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Fw: Variegation-Vaughn-1979-1980


Here another article from Dr. Vaughn on Variegation from the same issue of
the Journal.

Subject: Variegation-Vaughn-1979-1980


Found in THJ Bulletin 11, 1979-1980 pages 33-34


    VARIEGATION IN HOSTA-  by Kevin Vaughn

    When I first began my crossing experiments with hosta about 10 years
ago, I had but three plants---H. decorata, ventricosa, and undulata.
Thinking it would be interesting to produce more variegated plants,  I
tried selfing and intercrossing these clones.  My results, however, were
less than satisfying.   H. decorata gave all green seedlings; ventricosa
gave more ventricosas and undulata gave no seedlings at alI .

     Despite these results, I decided that I still liked HOSTA, was still
interested in studying the variegation, and subsequently obtained a working
collection.  From this crossing work and later work using techniques of
light and electron microscopy and biochemistry...the following conclusions
were drawn:

     Chodat in 1919 classified variegated patterns in Hosta in one of two
types:  marginata and mediovariegata.  The marginata type, represented by
such clones as 'Louisa' and 'Frances Williams' have conspicuously lighter
edges and a green center.  Self or cross-pollinated seed from these
varieties as the female parent give about 99% green and 1% mutant (yellow,
white, or variegated). Mediovariegata clones, such as 'Maya' and 'Kabitan',
in contrast to the marginata forms give a majority of mutant seedlings when
used as the pod parent.  All yellow Hosta behave in a similar manner to the
yellow-centered forms.  However, pollen from a variegated or yellow clone on
a green plant give all green progeny.

    When one thinks about the construction of the Hosta leaf and the
pollination process, a ready explanation pops forth for the breeding
behavior of the variegated Hosta.   The tissues from nearly all of the
higher plants are derived from 3 layers of cells which are present at the
growing tip of the plant.   In Hosta, the central  (L II)  layer, or second
layer forms the central area of the leaf and the ovules...while the L I
forms the marginal tissue of the leaf and the epidermis (skin) of the entire
plant. Thus, one would expect that since there are tiny proplastids or
immature chloroplasts (the chlorophyll-containing body of the cell) in  the
ovules  (derived  from L  II)  that  if  the  L  II  is mutant  so would the
seedlings.  The pollen parent appears to have little or no influence on
variegation.   Thus one would expect that since the pollen of Hosta has
probably no more than one proplastid and since many are supplied by the
ovule; there is good cytological reason why only the maternal parent
contributes the vast majority of variegation in Hosta  (several exceptions
are known, however).

    Thin sections through the capsules and leaves of Hosta  show that it is
possible for tissue from the L I to give rise to a small amount of the
tissue from central areas of the leaf and a fraction of the total number of
ovules.  This is caused by excessive growth of the L I of "Frances Williams"
and can give all golden sports whereas excessive growth of L II or a loss of
L I by injury can give an all green plant.
     Recently I have begun experimentation with a series of mutagens to
determine if it was possible to produce variegation in all  green Hosta.
Although the  results are preliminary,  a number of mutants have been
produced, but a high kill rate is still common.  By using less valuable
seeds such as radish and tobacco, we are gradually working out mutagen doses
which optimize mutation yet allow a normal amount of germination.

    Electron microscopy studies of mutant Hosta  are also in their infancy.
One of the main "whys" about Hosta was why the large number and variety of
plastid mutants.   I felt that an EM study would give us more of a clue.
Within each chloroplast there are membranous sacs where the pigments
(chlorophylls & carotenoids) are attached.   In green plants these
thylakoids lie in stacks whereas in the yellow 'Wogan Gold'  they lie spread
throughout the chloroplast.   Interestingly some species in the tropics have
similar levels of chloroplasts and thus these yellow Hosta may also be more
efficient in some environments at light-gathering.  Strikingly different are
the chloroplasts of the white edge and centers of the white variegated
clones.  These varieties start with normal thylakoids and later an extreme
swelling or 'exploding' of these thylakoids is apparent.  This allows the
pigments to be destroyed (which explains why these plastids show white
markings).  Various white clones vary to the extent of the 'exploding', with
the cleanest white clones exploding rapidly.  There appears to be even good
biochemical reasons why certain Hosta have white or yellow variegation and
this work will be reported in various other journals.

    This is the 100th anniversary of the use of Hosta as a research
organism.  With the many intriguing problems of this genus, it will surely
keep scientists hopping for another 100.





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