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Re: Re: Re One in 10,000


I agree with Tilney-Bassett.  It is clear how a nuclear mutation would end
up in the adult plant by when it occurs during cell division.  The quoted
article does a good job of explaining this.  With this we have no argument
as it is straight forward what happens.

My problem remains.  How do you get two populations of chloroplasts sorted
into the L1 and L2 of Hosta, and why do you need a streaked parent to get
streaked or variegated seedlings?

I can wait until tomorrow for your next installment.

Jim Anderson

-----Original Message-----
From: Jim Hawes <hawesj@gcnet.net>
To: hosta-open@mallorn.com <hosta-open@mallorn.com>; hawesj@gcnet.net
Date: Monday, August 16, 1999 6:07 PM
Subject: 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
> 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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