Subject: Mistletoe

From: Jeff Wagner <[email-masked]>
Date: Tue, 03 Dec 1996 10:40:24 -0600

Mistletoe is supposed to be a hemiparasite- it has chlorophyll but it
needs to have a host to absorb inorganic nutrients.  Is this correct,
and has there been any speculation about how this line of plants
descended from something else?

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Jeffrey J Wagner; P.O. Box 433, Niwot, CO 80544; 
phone: (303)652-3674; fax: (303) 652-3676; e-mail: [email-masked]
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From: Jan Wesenberg <[email-masked]>
Date: Wed, 04 Dec 1996 11:47:51 +0200

The mistletoe shares this "way of life" (named semiparasitism) with
quite a lot of other plants, many of them rather unrelated to the
mistletoe. This means that the transition to semiparasitism has occured
several separate times in different branches of the angiosperm tree.

The most widely known semiparasitic plants in the northern temperate
regions is the subfamily Rhinanthoideae of Scrophulariaceae, containing
genera such as Melampyrum, Euphrasia, Pedicularis, Castilleja and others
(I don't know the English names). They are herbaceous plants widespread
in northern mountain, forest, meadow and mire vegetation. They are
completely unrelated to mistletoe (that is, they are of course also
floweing plants).

Your question therefore has at least two independent meanings. The first
one is: how did the Mistletoe family arise? This is an ordinary
phylogenetic question, and the answer depends upon the answerer's point
of view upon plant systematics. First one must understand that the
ancestors of almost any modern plant group are extinct and what they
looked like is almost for certain completely unknown. So the only
possible way to search for the origin of a floweing plant family is in
most cases to search for sister groups, not ancestors. The standard
answer to this (apparently unchallanged by new systematic treatments) is
that Viscaceae (the mistletoe family) is closely related to several
other mostly parasitic/semiparasitic families and together with them
constitute the order Santalales. In traditional works Santalales are
regarded part of a large branch of angiosperms called the tetracyclic
rosids. In newer works this large branch is usually regarded an
artificial assemblage. R. F. Thorne (1992 - sorry, this is not his
latest version!), e.g., places Santalales together with the
holoparasitic (nonphotosynthetic) order Balanophorales in a superorder
of their own, Santalanae, and does not indicate the possible
relationships of this to other angiosperm superorders. In the now rather
famous molecular systematic treatment of Chase & al (1993, in Ann.
Missouri Bot. Gard.), Santalales (including Viscaceae) appear as a
sister group to a clade consisting of the caryophyllid families
(carnations, cacti, spinach and others), the Polygonales (rhubarbs,
sorrels, docks) and the insectivorous families Droseraceae and
Nepenthaceae. Together, this large assemblage constiutes one of the
major branches of the cladogram. If this picture will be confirmed by
future research, nobody knows.

In the other meaning, how did semiparasitism arise, it is a different
question. Of course, the transition to semiparasitism involves some
advanced ecological and physiological transformations, including the
ability to search for a host and to attach to it in order to get the
water and minerals from it. Such transformations do of course have a
genetic basis, and must have given the plants an increased fitness,
channeled through population genetic processes in some ancient
populations, in a concrete ancient environment. It is even possible that
several of the necessary steps arose as parts of adaptations to
completely other phenomena than semiparasitism - and only later on
served as preadaptations facilitating the transition. I am not very much
into what is known about this transition, perhaps you will receive
better answers to this than mine. But, on the other hand, there are so
many other strange adaptations among plants, also involving very precise
and advanced transformations (adaptations to full parasitism,
insectivory, to different modes of pollination and seed dispersal, to
arid climates, to aquatic life and so on) - so why not semiparasitism?
In this light it does not seem that strange or unique.

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From: [email-masked] (William H. Reid)
Date: 6 Dec 1996 03:18:51 GMT

> needs to have a host to absorb inorganic nutrients.  

It is, also, no doubt getting water from the tree, as on the local Juniper
there.
Don't know on evolutionary origins speculation, but there is a fair
literature with much onthe (also) highly evolved seed dispersal mechanisms
(explosive release, sticky seeds, etc.). Go over to EPO Biology at C.U.and
check with Yan Linhart or Jane Bock.
Bill
-- 
William H. Reid, Sr. Res. Sci.  <[email-masked]>
Environmental Technology, K6-84   
Battelle, Pacific Northwest National Laboratory
Richland, WA 99352-0999 USA <http://www.pnl.gov>