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Evolution of Lysichitum

  • Subject: Evolution of Lysichitum
  • From: "mossytrail" <mossytrail@hctc.com>
  • Date: Thu, 13 Mar 2008 18:02:07 -0800

I was in the wetlands today.  Lysichitum americanum was
abundantly in bloom.  I found one group with all-green
spathes.  Now that is probably not so unusual in itself -- a
spathe, after all, is not so very removed from a leaf.  But
still, most examples do have the clear, bright yellow
spathe.  These green-spathed ones appeared at first glance
not to be in bloom.  It made me wonder: what is the relative
importance of spathe color vs. plant odor in attracting
pollinators?  I am not even certain what the pollinators are
in this case -- perhaps the delicate, long-legged gnats I
saw in a few inflorescences?

As we know, the other member of the genus, L.
camtschatcense, has white spathes.  Presumably, it, too, is
quite capable of producing abnormal green-spathed forms. 
They would be a different shade of green -- a plain green
infusing an otherwise-unpigmented spathe, as opposed to the
green overlying yellow of L. americanus.

This started me pondering.  Presumably, either L. americanus
was the evolutionary ancestor of L. camtschatcense, or their
common ancestor also had a pigmented (probably yellow)
spathe.  This is simply because it is easier for a colored
plant part to lose its pigment, than for a white plant part
to evolve pigment.  Most likely, L. americanus will produce
an occasional "albino" or white-spathed individual; but the
likelihood of L. camtschatcense producing a yellow-spathed
individual would be far more remote.

There is of course no way to know the spathe color of the
fossils L. washingtonensis and L. nevadensis (and in any
case, these are only tentatively identified as Araceae), but
(assuming they are in fact of this genus) at least one is
likely to have had a yellow spathe.

As I continued pondering, I wondered where the genus
originated, and where it differentiated into the extant
species.  If the aforementioned fossils are valid, they
suggest a North American origin, which would be consistent
with the spathe colors: L. americanum having the primitive
character and being geographically nearer the point of
origin, L. camtschatcense having the derived character and
being in the geographically remote range.  One possibility
is that the white-spathed population had some ability to
spread to Asia that the yellow-spathed plants lacked.  On
the other hand, it is possible the genus had reached Asia
before differentiating, and that the Ice Age glaciations
then isolated the Asian population and allowed it to
speciate in situ.

The question then becomes, if we suppose the white spathe
was originally an abnormality in an otherwise yellow-spathed
population, what caused it to become the normal condition in
the one species but not the other?

An interesting aside: inside the spathes, I found small true
bugs which may have been Miridae.  If so, this is a parallel
to the Xanthosoma inflorescences I sudies in Monteverde,
Costa Rica, which had their distinctive Miridae communities
inside the spathes.  What is it with Miridae and Araceae
spathes?

Jason Hernandez
Naturalist-at-Large
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