Ghost pipe

I discussed symbiosis last week and mentioned the mutualistic relationship between a plant (Trout Lily) and an animal (an ant). There are, of course, other sorts of symbioses. Commensalism occurs when one participant benefits while the other is not harmed. And relationships are parasitic when one organism benefits, to the detriment of the other.

Ghost Pipe belongs to the genus Orobanche. This innocent looking woodland herb makes its living by parasitizing other plants. Individuals are entirely without chlorophyll and are not, therefore, capable of photosynthesis. They have modified roots, called haustoria, which attack, penetrate, and link to the plumbing (xylem (water supply) and phloem (nutrients)) of the host. Individuals are entirely dependent and are obligate, holoparasites.

If we assume that the progenitor of the genus was photosynthetic, I wonder how parasitism of this sort developed. It is difficult to imagine that a single mutation could have given rise to holoparasitic plants because all of the photosynthetic machinery would have had to have been be lost (mutationally) and, at the very same time, all of the machinery required for parasitism would have had to form. Although this seems unlikely, there are those who would argue that novel adaptive strategies can happen in this way, and all at once. Be that as it may, I imagine a slower, less hurried, move from one strategy to the other. I imagine that a fully photosynthetic ancestor gave rise to plants like Orobanche, but only through a hemiparasitic intermediate such as Mistletoe.  Although hemiparasitic plants are capable of providing nutrient via photosynthesis, they tap their hosts for water and mineral support. From there, the mutational jump to holoparasitism would seem more plausible.


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