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Summary Conclusions....State of the Science re Origin of Sports

  • Subject: Summary Conclusions....State of the Science re Origin of Sports
  • From: Jim Hawes <hawesj@atlantic.net>
  • Date: Sun, 01 Jul 2001 12:15:10 -0400


Because several questions have arisen this AM about "sorting out", I am
copying what a subpanel has submitted to summarize the current "State of
the Science" on this subject. If you will read it carefully, it might be
helpful in understanding the Origin of Sports better.Maybe Narda will
enjoy reading it again.

Jim Hawes


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

As stated in the summary conclusions regarding Zonneveld's Rule of Thumb
theories re origins of sports, this sub-panel has disagreed with his
theories. Therefore we have prepared a summary of what we think is the
current understanding of the causes for sports in plants. These current
understandings are believed to apply to hostas as expressed in personal
correspondence by Dr. Michael Marcotrigiano, world authority on
variegation in plants. The conventional wisdom as described follows
closely the processes he discusses in his 1997 HortScience article
"Chimeras and Variegation: Patterns of Deceit". 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 on origins of
sports. They do not resemble or appear to be even close to the theories
expressed by Zonneveld's Rule of Thumb.Therefore this sub-panel suggests
complete "displacement" of Zonneveld's theories ( from the inside to the
outside) to complete "replacement" of the Conventional Wisdom  school of
thought ( from the outside to the inside...as in tissue switching) as a
more acceptable doctrine to explain chimeras in hostas. A summary of
these processes follow.

In Patterns of Deceit, Marcotrigiano , leading representative and
authority on varigation in plants, 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 controlled.

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. (Note: Jim Hawes happens to know, being a fellow alumni
some years earlier from the same Department of Horticulture). In Hosta,
there are three layers of tissue in the meristem, but only two, LI and
LII,  exist in the hosta leaves according to Tilney-Bassett. 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 ocurred
(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 vegative 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 depending upon the proportion of green or
white (or yellow) proplastids in the eggs 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 described..

     (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.

     (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
chromasomal 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.

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. 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.

Jim Hawes
Bill Meyer
Dr.Joe Halinar
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