Re: Re: Re One in 10,000

Jim Anderson and open robins,

I have come out of my shade garden again into the dapple sunlight once
more...to be helpful, if I can..

In your post of Sunday night, you wrote:
<My main stumbling block is how the initial sorting is done in the
Zygote so that you get a <variegated seedling and how does sorting
relate to the maternal tissue being streaked? What <makes one type of
chloroplast go preferentially into one cell and another type of
chloroplast go <into another cell when initially they were all mixed
together in the zygote cell? A mechanism that <entails regulation of
chlorphyll synthesis ( or chloroplast development) is much easier to
visualize.>

 I sent to Joe a printout of a portion of an essay written by
Tilney-Bassett on "The Relationship Between the Position of Mutation in
Development and subsequent Chimeral Formation" . In it, the timing of
sorting out during embryogeneisis is discussed as it relates to eventual
chimeras that result in seedlings. I will quote a portion of it for you
and interested readers.

" As a result of spontaneous platid mutation, mixed cells can originate
at any stage of embryogenesis, or they may occur as mixed eggs owing to
incomplete sorting out in the mother, or as mixed zygotes through
hybridization. The first division of the fertilized egg produces a two
celled embryo consisting of a basal and terminal cell; the basal cell
gives rise to the suspensor and is no longer important for sorting- out.
If the terminal cell is a mixed cell the subsequent sorting- out pattern
will in most cases be reflected throughout the adult plant. But if the
initial plastid mutation takes place later in embryogenesis the
subsequent primary sorting-out lineage is less widespread.

The second successive division, which includes the first division of the
terminal cell, is either transverse or verticle, depending upon the
plant concerned. After a transverse division, the upper cell farthest
from the suspensor is the one which gives rise to the cotyledons and the
shoot, while the lower cell gives rise to the hypocotyl and roots. Hence
a mutation in the upper cell will usually give rise to a sorting-out
pattern reflected throughout the adult plant. But after a verticle
division of a terminal cell, a mutation occurring in either daughter
cell will subsequently show sorting-out  only on that side of the
devloping shoot; the embryo and later the adult plant will therefore be
sectorial  for a pure green half and a sorting out half. Up until the
verticle division of the terminal cell all nuclear mutations give rise
wholly to mutant lineages, but a mutation in one of the daughter cells
would give rise to a green-white sectorial chimera. Mutations in cells
giving rise to the root and the hypocotyl regions are unimportant in the
adult plant since we are only interested in the development of the
chlorophyll chimeras or the shoot.

During the next two successive divisions , the third and fourth, the two
terminal cells are split into four and then into eight cells to produce
an upper  and a lower quadrant  of four cells each. The lower quadrant
produces the root and hypocotyl regions  and the upper quadrant the
cotyledons and  shoot. A mutation in the lower group of cells is
unimportant, but a nuclear or plastid mutation  in one of the four upper
cells will give rise to respectively a white mutant lineage or a
sorting-out cell lineage encompassing the corresponding quarter sector
of the shoot. Hence the adult shoot will probably develop into a
sectorial chimera for approximately three-quarters green and one quarter
white or sorting out.....

After the fifth division new mutations  give rise not to true sectorial
chimeras but to mericlinal chimeras in which the  mutant sector is
actually periclinally divided. The fifth division divides each of the
octant cells periclinally into an outer and an inner cell. The outer
group give rise to the epidermis, L1, and the inner to the remaining
layers, L2 and L3. Hence a mutation in any of the eight upper cells
produces a mutant lineage that affects either L1 or the future L2 and
L3, but not all three layers together; it therefore gives rise to a
periclinal mutant sector. At a later stage of development the separation
between layers 2 and 3 takes place, unless there is no L 3.

Mutations in the shoot growing point

Only before the second or third successive division of the embryo is it
possible for a mutaion to affect all future cells of the shoot. After
the second or the third and before the fifth division a half or quarter
sector can be affected. Finally after the fifth successive division, all
mutations are restricted to one of the future germ layers so that only a
mericlinal sector is affected. In the two or three layered growing point
of the shoot, chimeras arising from single mutations are always of the
mericlinal kind, since a single mutation can occur in only one cell of
only one layer, although sometimes a second or third layer  becomes
affected as a result of subsequent layer alterations.

Most monocotyledons are two layered, and in white over green or green
over white periclinal chimeras (of  Hostas) ...it is clear that L1
regularly produces the marginal mesophyll tissue of the leaf as well as
the epidermis, while L2 produces the central mesophyll tissue. Hence,
mutations in the growing point of either layer are quickly revealed by
the appearance of conspicuous white sectors in the leaves.

The questions "What makes one type of chloroplast go into one cell
rather than another...and how does sorting - out relate to the maternal
tissue being streaked?" are questions for another time. Tomorrow, maybe.

................................

There is too much here for me to comment on at this time. I suggest
those interested,study it to get a good understanding of what
Tilney-Basstt is trying to explain about plastid and nuclear mutations
occuring during embrogenesis. This is just one of the examples of
classic knowledge of cytology and histology that determines the degree
of our understanding of variegation in hostas It is complex but worth
learning about.

Jim Hawes.


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