Pat Brennan wrote:
Hi Joanna,
If you look at a plant's DNA only a small percent will actually be part of a
gene and thus only a small percent is used in the coding for protein. If you
look at a chromosome (a DNA strand) it looks something like:
. . . "tr a s h" . precursor . gene . " t r a s h" . precursor .
gene. . . .
Actually, in plants it looks more like this:
"trash".precursor.part1ofgene.junk.part2ofgene.jun k.part3ofgene.junk".
There is a whole lot of nothin' that breaks up the actual coding parts
of a lot of genes. Those are called introns, and I don't know that
anybody knows what they are for, either. Bacteria don't have them and
they get along just fine.
About ten years ago (I am very out of date) I attended a seminar discussing
this "trash" DNA. (If they can not explain it, it must be trash --ha) One
theory was tied to evolution where some of the trash contained coding used
by ancient ancestors and no longer used. The same theory speculated that
coding that will be used in some future evolved generation is sitting in the
trash waiting to be turned on. Thus if a flowering plant evolved into a non
flowering plant, the non flowering plant could still carry the coding for a
protein required in a flower.
That was a theory at one point, and there may still be some validity
in it. Now it appears that the 'trash' DNA isn't old coding sequence.
Or at least most of it isn't. There are things called pseudogenes which
are copies of old genes which are no longer functional, presumably you
could get a back mutation in a pseudogene to make it functional again,
although I haven't seen any reports of that. But the vast majority of
the 'filler' DNA is pretty random 'noise', repeats, telomeres and
centromeres, etc. It could be that the filler DNA is structural in some
fashion, and that has been pretty well demonstrated for a percentage of
it. It could be that this 'filler' DNA is involved in gene regulation
somehow, at long distances. It could just be space to move in, allowing
chromosomes to recombine with less chance of disrupting a necessary
gene. It is a field which needs more study, that is for sure. I am
pretty convinced that this 'trash' DNA is not really trash, we just
don't understand it yet.
I do not think it is very likely some non flowering species is going to
evolve into an orchid. In an evolutionary time scale, I expect new orchid
species will result from current orchid species adapting to new niches and
global changes.
By definition, if it isn't an orchid already, it won't evolve into
one. Our concept of taxonomy requires that related organisms be
derived from a common ancestor. The degree of relationship is dependent
on the distance to the common ancestor. For example, two species within
the same genus have a fairly recent common ancestor, whereas Cattleya
labiata and Den. speciosum share a more distant ancestor. Cattleyas and
roses share an even more distant ancestor.... etc. There are instances
of different groups of plants evolving similar characteristics in
response to a common environmental condition, that is really another
beast entirely. But even two very similar plants, if they don't share a
common ancestor, cannot be put in the same taxon. Now, you might ask
how do we _know_ that they share a common ancestor? That is a whole
other box o' rocks. And mistakes have been made, of course. This is a
good place for the contribution of molecular phylogeny (using DNA
sequence instead of, or in addition to, physical characteristics).
To the other part of the question, if something is derived from an
orchid, but doesn't flower, it is still an orchid. Heck, I have many
plants that don't flower, and may never flower, but they are orchids.
In the jungle, we might not ever notice those plants, so they may not
get described. But chances are good that a skilled taxonomist could
recognise a plant as an orchid by its vegetative characteristics. I'm
pretty sure there are some orchid species described which haven't been
reported to flower, perhaps some of the 'jewel' orchids? If it can
propagate well vegetatively, it won't need to flower. Anyway, if a
critical gene for flowering has been silenced by a mutation, there is a
finite chance it will get turned back on in a subsequent generation.
Not a very good chance, but a chance.
Rob
--
Rob's Rules:
http://www.msu.edu/~halgren
1) There is always room for one more orchid
2) There is always room for two more orchids
2a. See rule 1
3) When one has insufficient credit to purchase
more orchids, obtain more credit