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Re: Clones; genetic variability and chimeras---Terms defined in a text from 2...


Hello Butch,
The text I sent a link to was in Adobe Reader which is a free download.
Below in the same text in HTML for those without Adobe.
Dan
================
Narda
When I try to open this I get crazy stuff like a picture that formed letters
instead of a picture.
Any idea what i did wrong?
lThanks
Butch Graves
================================
Hort 311
Winter 2001

                           Clones: Genetic Variability and Chimeras

Introduction
       Although clones are defined as a genetically uniform assemblage of
individuals,
genetic change doesnít stop when a clone is selected. Therefore, genetic
variation must
be identified and separated from the existing clone. However, distinguishing
genetic
variation from other types of variation is not always simple.
       In this lecture we will learn, 1) the structure of shoot apical
meristems, the source
of plant shoot tissues; 2) the types of plant chimeras associated with
meristematic
structure; and 3) how this chimeral structure is maintained through
propagation.

Reading HKDG pp. 244-250.
       If necessary, review the primary and secondary anatomy of plant stems
and the
general anatomical structure of leaves, roots and fruits.
       Marcotrigiano, J. 1997. Chimeras and variegation: patterns of deceit.
HortScience 32:773-784.

Objectives

1. Know the following terms:
       chimera
       periclinal chimera
       mericlinal chimera
       sectorial chimera
       meristem
       promeristem
       initial
       derivative
       tunica
       corpus
       histogenic layer

2. Describe the tunica-corpus organization of the shoot apical meristem and
its relation
to the histogenic layers.  Be able to diagram a typical plant promeristem,
including the
composition of the tunica and the corpus.  Indicate which cell layers are
the source of
the histogenic layers L-I, II and III.

3. Describe the histogenic composition of primary and secondary stems,
leaves, germ
and root tissues.  What tissues and organs arise from each of these
histogenic layers?

4. Which propagation methods are most likely to preserve chimeras?  Which
methods
are less likely to preserve chimeras?



Lecture Notes:
       Genetic changes can occur in clones, even though they are asexually
propagated. Mutations occasionally occur in cells, but we generally donít
see their
effects unless the mutated cell is part of a meristem. In that case, the
cell undergoes
mitosis, producing many copies of the mutated genome.
       Even if a mutant cell is incorporated into a meristem and multiplied,
we may not
recognize the change. In the first place, most mutations are probably
recessive. The
other copy of the gene carries on the function of the gene pair. (In this
case, the
recessive mutation will never be expressed unless seed formation results in
it being
paired with a similarly mutated partner.) Some mutations may be lethal, in
which case,
the cell dies and other non-mutated cells take its place. Other mutations
result in subtle
changes that are either not visible or are difficult to distinguish from
other types of
variation. A small fraction of mutations are expressed, however, and result
in a visibly
distinct phenotype. When this happens, the mutant is separated from the
parent clone
and either culled or established as a new clone.
       Mutants may be crippling or degenerative and of no horticultural
interest. Others,
however, have considerable value. Altered growth habit, such as weeping or
prostrate
forms, and some types of variegation may originate as mutations and find use
as
ornamentals. In the fruit industry, mutations for fruit color, fruit quality
and dwarf stature,
have resulted in important new clones.
       Mutations enter meristems in either of two ways. 1) A cell in a shoot
meristem is
mutated, or 2) a mutated cell in the stem or root is incorporated into an
adventitious
meristem. In either of these situations, the meristem must develop into a
shoot or a
branch for the mutant phenotype to be seen. Such a mutant branch on a tree
is referred
to as a sport or bud-sport. Sports are an important source of new clones.
       Mutant meristems often contain both mutant and normal cells, at least
initially.
Plants in which two or more genotypes are growing separately, but
adjacently, are
called chimeras. Many horticulturally important clones are chimeras-
including some
types of variegated foliage, some nectarines, red color sports of apple,
dwarf growth
habit of some species, thornless blackberries and the original pink-fleshed
grapefruit.
Thus, chimeras are both common and important.
       In order to understand how we can have two or more genotypes growing
separately in the same meristem, we must understand how meristems are
organized.
The key to this is in the area containing the most primitive cells of the
meristem. This
area is called the promeristem and is found at the very tip of the shoot.
       The cells in the promeristem are composed of initials and
derivatives. Even
though they all look the same, they behave differently. The initials remain
in the
promeristem and keep dividing. They are called initials, because they are
the initial (or
ultimate) source of new cells in the plant. The other cells in the
promeristem are called
derivatives, because they are derived from the initials. Other than a few
initials in each
promeristem on the plant, all of the plantís cells are derivatives.
Derivatives in the
meristem undergo additional cell divisions while they remain in the
meristem, but they
eventually differentiate into the various cell-types of the stem, leaves and
buds.
       When we look under a microscope at sections of the promeristem we see
a
distinctive arrangement of cells. Usually there are two layers of cells
overlaying a less
well-organized group of cells. This pattern is called tunica-corpus, meaning
a layer and



