alternative" cancer treatment based on mistletoe, "Iscador", that was used by Suzanne Somers instead of a conventional chemotherapy. However, independent clinical trials does not support its use or efficacy.
So here's the interesting part. Parasitism has evolved independently nine times in the plant kingdom and five of those are mistletoes! Five independent evolutionary paths -amazing. Of course, it is not a coincidence that the products of similar evolutionary paths been grouped together into similar families.
In the plant kingdom, parasites are classified in three different dimensions. Parasites are other Obligate or Facultative; depending on whether the require a host or can complete their life cycle without a host. A parasite can be either a Stem or Root Parasite. Finally, a Holoparasite is completely parasitic and has no chlorophyll of its own while a Hemiparasite is normally parasitic, but is capable of photosynthetic energy production on its own. So Mistletoe is an obligate stem hemiparasite.
One of the odd idiosyncrasies of the biological literature is the way the mistletoe lives is called its "Habit". I presume there are other types of parasitic habits, as well. How did this "habit" evolve? How did mistletoes become aerial parasites? Two scenarios seem possible. Firstly, they could have climbed trees first, then become parasites. In other words, mistletoes could have evolved from free-living ancestors which were "epiphytic" on tree branches in humid forests. An epiphytic plant is a plant that derives moisture and nutrients from the air and rain; usually grows on another plant but is not parasitic on it. Alternatively, they could have become parasites first, then climbed trees. In other words, mistletoes could have evolved from terrestrial root-parasites by somehow adapting as parasites on tree branches. Apparently, most evidence favors the second hypothesis; that is, root parasites learned to climb trees.
Given this conclusion, there are further questions of more specific evolutionary paths. How did ground plants get up into the branches? One possibility is that they could have proceeded through a vine-like intermediate that gradually left the ground altogether. Alternatively, a more direct route could have been followed in which seeds developed the ability to germinate on the branches of host trees. Again, most evidence favors the second conclusion.
More to the point, why do we kiss under the Mistletoe? The reason is based on a relatively bizarre (as myths go) story from Norse Mythology-filtered through Druid rights. Loki (that Joker!) tricked a blind god named Hod into killing Baldr with an arrow made of mistletoe. Through a convoluted series of magical interludes, Baldr was bought back to life. Frigga, Badr's Mother and Norse Goddess of Beauty, was so happy and thankful that she made the mistletoe sacred and commanded humans to kiss whenever they passed beneath. As with many religions and mythologies, the logic's a bit sketchy-just take advantage of the opportunity and enjoy the kisses.
While I'm on the topic of parasites, I might mention that Castilleja or Indian Paintbrush is an obligate root hemiparasite.
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