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re: mitotic recombination


>I had understood this phenomenon to be an accepted operating 
>mechanism in Genetics to explain how closely linked genetic  
>characteristics  on the same chromosome may change their
>relationships by mechanical changes in their relative  locations.

Crossing over is common during meiosis when homologous chromosomes 
pair.  However, during mitosis homologous chromosomes do not normally 
pair, and as far as I know no one has ever seen cells undergoing 
mitosis where there was visible homologous chromosome pairing.  
Somatic crossing over has been demonstrated in yeast, a few fungi and 
fruit flies using very special conditions in which to detect the 
crossing over.  However, it takes CONSIDERABLE effort to demonstrate 
this and even then the rate of crossing over is VERY low and ususally 
needs to be induced by irradiation.  

>4. its explanation as a causal factor for sports is pure theory, not
>based upon fact, without proof of any kind.

I seriously doubt that there are any variegated hostas that are the 
result of mitotic or somatic crossing over.  The possibility that this 
can be a cause of variegation isn't zero, but it is pretty close to 

Where Ben makes his error is that he is thinking in terms of single 
cell organisms.  We have to remember that Ben is a yeast geneticist, 
NOT a botanist.  In a yeast, which is a single cell organism, IF you 
do have a mitotic recombination, one daughter cell gets the recombined 
genotype and you end up with a twin sector.  For example, take a very 
simple hypothetical example where you have a genotype Rr where RR or 
Rr gives a red colored colony while rr gives a yellow colored colony. 
Now, if you get mitotic crossing over so that you get daughter cells, 
one of which is RR and one of which is rr, you will end up with a 
colony that is half red and half yellow.  With a single cell organism 
like yeast you only have to worry about a single cell.

Where Ben is making a big error is in not understanding that a hosta 
is a multicelled organism.  Not only is mitotic crossing over VERY 
rare, but in a multicelled organism it has to occure in a cell where 
the results can be observered.  It makes little difference if a 
mitotic recombination for leaf color takes place in a root cell.  Lets 
assume that we have a yellow leafed hosta with genotype Yy and lets 
assume that we do get a mitotic recombination such that a daughter 
cell ends up with genotype yy, green leaves.  Well, unless that cell 
undergoes further mitotic cell divisions we probably won't even be 
able to see the ONE green cell.  Even if it does undergo further cell 
divisions all we most likely would see at best is a spot of green on a 
yellow leaf.  To be able to get a mitotic recombination to result in a 
L1-L2 chimera variegated plant would require the recombination to 
occure in an apex meristem cell, or close enough to the apex cells to 
form a mericlinal chimera that could later be converted to a 
preclinical chimera.  

Now, if Ben has done any research on mitotic crossing over in hosta I 
wish he would present his results to a peer review scientic journal.

Joe Halinar

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