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Re: Discussion on Origin of Sports...2nd causal factor


Jim:

>According to Ben's Rule of Thumb mentioned several times in his 
>articles is Chimeral Rearrangement.

I can't see how shifting or rearrangments of histogenic layers can be 
considered a cause of sports in the sense of creating variegation.  
Shifting of tissue layers is significently from a horticulture point 
of view, but simple shifting of tissue layers can't account for the 
wide diversity we see in hostas.  Basic L1-L2 shifting really 
shouldn't give more then two or three different types of sport for any 
one original hosta if we are considering that mutated chloroplast are 
the only factor causing the variegation.  If you start with a green 
leaf and it sports to a white edge, then the L1 and L2 layers can 
switch or displace so that you get back the green form, an all white 
form (dies) or a white centered form.  No matter how many times the 
shifting occured you should only see the same plant showing up.  Now, 
many times different people will get what appears to be the same sport 
via tissue shifting, rearrangement or displacment, but sometimes the 
plants are not the same.  Before going into further detail I want to 
cover some basics of genetics and mutations so that those without a 
scientific background can follow along.  I'll try to keep it at a 
simple a level as possible, but that may not always be possible.

Up until not too long ago geneticists believed that one gene produced 
one enzyme.  Today we are beginning to understand that it may be one 
gene producing 20 or even 30 enzymes or several genes producing one 
enzyme.  That aside, if the sequence of the base pairs of the DNA are 
changed, then a gene will produce an enzyme that has a different amino 
acid sequence and this different protein may not function as well as 
the original enzyme, or not function at all, or it could even be more 
effecent.  What we are mainly concerned with is chlorophyll synthesis. 
Chlorophyll synyhesis is a VERY complex bichemical process and any 
number of changes in the enzymes that are involved in chlorophyll 
synthesis can result in albino or xanthous leaves or variegation if 
the mutation occures in only one tissue layer.

Now, just what constitues a mutation can be debated to some extent.  A 
point mutation where one of the base pairs is changed so that a 
different ammino acid is inserted into the enzyme is a example of a 
simple mutation.  However, some base pairs could be removed from the 
gene or base pairs could be added; either could result in a different 
enzyme being produced that becomes nonfunctional.  Interestingly, if 
this occures in a duplicated gene, then that new enzyme can take on 
new functions because the original gene is still functional and doing 
the job it is suppose to be doing.  You also have the situtation where 
loss of pieces of chromosomes will also result in loss of a gene or 
genes.  There are also inversions where a piece of chromosome is 
removed and then reinserted backwards and translocations where a piece 
of one chromosome breaks off and attaches to another chromosome.  All 
of these chromosome changes can have an effect on gene function, 
either by the loss of a gene or the duplication of a gene, and can 
also have an effect on fertility.

Then we get into the area of transposible elements where a piece of 
DNA can duplicate itself and then transpose itself to a different 
location in the genome.  If a transposible element inserts itself into 
an existing gene, then it can render that gene inoperatable.  However, 
a transposible element can also insert itself in such a way that it 
doesn't change the DNA sequence of a gene, but it can still effect a 
gene by a position effect.  

Then to make it even more interesting, chloroplasts have a small 
amount of their own DNA.  It is often thought that at a VERY long time 
ago a simple "algae" like organism formed a symbolic relationship with 
cells that didn't have photosynthetic ability and then "evolved" into 
what we see as chloroplast today, thus explaining the small amount of 
DNA they contain.  Thus, any nuclear gene mutation that effects 
chlorophyll synthesis or the structural integrity of chloroplast can 
result in variegation if it only occures in one tissue layer.  Changes 
in the chloroplast DNA can also have a negative impact on the 
functioning of chloroplasts.  

However, we now face another problem in that chloroplasts only have a 
VERY small amount of DNA.  I don't have a figure off hand, maybe Ben 
can give us a better figure, but I think that the amount of DNA in the 
chloroplasts is in the order of one tenth of one percent or less of 
the total nuclear DNA content.  Thus, the probability that a mutation 
will occure in the chloroplasts is very low compared to a mutation 
occuring in the nucleus.

I think there are two different questions we are looking at.  The 
first is how do we get from an all green form to a variegated form.  
The second question is once we have a variegated form, how do we get 
the wide assortment of variegated sports from the original variegated 
form that we see.  

Joe Halinar

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