Chimeras. Who was Haig Dermen? by Jim Hawes

The following is a presentation given by Jim Hawes at the Winter
Scientific Meeting in Chicago, Feburary 1998.


hawesj@gcnet.net wrote:

 Dan,  I have written the following as my 15 minute presentation at
the Winter Sci. Meet. Please copy and send to Alex for his
information...yours too, of course....Jim

             Who was Haig Dermen?    by Jim Hawes, Oakland MD

 I remember Haig from about 45 years ago. That was when, as a Graduate
 Research Assistant in the Horticulture Department at the University of
 Maryland, I was loaned out to the Small Fruit Division of the Bureau of
 Plant Industry, USDA in Beltsville, MD, headed by Chief of Department
 Dr. George M. Darrow. I was the University's contribution to a joint
 project in Virus Indexing of Strawberries. I met Haig there and also
 several other times at Dr. Darrow's home in Glenn Dale, Maryland on
 Sunday afternoons. We, a group of neighbors and co-workers, played
table
 tennis in Dr. Darrow's basement regularly. I was the youngest member of
 the group. Haig and his family lived upstairs in an apartment rented
 from my brother-in-law, Dr. William H. Cowgill, who was my Horticulture
 career role model.

 Haig came from Armenia and had this black mustache...quite unusual in
 the 1950's. He studied cytology and  became Senior Cytologist at
 Beltsville. He conducted cytological research in colchiploidy of
grapes,
 periclinal cytochimeras of cranberries and peaches, histological origin
 of plant sports and chromosomal counts in cells of numerous small
 fruits. In my opinion, his most outstanding contribution to plant
 science was his work in meristem histology involving chimeras.

 Dr. Kevin Vaughn referred to Dermen's work in his article "Variegation
 in Hostas" in the AHS Bulletin 11 acknowledging Dermen's concept of
 differences in the histological layers of tissue (known as Layer 1,
 Layer 2 and Layer 3) in meristematic dome tissue of dicots and
monocots.
 These differences in layers were identified as the causal factors for
 development of varying chimeras in plants. Vaughn borrowed these
 concepts and applied them to his variegation studies in hostas. The
work
 of both scientists represents classical research which led to a better
 understanding of variegation in hostas. I found these related subjects
 fascinating and have continued studying them. I have also written a few
 articles on these subjects for the Hosta Journal and other
publications,
 all based upon the concepts that originated with Dermen in the 1950's.

 What are chimeras? No, they are not the two headed monsters that we
read
 about in mythology. They are plants that have two (or more) distinctly
 different layers of tissue adjacent to each other. Variegated hostas
are
 typical chimeras...they sometimes have green, white and yellow tissues
 in the same leaf. The tissues may be similar genetically  but differ in
 the kinds and numbers of plastids in the cytoplasm of the cells. Green
 colored tissue is green because of the high population of chloroplasts
 in the cells. In white tissues, chloroplasts are absent or limited in
numbers. In yellow tissues, carotenoid and chlorophyll b pigments are
 responsible for such coloration. Populations of various plastids are
 everchanging due to environmental and genetic controlling factors, thus
 colors often change during the growing season.

 Vaughn pointed out that the variegation patterns in hostas are due to
 the arrangement of tissues in Layers 1 and 2 of the meristems. Layer 1,
 as described by Dermen, is the one row of epidermal cells on the
outside
 covering of the meristem. As this row of cells grows, it becomes the
 outside tissue (border) of leaves and stems. If L1 lacks chlorophyll,
it
 is white. If carotenoid or chlorophyll pigments are present, the border
 tissue is cream or yellow in color.
 L2 in monocots was described by Dermen as the one layer of cells below
 and parallel to the epidermis. It differentiates and develops into the
 inside area of the leaf, (for example, the mediovariegated portion of
 the hosta leaf) and the petiole, part of the scape tissue and part of
 the florescence including the ovules. If L2 lacks chlorophyll, it is
 usually white, ivory, cream or yellow in color.
 L3 in monocots, according to Dermen, differentiates into other interior
 tissue of the leaf not involving color, the rhizome and root tissue.
 Since the Vaughn period of investigation, several authors have written
 about the Benedict Cross involving sports derived from a common
 progenitor. I have written "Clans of Sporting Clones", "Using the
 Artist's Palette to Classify Hosta Sports" and other not yet published
 articles based upon the concepts of Dermen. I have categorized
cultivars
 and species of variegated hostas into logical groups based on their
 origin and characterized by their plastid types, numbers and color
 differences. All of these differences take into account the
histological
 Layers 1 and 2 in chimeras, all based on Dermen's concepts. These
 concepts must be understood when undertaking any intelligent discussion
 of types of chimeras. The various types include periclinal, mericlinal
 and sectorial chimeras.

 These types of chimeras are recognized by viewing a meristem dome in
cross section or from above and identifying areas of the dome with
 original green tissue. The rest of the dome is different in color (let
 us say white) because a mutation has changed the ability of tissues in
 ths area of the meristematic dome to produce chloroplasts. In viewing
 the meristematic dome from above and making a sketch of it, the lower
 half could be the green, unmutated portion. The upper half could
 represent the mutated white area. Any buds derived from the lower half
 will produce green plants. Any bud derived from the white  upper half
 may produce plants with white leaves but they probably will not live
 because of lack of chlorophyll. Any buds sprouting from areas where
 white and green overlap will develop into periclinal and mericlinal
 chimeras. Periclinal chimeras have lighter colored borders. Mericlinal
 chimeras have lighter colored centers. L1 tissue is always on the
border
 in the case of marginal variegation patterns. L1 tissue is always in
the  center in mediovariegated patterns. Chimeras which are half and
half
 patterns of tissue in leaves are known as sectorial chimeras. They
arise
 in areas of the dome where mutated and non-mutated tissues join.
 Striated hostas usually have irregular, disorganized streaked areas in
 L1 and L2 tissue throughout the leaf, making them somewhat like
sectorial chimeras.
 There are other chimeras also. Dermen described how one layer of
diploid
 cells may exist adjacent to a layer of tetraploid cells. He used the
 term "cytochimeras" in the titles of several of his research
 publications indicating that the two tissue layers were distinctly
 different with respect to chromosome numbers in cells. Dermen described
 a condition where L1 could be diploid and L2 could be tetraploid,
 induced by colchicine treatment. If L3 remained unchanged, this plant
 was described as a 2-4-2 cytochimera plant. Depending upon how the
cells
 divided (anticlinally at right angles or periclinally , parellel to the
 outer epidermal layer), other types of cytochimeral plants were
 possible, i.e.2-4-4,4-2-2 or 4-4-2 type cytochimeras.

 Do cytochimeras exist in hostas? Are H.'Embroidery' and H.'Emerald
 Necklace' examples of cytochimeras demonstrating  heavy tetraploid
 tissue around the perifery of leaves causing extreme ruffling? I don't
 know but suspect this to be the case. Some research work is needed. But
not by me...I'm busy thinking and writing.

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