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Mutations in hostas

I have a few questions that I'm hoping will start a discussion that we 
can expand on later.  I've been interested in hostas for maybe 20-30 
years, but haven't done much with them until the last few years.  The 
rate at which hostas sport has been of some interest to me from a 
scientific point of view.  My background is in botany, plant breeding 
and genetics, so it has been of some interest to me as to why hostas 
sport so easily and frequently.  I'm planning on doing some 
experiments that I don't want to go into in any great detail right 
now, but I hope will answer some of my questions.  

I've noticed the frequent use of the term plasma mutations as the 
cause of variegation in hostas, in conjunction with L1/L2 periclinal 
chimeras.  Some hostas seem to be reasonably stable periclinal 
chimeras where the leaves are more or less the same, although the L1 
and L2 layers can and do sometimes switch positions or one or the 
other is lost.  Other hostas seem to have unstable variegation and 
every leaf seems to be different.  I've heard the term unstable plasma 
mutations used for these unstable variegations.

Now, variegation can be caused by either nuclear mutations affecting 
chlorophyll synthesis or plasma structure; by mutations in the small 
amount of DNA in the chloroplasts or by virus infection.  I keep 
seeing the use of the term plasma mutations or unstable chloroplasts, 
but am wondering in just what context those terms are used.  Has there 
been any good studies to determine if variegated sports that are not 
virus infections are really caused by defective chloroplasts or if the 
variegation is caused by mutations of nuclear genes affecting 
chlorophyll biosynthesis.

The other question is why hostas mutate or sport so easily.  There is 
no reason to expect that hosta DNA is any more susceptible to 
mutations then any other plant or that hosta chloroplasts are more 
susceptible to mutations then other plants.  Has anyone studied the 
possibility that hosta mutations may be caused by transgressive 
elements - small pieces of nuclear DNA that can move from one 
chromosome to another?  Can anyone articulate the current theories as 
to why hostas mutate so easily.  I have some experiments I'm working 
on to help answer some of these questions, but I figure it will 
probably take 10 years to get some meaningful results.

Joe Halinar


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