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  Lemna and other duckweeds
From: "Eduardo Goncalves" edggon at hotmail.com> on 2000.06.21 at 01:45:33(4863)
Dear Aroiders,

Since I received some private messages concerning my last not-so-funny
joke, I think I should present this aspect to the whole list. Some say they
are shooked-up about the fact that Amorphophallus and Lemna being close
relatives. Others corrected me, saying that Lemna is a genus of Lemnaceae
(duckweed family), not an aroid. Ok, I have a bombastic new to the members
that have been sleeping for the last five years. Yes, all duckweeds are true
aroids! In fact, duckweeds can be better considered like an aroid than
Gymnostachys, Orontium and Symplocarpus! I will try to explain it (before
you try to beat me)...
Taxonomy has suffered an irreversible micro-revolution in recent
years. Since Linnean times, taxonomy deals exclusively with morphological
aspects like shape of organs, color, etc. The taxonomic dataset has been
improved by plant anatomy (or micro-morphology), chromosomic features and
phytochemistry, but all of this aspects concern phenotypic information.
After the discovery of the structure of the DNA by Watson & Crick, we became
able to take a closer look to the essence of the living beings, i.e. we can
'read' their genes. It is true that it is not so simple to access and
compare genes, but it is potencially a very powerful tool.
Lemnaceae has been considered a "good" family for much time, since
they are all free floating plants, with similar reduced flowers. Until very
recently, most taxonomic treatments include Lemnaceae as a distinct family,
always based on phenotypic features. If I am not confused, in the book
"Families of Monocotyledons", R. Dahlgren and colleagues included Lemnaceae
in the order Arales, together with Araceae.
In French's work (with collaborators) concerning the cladistic
analysis of restriction site changes in Araceae, the genus Lemna was
included. Interestly, it appeared well nested within the "advanced" aroid
genera, being closer to a "twig" that includes Amorphophallus, all
Caladieae, all Areae and all Colocasieae. Just for information, the "twig"
with Gymnostachys, Orontium and Symplocarpus ("Proto-Aroids") are very far
from it, and seems to be almost as a syster family. The same you can see if
you analyse the sequence of the genetic marker rbcL. These data with rbcL
were not published, but I used the sequences I imported from GenBank, just
for fun. I surveyed Lemna and Spirodela (both from the Lemnaceae) and both
appeared like "advanced" aroids. Once again, the Proto-Aroids appeared so
related to the other aroids as a Potamogeton I used in this analysis! This
is to show you that if you put Lemnaceae as a distinct family, you should
also separate Gymnostachys, Orontium and Symplocarpus in a distinct family,
and probably a miriad of other "small families". I don't think it would be a
good idea, since I love aroids with all currently recognized genera!
I know that Lemna and all the other genera in Lemnaceae don't look
exactly like an aroid. They are a very specialized group of free-floating
plants, with very fast vegetative reproduction. The reduction in their
reproductive organs (maybe because big flowers make small plants to sink in
the water) seems to be strongly adaptative, and resulted in the poor
recognition of these plants like true aroids. However, we can't say that
they are not aroids only because WE couldn't recognize them before! If we
want a taxonomy based on the evolutive history, I think we should consider
those diminute duckweeds like true aroids, just like we recognize an
outrageous A. titanum! Ok, who will be the fist one to shoot me?

Best wishes,


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