a body. The cells of the angiosperm promeristem have a tunica-corpus
organization.
Promeristems in other groups of plants are organized differently.
       The tunica layers divide mostly perpendicularly to the plant surface,
so these
layers remain thin. The corpus divides in many planes and forms the bulk of
the plant
body. The layers of cells (two tunica plus one corpus) are called histogenic
layers,
because they are the source of plant tissue. The layers are designated from
outer to
inner as L-I, L-II and L-III. Very importantly, each histogenic layer has
its own set of
initials.The cells derived from each layer stay in relatively well-defined
zones as the
plant develops. Using genetic markers, biologists have traced cells from
each
histogenic layer into the cells of the mature plant. Thus-
1)  The L-I layer gives rise to the epidermis, the single cell layer
covering the entire
   primary structure of the plant (recall primary versus secondary plant
structure),
   stems, leaves, flowers and fruit. The L-I layer of a shoot is also the
source of L-I
   initials in the axillary buds formed on that shoot.
2)  Derivatives of the L-II layer form a layer a few cells thick just under
the epidermis. In
   stems this becomes the cortex. Sometimes L-II derivatives form part of
the phloem.
   In leaves, L-II derivatives are found in the outer and upper portions of
the leaf blade.
   In flowers, the gametes, both male and female, are derived from L-II. The
L-II layer
   of a shoot is the source of the L-II initials in the axillary buds on
that shoot.
3)  Derivatives of L-III comprise the core of the stem. From outer to inner
this would be
   part of the cortex, most or all of the phloem and cambium (if the plant
has a
   cambium), the xylem and the pith. In the leaf, L-III derivatives comprise
the central
   and lower portions. Flowers and fruits are complicated, the key is that
L-II gives rise
   to the gametes. Finally, L-III derivatives form the L-III initials in
axillary buds.

       The figures following these notes are very helpful in clarifying
these
relationships.

       You should now have a three-dimensional picture of a primary stem
composed
of cylinders of cells derived from each histogenic layer. The central
cylinder was derived
from L-III is overlain by the L-II cylinder which is in turn covered by the
L-I cylinder.
These cylinders are not perfect, the boundaries between layers are somewhat
irregular,
but they are predictable.
       Totipotency is at work here. L-I cells do not become epidermal cells
because of a
genetic predisposition. They become epidermal cells, because they are where
the
epidermis forms. If we transplanted an L-II cell into the L-I, it would
become an
epidermal cell. L-II cells in the stem are in the area where the cortex
forms. An irregular
growth of L-II cells might result in L-II cells being in part of the area
where vascular
tissue forms. L-III cells may differentiate into cortex, phloem, cambium,
xylem or pith. If
we transplanted an L-I or L-II cell into the L-III area, the cell would
become a cortex,
phloem, cambium, xylem or pith. Differentiation is based on position.
       Similar differentiation occurs in leaves, flowers and axillary buds.
       In woody plants, the cambium is a lateral meristem that forms new
phloem and
xylem. The cambium is usually derived from L-III. As a tree trunk grows and
adds new



layers each year, the new cells are derived from L-III. If L-II cells
happened to form part
of the cambium, then there would be a segment of L-II derivatives, as well.
          Now we come back to chimeras- plants composed of two or more
genotypes
growing separately, but adjacently. Given this angiosperm promeristem
structure with
each histogenic layer having its own initials, we have a means by which two
genotypes
can coexist in the same meristem.

 Examples of possible chimeral genotypes. A and B represent two different
 genotypes. Either all of the initials are one genotype or the other as in
periclinal, or
 there are both mutant and normal initials in one or more layers as in
mericlinal and
 sectorial.
                          Periclinal                         Mericlinal
       Sectorial
 L-I               A           A          B          A/B          A
A          A/B
 L-II              B           A          A           A          A/B
A/B         A/B
 L-III             A           B          A           A           A
A/B         A/B

