hort.net Seasonal photo, (c) 2006 Christopher P. Lindsey, All Rights Reserved: do not copy
articles | gallery of plants | blog | tech blog | plant profiles | patents | mailing lists | top stories | links | shorturl service | tom clothier's archive0
Gallery of Plants
Tech Blog
Plant Profiles
Mailing Lists
    Search ALL lists
    Search help
    Subscription info
Top Stories
sHORTurl service
Tom Clothier's Archive
 Top Stories
New Trillium species discovered

Disease could hit Britain's trees hard

Ten of the best snowdrop cultivars

Plant protein database helps identify plant gene functions

Dendroclimatologists record history through trees

Potato beetle could be thwarted through gene manipulation

Hawaii expands coffee farm quarantine

Study explains flower petal loss

RSS story archive

Discussion on Origin of Sports...Summary Conclusions re State of the Sciencem

Discussion on Origin of Sports---Summary Conclusions re State of the

       We have prepared a summary of what we think is the current
understanding of the causes for
sports in plants, after a review of the research done in this field.
Currently the outstanding scientist
working in plant chimera research is Dr. Michael Marcotrigiano. He has
published numerous scientific
articles on the subject, dating from the late '80's to the present. As
of now he has not researched Hosta
variegation specifically, but we can hope that sometime in the future he
will. In the beginning of this
discussion, reference was made to the "Conventional Wisdom" about the
causes of variegation in plants.
In his 1997 HortScience article "Chimeras and Variegation: Patterns of
Deceit", Dr. Marcotrigiano
gives an overview of this subject.

     A  debt of gratitude is due Dr. Marcotrigiano and other co-workers
and fellow scientists in the field
of variegation in plants. All of these scientists appear to agree in
principle with the processes that will be
described in this brief summary. They represent our best summary of an
explanation of the origins of
sports, based on the research that has been done and reported on to

A summary of these processes follows:

In "...Patterns of Deceit", Dr.Marcotrigiano describes the several
processes and inter-related
mechanisms that exist in plants. He attempts to explain the
misconceptions and misuses of terms
frequently made in discussing causes of variegation and chimeras. He
defines the terms and describes
the processes that are responsible. He discusses the genetic and
anatomical reasons for both cell lineage
and non-cell lineage types of variegation and chimeras in plants as
categorized by Kirk and
Tilney-Bassett (1978). Hostas fall within the cell lineage type of
variegation types  that are genetically

Cell lineage variegation occurs in genetic mosaics ( that is, plants in
which cells of one color are clonally
related). Marcotrigiano defines chimeras as plants which contain cells
that are not genetically similar in
the apical meristem. He describes the "tunica-corpus" structure of the
meristem (according to Schmidt,
1924) in which distinct layers exist and usually maintain their
integrity and independence from each other
(Stewart and Dermen, 1979). Stewart, BTW,  was Marcotrigiano's principal
advisor for his Doctorate
work at the University of Maryland in the early 1980's.

In Hosta, there are three
layers of tissue in the meristem, but only two, LI and LII,  exist in
the hosta leaves according to
Vaughn, (1980). Stewart and Dermen  (1979) determined that periclinal
chimeras are the most stable
form since apical layers remain independent of each other and auxillary
buds possess the same apical
organization as the terminal buds from which they were generated.
Marcotrigiano explained that there
exist apical initial cells within each apical cell layer which give rise
to all the cells of that layer and
ultimately the entire body of the plant. When the cells from an interior
layer take over the position of  the
cells from an outer layer,"displacement" occurs. When the opposite
tissue position occurs through
periclinal division (parallel to the perimeter) , "replacement" is said
to occur. (this terminology from
Stewart and Dermen,1970). Thus different bud sports may occur through
layer switching. The stability
of chimeras is largely dependent on the stability of the apical initial
cells.Chimeras differ not through
gene expression but by stability of apical cell layers.

In a sub-section of his article, entitled Origin of Chimeras,
Marcotrigiano lists and describes several
causes for their origin. Let us review the most important.

