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  Chimeric variegation and its pitfalls
From: "Sean O'Hara" <SAOUC at UCCMVSA.UCOP.EDU> on 1997.11.22 at 10:07:36(1647)
Fellow Varoidophiles -

All this talk of variegated plants and their instability is not a
new topic on internet forums - it comes up routinely where
variegated plants are discussed. Among plantspersons I know, there
are often strong views about what causes variegation in plants,
usually tending towards the virus factor. I know little about viral
variegation myself, but I have read a bit on the chimeric type,
which I find quite interesting. My own observations indicate to me
that most variegation in plants, especially those that tend to
revert or 'come and go', is of this type.

Chimeric variegation is due to a partial mutation IN SOME LAYERS of
the plants meristem (growing tip) tissue. Even within one layer,
the mutation may only be towards one side or only effecting a
percentage of the total tissue layer. This phenomenon takes its
name from the mythic beast, the Chimera, who was made up of
different parts of various animals, alluding to the coexistence of
different tissue in the same plant. Sometimes these mutations are
visible, as with variegation, or more subtly as with contorted
plants (one side of the stem grows faster than the other, causing
the twisting of the growth). Sometimes the mutation is not visible
in any way, but may still represent a different tissue type only
detectable by chemical analysis.

As the tissue grows, only some portions of these layers are
effected. As tht tip elongates, or new stems arise, the ratio of
normal to mutated tissue can change and alter sides, etc. This
causes the random 'splashing' and is the least stable type of
variegation. Other chimeric variegation can envolve entire selected
layers of tissue, creating a more regular variegation pattern (e.g.
pale stems/petioles and central 'flame' variegation on each leaf, as
both these areas arise from the same meristem layer).

While some chimeric variegation can be more stable that others, all
might revert at some point due to various factors (such as those
discussed - stress, light change, fertilization, etc.) which can
change the grown rate of the meristem and alter the mix of regular
and mutant tissue. They can also revert without any apparent
stimulus due to the happenstance of growth taking place in an area
free of mutation, usually much more vigorous and therefore likely to
quickly take over the plant. Careful monitoring of these plants and
selection of the best variegated parts (along with removal of
reversions to non-variegated) is the means to sustain the clone.
Often, new clones can arise from these unstable situations, or more
stable forms might be selected. It is worth observing.

Sean A. O'Hara sean.ohara@ucop.edu
710 Jean Street (510) 987-0577
Oakland, California 94610-1459 h o r t u l u s a p t u s
U.S.A. 'a garden suited to its purpose'
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Ask me about the worldwide Mediterannean gardening discussion group

From: Rand Nicholson <writserv at nbnet.nb.ca> on 1997.11.22 at 16:02:42(1652)
>Fellow Varoidophiles -
>
>All this talk of variegated plants and their instability is not a
>new topic on internet forums - it comes up routinely where
>variegated plants are discussed. Among plantspersons I know, there
>are often strong views about what causes variegation in plants,
>usually tending towards the virus factor. I know little about viral
>variegation myself, but I have read a bit on the chimeric type,
>which I find quite interesting. My own observations indicate to me
>that most variegation in plants, especially those that tend to
>revert or 'come and go', is of this type.
>
>Chimeric variegation is due to a partial mutation IN SOME LAYERS of
>the plants meristem (growing tip) tissue. Even within one layer,
>the mutation may only be towards one side or only effecting a
>percentage of the total tissue layer. This phenomenon takes its
>name from the mythic beast, the Chimera, who was made up of
>different parts of various animals, alluding to the coexistence of
>different tissue in the same plant. Sometimes these mutations are
>visible, as with variegation, or more subtly as with contorted
>plants (one side of the stem grows faster than the other, causing
>the twisting of the growth). Sometimes the mutation is not visible
>in any way, but may still represent a different tissue type only
>detectable by chemical analysis.
>
>As the tissue grows, only some portions of these layers are
>effected. As tht tip elongates, or new stems arise, the ratio of
>normal to mutated tissue can change and alter sides, etc. This
>causes the random 'splashing' and is the least stable type of
>variegation. Other chimeric variegation can envolve entire selected
>layers of tissue, creating a more regular variegation pattern (e.g.
>pale stems/petioles and central 'flame' variegation on each leaf, as
>both these areas arise from the same meristem layer).
>
>While some chimeric variegation can be more stable that others, all
>might revert at some point due to various factors (such as those
>discussed - stress, light change, fertilization, etc.) which can
>change the grown rate of the meristem and alter the mix of regular
>and mutant tissue. They can also revert without any apparent
>stimulus due to the happenstance of growth taking place in an area
>free of mutation, usually much more vigorous and therefore likely to
>quickly take over the plant. Careful monitoring of these plants and
>selection of the best variegated parts (along with removal of
>reversions to non-variegated) is the means to sustain the clone.
>Often, new clones can arise from these unstable situations, or more
>stable forms might be selected. It is worth observing.
>
> Sean A. O'Hara sean.ohara@ucop.edu

Now ...; there's an answer ... :)

Rand

From: Steve Marak <samarak at arachne.uark.edu> on 1997.11.22 at 18:37:34(1653)
I have a fascination with variegation (as well as contorted plant forms)
and have been waiting for the weekend and some time to get in on this
discussion.

For those who would like to read more about chimeric variegation, I
recommend:

http://aggie-horticulture.tamu.edu/syllabi/clasnotes/201notes/chimeralec/
chimeras.html

(Please note that's all one long line.) This is a lecture by Dan
Lineberger at Texas A&M, and I found it a concise summary. Unless you are
already very familiar with this subject, though, don't expect to speed
read it - it takes some thought.

It's easy to see how variegation can result from a chimera in which one
group of cells for some reason lacks (some or all) chlorophyll or other
pigment. It's also easy to understand how such a chimera might "revert" -
any time the plant is grown under conditions which sufficiently favor
"normal" cells, the variegated line of cells might well die out, leaving
only the normal.

Often I have seen, in the growing hints for variegated plants, something
to the effect of "needs more light than the normal form to retain good
variegation". I've always assumed that this was because in high light the
normal cells could produce plenty of food to go around, while in lower
light those cells which must live on charity find survival much harder. I
haven't seen anyone actually state this, though it seems consistent with
observed results: leaves of variegated plants grown in low light lose
some or all of their variegation, compared with older leaves which formed
in higher light, and when moved back to high light, new leaves usually
regain their abnormal coloring in all its glory. Once any leaf has opened,
however, its coloring is fixed and won't be affected by changes in light
level.

My further assumption is that even if a variegated plant is grown in low
light for some time, so long as the variegated cell line is still present
in the meristem, the plant can regain its coloration when moved to higher
light. A question: does that mean that such a plant, if left in low light
conditions long enough, would completely revert to normal because all
variegated cells, even in the meristem, would die? I'm not sure I can
accept it - it doesn't seem like the cells in the meristem would be
selected against for presence or absence of chlorophyll.

The URL given above also discusses briefly why some kinds of variegation
are more stable than others - some are very consistent, while others
routinely produce both non-variegated shoots which much be pruned off and
completely achlorophyllous shoots, as well as shoots with varying amounts
of "normal" variegation.

Apparently there are several other causes of variegation, including the
viruses that Sean mentioned, something called "chloroplast mosaics" about
which I know nothing, and of course genetics. I believe Jim Waddick gave
the statistic that 95% of all variegation was NOT caused by virus, which I
am pleased to hear.

I'd appreciate any pointers from the group to good resources on
variegation and contortion, whether on the Web or not.

Steve

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