          If the initials of L-I are all genotype A, the initials of L-II
are all genotype B and
the initials of L-III are all genotype A, then we have a chimera. Other
combinations of
two genotypes could also occur- BBA, BAB, ABB, AAB, etc. A chimera in which
all of
the initials in each layer are either one genotype or the other (either A or
B) is called a
periclinal chimera. Periclinal chimeras are the most common chimera in
horticulture,
because they are the most stable and can be easily propagated.
          How do we recognize a periclinal chimera? The most common have
variegated
foliage. Earlier, I said that leaves are composed of derivatives from all
three histogenic
layers. If the L-II initials have a mutation that blocks chlorophyll
biosynthesis, then the
leaf cells that are derived from L-II will be white or yellow. These cells
are found in the
outer and upper portion of the leaf blade. The L-III initials are still
genetically capable of
producing chlorophyll, so those derivatives in the leaf are still green.
(L-I derivatives
form the epidermis, which is clear.) The proportion and pattern of L-II and
L-III varies
somewhat from species to species, but the chimeral pattern is easily
recognizable.
          If L-II able to synthesize chlorophyll, but L-III unable, the
pattern would be
reversed.
          Another chimeral trait is thornless blackberries. Thorns are an
epidermal trait, so
if L-I has a thornless mutation, the plant is thornless. For some reason,
thornless
blackberries have so far only occurred as chimeras. L-I is thornless, but
L-II and L-III
are thorny. This has greatly complicated the culture of thornless
blackberries, as we
shall see.
          If a mutation occurs in a histogenic layer, where that trait is
ordinarily expressed,
then we will detect the chimeral phenotype. A thornless mutation in L-II or
L-III would
not be immediately detectable, for instance.
          Other types of chimeras also occur. Some of the initials in one
layer may be
ìnormalî and others mutant. This can occur in one or two layers and these
chimeras are
mericlinal. A chimera in which all three layers contain both genotypes is a
sectorial
chimera. Many chimeras often originate as mericlinal or sectorial, but
stabilize as
periclinal.



       Chimeras are propagated by methods which maintain the two genotypes
in the
same relationship in their respective histogenic layers. These methods are
those
asexual techniques which propagate axillary buds. Remember that we said that
L-I
initials in axillary buds were derived from L-I initials in the apical bud,
L-II from L-II and
L-III from L-III. This pattern of development preserves genetic
relationships such as the
periclinal patterns shown in the table above. Cuttings, grafting, layering
and careful
micropropagation propagate periclinal chimeras.
       Adventitious buds do not propagate chimeras. The reason is that
adventitious
buds initiate from cells from only one histogenic layer. Adventitious buds
usually arise
near the vascular system which is usually composed of L-III derivatives.
Depending on
the type of periclinal chimera, adventitious buds will be genetically A or
B, but not both.
(Look at the table under periclinal chimeras, L-III.) This does not mean
that adventitious
buds only have one histogenic layer. They have three histogenic layers, but
they are all
the same genotype.
       Likewise, seed formation does not propagate chimeras. Gametes are
L-II
derivatives, they will be one genotype or the other, either A or B.
Self-pollination will
result in a diploid AA or BB, depending on the type of chimera. The
histogenic layers of
the seedlings will be AAA or BBB for L-I, II and III, respectively. If an A
gamete fertilized
a B gamete, or vice versa, we would have a diploid AB, which is
heterozygous, not
chimeral. The seedlings would be AAA for L-I, II and III, assuming that A
dominates B.
       Mericlinal and sectorial chimeras are unstable and difficult to
propagate. This will
be clearer after the demonstrations in class. However, picture an L-I
histogenic layer as
a plate. Part of the plate is genotype A, the other part genotype B. If
axillary buds form
on the A/B boundary, then some of the axillary initials will be A and some B
and will still
be a mericlinal chimera. In fact, axillary buds are more likely to be
entirely A or entirely
B, because axillary buds usually donít form on the AB boundary.

Assignment 1: Due Friday, February 2, 1 page. 25 points.
       Clearly describe the tunica-corpus organization of the angiosperm
meristem and
how derivatives from the meristemís histogenic layers are arranged. What
tissues arise
from the derivatives of each histogenic layer? How do different cell types
form from
derivatives of a single histogenic layer?

Assignment 2: Due Monday, February 5, 2 pages. 25 points (2 points based on
the
holly question).
       Clearly describe periclinal, mericlinal and sectorial chimeras in
relation to the
angiosperm promeristem. (You described the promeristem in the previous
assignment.
Assume the reader has just read that.) You may include illustrative examples
from class
or elsewhere to aid this explanation. Explain why or why not axillary buds,
adventitious
buds and seeds propagate chimeras.
       Remember the photo of the holly seedlings that I showed in class? (A
black and
white copy is attached to Marcotrigianoís article.) These yellow seedlings
germinated
from seed collected from an L-II chlorophyll chimera (GYG). Holly is
dioecious, so the
pollen came from a male plant somewhere. I guarantee you that the pollen
that
fertilized the chimeral flowers was genetically green. If green dominates
yellow, how did



we get yellow seedlings from a G x Y cross? The answer is in Marcotrigianoís
article
ìChimeras and variegation: patterns of deceit.î





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