      !. Genetic changes in nuclear or chloroplast genome. He states
that any change on one or more cells
in a shoot meristem can result in a chimera. Nuclear mutations or
plastid mutations can result in changes
that are obvious, such as pigment mutations or changes that are not
noticable. Marcotrigiano states that
the most common cause of chimeral variegation is spontaneous mutation
affecting chlorophyll synthesis
or plastid morphology. These mutations are chloroplast mutations rather
than nuclear mutations. Such
mutations produce cells with two types of plastids called
"heteroplastidic"  cells. . Chloroplasts divide,
increase in population within the cell and sort themselves out by the
formation of cell walls between
cells, producing a cell line with only one type of chloroplast. When
sorting-out is completed, all plastid
decendants in the cell line are identical, resulting in "homoplastidic"
cells, producing a mosaic of green
and white (or yellow) areas of various sizes and shapes, depending upon
the time sorting-out occurred
(Jogannathasn and Marcotrigiano,1986). Large areas of white or yellow
represent early sorting-out or
rapid growth of some of the mosaic tissue. Stable periclinal chimeras
develop only after apical initial
cells are sorted-out. If a non sorted-out vegetative meristem becomes a
floral meristem, the eggs may
contain a mixture of mutant and non mutant plastids in different
proportions, depending upon the
number of proplastids contained in the eggs. Mosaic seedlings which
result will contain varying amounts
of green
and white (or yellow) tissue. They do not follow Mendelian patterns of
inheritance. Maternal inheritance is the common
form because of the influence of proplastids in the cytoplasm which  is
perpetuated in the new cells as
they divide and grow.

Other processes responsible for chimeras are listed by Marcotrigiano.
Aside from:
     (1)  Genetic changes in nuclear or chloroplast genomes-

other possible processes responsible for chimeras are listed and

     (2) Transposable genetic elements. In some species a series of
genetic elements can be transposed
from one position or may cause chromosome breaks resulting in mosaics.
This is reported to have
happened in maize, petunia and Ipomea but not yet in hostas. This is a
new area of research which
shows some promise of offering some explanation of the causes of plant

     (3) Graft induced chimeras. Adventious shoots arising from tissue
at graft unions in some species
 has been reported to result in chimeras. This has not occurred in

     (4) Semigamy is a rare phenomenon involving incomplete
fertilization resulting in  haploid chimeric
embryo with patches of tissue derived from maternal and paternal
sources. It is not applicable to hostas.

     (5) Tissue culture proceedures to develop chimeric plants from cell
culture has not been very
successful to date.

     (6) All other proceedures mentioned by other scientists include
induced  changes by chemical  or
irradiation treatment to bring about chromosomal or cellular
modifications creating point mutations,
gene deletions, duplications, inversions, chromosome crossing over or
polyploidy. These proceedures
have not attained common usage in hosta research or development.

Items 3. to 6. are not considered germane to this discussion. It should
be noted that mitotic
recombination is not included for consideration as a significant cause
of plant chimeras. The sub-panel
points out that the processes described are not our own opinions, nor
even those of one authority.
Rather they are the composite views of a group of respected scientists.
The bibliography appended to
Dr. Marcotrigiano's article is thorough and extensive and includes a
virtual history of scientific research
into the phenomenon of plant chimeras. Their research has withstood the
scrutiny of their peers over
decades. This summary simply reports on currently accepted views of the
current state of the science.

To put it simply, the main causes of plant chimeras as they are
understood today are:

1. Chloroplast Mutations

2. Nuclear Mutations

There is some reason to think that transposable elements may in the
future be found to play an important
role as well. Additionally it is important to note that tissue transfer
or chimeral rearrangement plays an
important part in what we see in Hosta. It is not listed above as a
cause of variegation because this process
as described by Stewart and Dermen involves the switching of already
existing tissues.

Jim Hawes
Dr.Joe Halinar

Bill Meyer

To sign-off this list, send email to majordomo@mallorn.com with the

 © 1995-2017 Mallorn Computing, Inc.All Rights Reserved.
Our Privacy Statement
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index