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[Fwd: one in 10,000]





Jim, (and others still with us),

If you agree with Tilney-Bassett, then you now agree that mixed cells
can originate
   - at any stage of embryogenesis
   -may occur as mixed eggs owing to incomplete sorting out
   -or as mixed zygotes via hybridization
as a result of spontaneous plastid mutations (and sometimes as nuclear
mutations as well).

You ask specifically how do you get  populations of chloroplasts sorted
into L1 and L2 tissue. The quick answer is that they get sorted first
into a few cells, then these cells later differentiate and then  develop
into L1 and L2 tissue. But lets do it the professional way also. Let's
see what Marcotrigiano says about it in his article frequently quoted
previously.

 Under Origins of Chimeras,  he says referring to "Genetic changes in
nuclear or chloroplast genome"  (either spontaneous or induced)..

"Any genetic change in one or more cells in a multicellular shoot
meristem can result in the generation of a chimera. Pigment mutations
are obvious and one could mistakenly conclude that they occur more
frequently than other mutations. Mutations also result in the absence of
morphological features...(he gives some examples which we need not
discuss).

The most common cause of chimeral variegation is spontaneous mutation in
the pathway of chlorophyll synthesis or plastid morphology. These
mutations are generally chloroplast mutations rather than nuclear
mutations. Nuclear mutation is a  less likely cause of variegation in
chimeras for two main reasons. First, since most albino mutations are
recessive, spontaneous somatic mutations of the nucleus would lead to a
heterozygous condition  and the affected cell would remain green.
Second, If the heterozygous mutant cell eventually gave rise to gametes,
the next generation would  yield a population of albino seedlings and
green seedlings instead of variegated seedlings. In contrast, a mutation
in chloroplast DNA would lead to a single albino chloroplast coexisting
in a cell dominated by green chloroplasts. A cell having two types of
plastids is said to be "heteroplastidic" . During cell division,
chloroplasts are also dividing and stochastic processes lead to a
"sorting out" that eventually results in a production of a cell line
with only one type of chloroplasts. When a cell is completely "sorted
out"( i.e. contains all normal or all mutant plastids) , all of its
decendants will be identical to it and the cells will be
"homoplastidic". The plant's phenotype will be a mosaic of green and
white or yellow patches of various sizes depending upon the time the
sorting out occurred. For example, if a white cell line is established
early in leaf development, a large white patch will appear on the
leaf.Only after the apical initial cells are sorted out can a stable
periclinal chimera  be generated. In some cases this does not happen
before the vegetative meristem  becomes a floral meristem and eggs,
containing a mixture of  mutant and normal proplastids, are generated.
These eggs will generate mosaic seedlings with the relative amount of
white or green tissues being influenced by the relative proportion of
green and white proplastids in the egg cell. In most plants, plastids
are maternally inherited, so crosses using a green female and a
nonsorted variegated male yield only green offspring. The reciprical
cross, however will yield green, white and variegated seedlings in a non
Mendelian ratio  that is influenced by  the plastid composition of the
individual eggs within the egg population....".
..................................................................................................................................

I make the observation from the above description from Marcotrigiano
that it is almost identical to the description given by Kevin Vaughn in
the AHS  Bulletin 11, of 1982....perhaps more comprehensive This subject
was the research problem that Kevin worked on for his Doctorate. I have
lost my copy of Bulletin 11 which contained  his articles on plastid
mutants...but I have read it and written about it so many times, that I
have almost memorized it by heart. Vaughn and Marcotrigiano say
virtually the same thing with Marcotrigiano describing this phenomenon
in many Genera while Vaughn's studies concentrated on hostas
specifically.

Do these portions of Marcotrigiano's article and comments help in your
understanding and in answering some of your questions? I hope so.

Jim Hawes.



plem study













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