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Re: hosta-open DIGEST V1 #322


Joanne:

>In his handout at Hosta College Jim Wilkins mentioned that 
>yellow/gold color is dominant over blue or green.  And in his Journal 
>article Ben said blue was dominant over green.  If these are true why 
>are most hostas green?

The answer to your question is based on understanding population 
genetics and selection.  In a population of randomly intermating 
individuals and in the absence of selection for or against a trait, 
then the frequency of particular alleles within the population will 
stay the same from generation to generation.  Whether or not a trait 
is dominant or recessive doesn't have any bearing on how common it 
will be in a population or whether or not it will be selected for or 
against.  

The dominant yellow gene in hostas is most likely a dominant inhibitor 
gene that stops chlorophyll synthesis prior to the formation of the 
final chlorophyll molecule at a xanthous pre-chlorophyll molecule.  
This inhibitor gene is probably temperature sensitive - that's why 
many yellow/gold hostas eventually green up later in the season (when 
the temperature is warmer).  This inhibitor may also exist as several 
different alleles with different temperature sensitivity.  This 
xanthous pre-chlorophyll molecule isn't going to be as effecent at 
photosynthesizing as chrolophyll, so there will be some natural 
selection against these plants in nature.  Thus, you would expect to 
see fewer of them in nature.  However, this has nothing to do with 
this allele being dominant.  A recessive yellow gene would be selected 
against in nature just as equally as the dominant inhibitor gene.

The blue color is a waxy covering and has nothing to do with 
chlorophyll.  This is a good example of how phenotypes can get 
confused.  Blue color is a completely different trait then color 
caused by chlorophyll.  You really shouldn't look at hosta leaves as 
being either green or blue.  They are either yellow/gold or green and 
either waxy or not waxy.  All complex traits can be broken down into a 
series of simpler traits.  

The synthesis of chlorophyll is very complex and the chlorophyll has 
to be organized in a complex manner in the chlorplasts, so there are 
probably maybe 100 or more genes that might interfer with chlorophyll 
synthesis.

Joe Halinar

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