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Re:Discussion on Origin of Sports..3rd causal factor


Scholars:

I am flabbergasted that everyone is doing their homework assignments and
coming to class all prepared. I have never seen hosta-open so
enthusiastic in sharing their experiences and knowledge. Keep up the
enthusiasm so that I can become a lazy lurker once more.

Joe, your presentation in summarizing the current thinking on how the
genetic system works seemed to correspond with my understanding also. We
are on the very same wavelength, so to speak. Let's continue along these
same pathways  and we will get to the same destination.

I hope we can continue with open minds. Where are we? I think we have
covered  (disposed of idea perhaps ?)  that (1) mitotic recombination is
questionable as a causal factor for origin of sports, that (2) Stewart
and Dermen (1970) are the appropriate authorities who tell us that
displacement and replacement cause " layer switching" in periclinal
chimeras frequently resulting in "bud sports" on chimeral plants. Now we
are at mutations ( both nuclear  and chloroplastidic). If you will
permit, I will transmit verbatim from a section of Marcotrigiano's
article published in Hort Science Vol 32(5) dated August 1997, what he
says about mutations as a causal factor  in his section entitled "
Origins of Chimeras".I will redact portions that obviously have little
or no relationship to hostas, such as the case with LI tissues  in
thornless blackberries.

"Any genetic changes 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 ......(non relevant.portion redacted) .

" The most common cause of chimeral variegation is spontaneous mutation
in the pathway to chlorophyll synthesis or plastid morphology. THESE
MUTATIONS ARE GENERALLY CHLOROPLAST MUTATIONS RATHER NUCLEAR MUTATIONS (
caps are mine to shout out this message)...........(redacted portion
citing reasons for this conclusion)

" A cell having two types of plastids is said to be 'heteroplastidic" .
During cell division, chloroplasts are also dividing and, in time,
stochastic processes lead to a "sorting out" that eventually results in
the production of a cell line with only one type of chloroplast..  When
a cell is completely sorted out ( 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 on the
time the sorting out occurred ( Jagannathan and Marcotrigiano, 1986) .
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 with eggs, containing a mixture of
mutant and normal protoplastids, are generated . These eggs will
generate mosaic seedlings with the relative amount of white or green
tissue being influenced by the relative porportion of green and white
proplastids in the egg cell. In most plants, plastids are maternally
inherited ( Kirk and Tilney- Bassett, 1978), so crosses using a green
female and a nonsorted variegated male yield only green offspring. The
reciprocal 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 in the egg population............(The
rest of the article is redacted in this discussion. I do wish to point
out that in this section, he mentions  the following  causes ( Origins)
of chimeras. We do not discuss them here because they are not very
relevant to our discussion in my opinion.
   2. Transposable genetic elements
   3. Graft induced chimeras
   4 .Semigamy
   5. Tissue culture synthesis of chimeric meristems.  He states that
intentional synthesis in cell culture has met with very limited success
( Marcotrigiano, 1990).

So there, ladies and gentlemen, you have an authority's views on the
origins of chimeras. They correspond to those of Dr. Kevin Vaughn's
views express 20 years earlier. I consider this the convention wisdom
and I consider Ben Zonneveld's explanation for sports incomplete and
faulty.

Let me try to explain how I understand it to work in plain old
non-scientific language.

The main causes for sports are the many mutant forms of plastids
expressed by the multitudinous mixtures of plastids  populations
composed of  normal green and mutated non-green plastids which will
undergo sorting out separation into the various cells in LI tissue and
LII tissues. Either LI or LII may be mutant tissues. They will grow in a
fashion depending upon the tissue shifting that occurs. All of these
phenonmena are highly variable giving us many different color
combinations and patterns that are caused by the highly variable timing
of the events that occur in the phenomena described. It is all very
complex but understandable when studied in the manner that has been done
by the various scientists involved over the decades. The work of each
scientist built upon the work of others They communicated with each
other. They shared information with each other in the form of a
literature review and the recognition of each other's work. This is the
scientific ethic which allowed science to operate efficiently. Sorry,
folks, I will get off of my soap box...I was getting carried away with
morality and ethics issues in science.....actually,  there has not been
an over abundance of it lately in the hosta community.

Jim Hawes.


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