Re: Origin of Sports
Dear Kay Dye, and others interested in sports,
This is the second e-mail I mentioned which was sent to Hosta Friends on
Mar. 30 1997. I answered several questions raised by Kay.
Jim Hawes wrote:
> Dear Kay,
> On Easter Sunday morning you wrote a message to honza raising some very
> key questions about sports. To my knowledge, no one has yet answered all
> of your questions. You asked about the origin of sports; what caused
> sports to arise?; did genetic changes take place?; if two sports arise
> about the same time in two geographical locations and are similar in
> appearance, are they close genetically? and do we need more sports? The
> last question is very easy. Of course we need more sports!
> But what a great series of questions. I will try to answer them as best
> I can. The following presentation about the origin of sports is compiled
> mostly from articles and talks I have prepared previously...a cut and
> paste effort. If it does not flow, blame it on the glue.
> Sports are new, different looking plants that arise from buds of parent
> plants. Differences are largely in coloration of leaves. Let's look at
> the differences from the histological (tissue and cellular level)
> viewpoint. Color differences occur because the mix of plastids (and
> their corresponding pigments) in cells change. The change in plastids
> occurred because of several possible phenomena. These include:
> Mutations of DNA in chromatin material of the nucleus of cells
> Mutations of DNA material in the plastids themselves
> Cell division and differentiation phenomena that create variegation
> patterns in leaves
> Shifts in tissue layers that create changes in variegation patterns
> (known as chimeral aberrations)
> Crossingover of chromosomes during mitosis (somatic cell division)
> creating new combination of genes in affected chromosomes which may
> influence plastid types and numbers (known as recombinants)
> Overgrowth of one tissue layer over another creating new color
> patterns (reversions, or sporting back to original forms)
> Mutations of DNA in mitochondria may cause changes in vigor but not
> necessarily in color.
> The above listed histological and genetic causes of sporting are my own
> opinions based upon rather comprehensive study of the subject of sports.
> They differ somewhat from the views of Ben Zonneveld as presented in his
> Journal Article "Mutations, Recombinations, Sports and Chimaerae"(see
> Vol.27, No.1), but not significantly.
> But how do these sports start in the buds in the first place? An
> understanding of histological growth and development of cells and
> tissues in the meristem is fundemental if one is to understand the
> phenomenon of variegation and sporting in Hostas.
> If we were to examine with a microscope a lateral section of the
> meristematic dome tissue in the minute growing point of a growing Hosta
> shoot, we would find that the growing point is composed of three
> parallel layers of cells. The first layer of cells (L1) is the epidermal
> layer. The second (L2) is the sub-epidermal layer of cells. The third
> layer (L3) is not involved in growth and development of leaf tissue. In
> each layer of cells, only a few cells divide continually producing other
> cells which then differentiate into functional tissues such as leaf
> primordia (new tiny leaves) and bud primordia (new tiny lateral buds in
> axils of new leaves). According to work of Dermen (1960) in monocots,
> cells of L1 origin develop into epidermal and border tissue of the leaf.
> Cells of L2 origin differentiate into tissue occupying the central
> portion of the leaf, streaked tissue of the flower scapes and ovary
> tissue inside of flower buds. Cells of L3 origin differentiate into
> tissue in the rest of the plant, such as rhizome and root tissue.
> If the DNA contained in a chloroplast in a specific dividing cell in L1
> mutates to create a different set of enzymes, amino acids and proteins
> that affect the chloroplast, it may change and grow in numbers. If the
> cell containing these new plastids then divides several
> times,differentiates and demonstrates new genetic characteristics, it
> may develop into border tissue of the tiny primordial leaf with a color
> different from its parent. A plastid mutation has create a new leaf
> sport. L2 tissue has remained the same. Only L1 tissue has changed. An
> axillary primordial bud grew simultaneously as the primordial leaf grew.
> It has identical tissue as the primordail leaf (It has border tissue
> with plastids that have changed as a result of the plastid DNA
> mutation). If it ever grows, it will be identical in appearance to the
> leaf sport which we observed.
> Another scenario is that the chromatin material in the nucleus of a
> critical cell in L2 mutated to create other major changes in the
> plastid's types or numbers. Perhaps the changes were even more drastic
> than the previous example. Controlling enzymes, amino acid and new
> proteins might cause the previous chloroplast to self- destruct, thus
> reducing the amount of chlorophyll a and b in the cell where the nuclear
> mutation occurred. As the cell divides, differentiates and grows into a
> part of the leaf primordial tissue, the leaf may grow to have a central
> portion with less chlorophyll than the rest of the leaf.The resulting
> primordial leaf has developed into a medio-variegated form. Since the
> primordial axillary bud at the base of the leaf originated from the same
> original mutating cell, it is identical genetically and, if it grows in
> the future, leaves from the bud will be identical in appearance to the
> leaf sport. The plot has become very complicated, has it not, Kay?
> These are only two scenarios, but we could continue and develop others
> to explain all of the various histological and genetic causes of sports
> mentioned previously. To do so would make this presentation extremely
> complex and long. I would prefer to postpone a discussion of some of the
> causes to another occasion....for example, when we get into tissue
> culture aberrations, that would be the better time to discuss Chimeras
> as causes of sports.
> Some of your questions are yet unanswered. What causes these sports that
> may be the result of nuclear or plastid mutations of DNA? It is well
> known that mutagens of various types cause changes in the structure of
> the long organic molecule that comprises DNA. These mutagens could be
> chemicals, biological agents, physical shocks of various kinds and
> radiation particles, such a X-rays, gamma rays, beta rays, cosmic rays,
> ultra-violet light rays and perhaps others. Mutagens change the sequence
> of constituent components of the DNA molecules. Changes in sequence
> produce different enzyme, amino acid and protein end products in the
> cells giving them different characteristics. This is a very simplistic
> explanation but is perhaps adequate at our current level of discussion.
> What about sports that occur almost simultaneously at different
> geographical locations? Are they identical? A correct answer to this
> question would be very speculative. A correct answer could only be
> determined by comparative testing and observations of the sports
> involved. We don't have experiment stations set up to do this
> investigations. The obvious alternative is what we are presently doing,
> comparing notes with each other, growing similar sports side by side (as
> I am doing with 'September Sun','Lunar Orbit' and 'Lunar Magic'), and
> reporting on similar sports when we find them. All of this is
> interesting and fun. An understanding of the phenomenon of sports helps
> it become even more fun, don't you think?
> I hope this discussion has been interesting and helpful. I few details
> may be somewhat controversial but we can discuss all of these details in
> time to have even a better understanding of what is happening down
> inside of the meristem, into the cells, along side the plastids, and
> even into the DNA molecules themselves. Happy Easter everyone!
> Jim Hawes
> Oakland MD
> email@example.com...now <firstname.lastname@example.org>
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