Re: Clones; genetic variability and chimeras---Terms defined in a text from 2...
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Clones: Genetic Variability and Chimeras
Although clones are defined as a genetically uniform assemblage of
genetic change doesnít stop when a clone is selected. Therefore, genetic
be identified and separated from the existing clone. However, distinguishing
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
structure; and 3) how this chimeral structure is maintained through
Reading HKDG pp. 244-250.
If necessary, review the primary and secondary anatomy of plant stems
general anatomical structure of leaves, roots and fruits.
Marcotrigiano, J. 1997. Chimeras and variegation: patterns of deceit.
1. Know the following terms:
2. Describe the tunica-corpus organization of the shoot apical meristem and
to the histogenic layers. Be able to diagram a typical plant promeristem,
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,
and root tissues. What tissues and organs arise from each of these
4. Which propagation methods are most likely to preserve chimeras? Which
are less likely to preserve chimeras?
Genetic changes can occur in clones, even though they are asexually
propagated. Mutations occasionally occur in cells, but we generally donít
effects unless the mutated cell is part of a meristem. In that case, the
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
other copy of the gene carries on the function of the gene pair. (In this
recessive mutation will never be expressed unless seed formation results in
paired with a similarly mutated partner.) Some mutations may be lethal, in
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
and either culled or established as a new clone.
Mutants may be crippling or degenerative and of no horticultural
however, have considerable value. Altered growth habit, such as weeping or
forms, and some types of variegation may originate as mutations and find use
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
mutated, or 2) a mutated cell in the stem or root is incorporated into an
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
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
Plants in which two or more genotypes are growing separately, but
called chimeras. Many horticulturally important clones are chimeras-
types of variegated foliage, some nectarines, red color sports of apple,
habit of some species, thornless blackberries and the original pink-fleshed
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
The key to this is in the area containing the most primitive cells of the
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
though they all look the same, they behave differently. The initials remain
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
When we look under a microscope at sections of the promeristem we see
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
Promeristems in other groups of plants are organized differently.
The tunica layers divide mostly perpendicularly to the plant surface,
layers remain thin. The corpus divides in many planes and forms the bulk of
body. The layers of cells (two tunica plus one corpus) are called histogenic
because they are the source of plant tissue. The layers are designated from
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
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
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
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
of a shoot is the source of the L-II initials in the axillary buds on
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
cambium), the xylem and the pith. In the leaf, L-III derivatives comprise
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
The figures following these notes are very helpful in clarifying
You should now have a three-dimensional picture of a primary stem
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
These cylinders are not perfect, the boundaries between layers are somewhat
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
epidermis forms. If we transplanted an L-II cell into the L-I, it would
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
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
xylem. The cambium is usually derived from L-III. As a tree trunk grows and
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
growing separately, but adjacently. Given this angiosperm promeristem
each histogenic layer having its own initials, we have a means by which two
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
there are both mutant and normal initials in one or more layers as in
L-I A A B A/B A
L-II B A A A A/B
L-III A B A A A
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
two genotypes could also occur- BBA, BAB, ABB, AAB, etc. A chimera in which
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
because they are the most stable and can be easily propagated.
How do we recognize a periclinal chimera? The most common have
foliage. Earlier, I said that leaves are composed of derivatives from all
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.
form the epidermis, which is clear.) The proportion and pattern of L-II and
somewhat from species to species, but the chimeral pattern is easily
If L-II able to synthesize chlorophyll, but L-III unable, the
pattern would be
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,
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
If a mutation occurs in a histogenic layer, where that trait is
then we will detect the chimeral phenotype. A thornless mutation in L-II or
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
mericlinal. A chimera in which all three layers contain both genotypes is a
chimera. Many chimeras often originate as mericlinal or sectorial, but
Chimeras are propagated by methods which maintain the two genotypes
same relationship in their respective histogenic layers. These methods are
asexual techniques which propagate axillary buds. Remember that we said that
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
micropropagation propagate periclinal chimeras.
Adventitious buds do not propagate chimeras. The reason is that
buds initiate from cells from only one histogenic layer. Adventitious buds
near the vascular system which is usually composed of L-III derivatives.
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
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
derivatives, they will be one genotype or the other, either A or B.
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
a B gamete, or vice versa, we would have a diploid AB, which is
chimeral. The seedlings would be AAA for L-I, II and III, assuming that A
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
how derivatives from the meristemís histogenic layers are arranged. What
from the derivatives of each histogenic layer? How do different cell types
derivatives of a single histogenic layer?
Assignment 2: Due Monday, February 5, 2 pages. 25 points (2 points based on
Clearly describe periclinal, mericlinal and sectorial chimeras in
relation to the
angiosperm promeristem. (You described the promeristem in the previous
Assume the reader has just read that.) You may include illustrative examples
or elsewhere to aid this explanation. Explain why or why not axillary buds,
buds and seeds propagate chimeras.
Remember the photo of the holly seedlings that I showed in class? (A
white copy is attached to Marcotrigianoís article.) These yellow seedlings
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
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
ìChimeras and variegation: patterns of deceit.î
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