For those whose search engines
may have missed it, this is Ben's article which turned up when
searched using his parameters. This article is the first where
Dr. Zonneveld puts forth his theories on the origin of sports. Ben, do you still
hold to the things you have said here, or has later research caused you to
reconsider some of these points? This article was written several
years ago. Also, has anyone found any good
articles on plant research with this search? Most of what I found so far is
human, fruit fly, and yeast articles.
............ Bill Meyer
Mutations, Recombinations, Sports and chimaerae
By Ben J. M. Zonneveld
Leiden, The Netherlands
These four processes cause hereditary changes in hostas. The questions
discussed here are: When do these changes occur, and what causes them? I have
seen several definitions of these terms in the Hosta Journal but they are not
very clear-cut. Our purpose is to define the terms more precisely.
Processes responsible for deviation observed
Mutation is a change in DNA in a plant that can be transmitted through its
offspring. It is a rare process taking place with a frequency of about 1 in
100,000. DNA in plants can be found in the nucleus, in chloroplasts and in
mitochondria. The mutation can take place in a leaf bud during mitose (a somatic
mutation) resulting usually in a change in part of an organism. A mutation in
the flower during meiose (pollen or egg cell formation) leads to a change in the
whole plant (usually visible only in the second generation).
Mutations can also be divided into two types: nuclear mutations that inherit
via both parents, and cytoplasmic mutations i.e., in the DNA of
chloroplast or mitochondria that only can be transmitted via the mother
(seedparent). If a leaf changes from green to yellow it can be caused indirectly
by a nuclear mutation (as is usually the case in hostas) or directly by a
mutation in the chloroplast DNA. So the fact that the chloroplast is in the
cytoplasm does not mean that a change in its color always gives a non-mendelian
inheritance. On the contrary in hostas the change from green to yellow is nearly
always due to a nuclear mutation. Without mutation there is no
When a mutant mutates back to wildtype, such is backmutation. Contrary to
generally held opinions, back mutations are as rare as forward - (new)
mutations. A yellow-edged plant going back to all green is usually not a back
mutation but a chimaeral rearrangement (see Below).
A yellow plant getting a green edge is not backmutated either (or rarely so)
but rather is the result of recombination (see bolow).
Recombination or Crossing Over
In diploid plants (and except H. ventricosa and H. Clausa all
wild hostas are diploid) each character (gene) is paired with another. If one of
a gene pair is mutated usually we do not see anything unless the gene is
dominant like most yellow genes in hostas; however, it is possible due to
exchange (mitotic recombination ~ somatic crossing over) of part of chromosomes
that the vegetative offspring of such a cell gives cells in which both genes are
mutated and cells with both genes not mutated. If this is happening in leafbud
of a yellow plant this can lead to a green (white) center or edge in a
Hosta leaves have a three layered structure, but the third layer does not
contribute to leaf color. Usually, with respect to hostas we have periclinal
chimaeras, but a plant like H. 'Cheesecake' seems to be a sectorial
chimaera. The first layer covers the second inner layer like a glove. A
chimaeral rearrangement is an exchange of cells in a meristem of a leafbud from
one layer to another layer. The same result is observed when a new leafbud
starts from only one of the layers. An example is a yellow-edged plant like
H. 'Frances Williams' getting a completely yellow offset like H.
'Golden Sunburst'. This can only take place when the plant is already a chimaera
and a plant can only be a chimaera if earlier a mutation has taken place.
Chloroplast Loss or Inactivation
Especially in tissue culture, it seems that chloroplasts are easily lost or
inactivated in the outer layer leading to a whitish edge. This might have to do
with the fact that sometimes meristems are started from flowerbuds.
Frequency of the Different Processes
The frequency of recombination and chimaeral rearrangment are much higher
than the frequency of forward and back mutation. Whereas the frequency of
recombination and chimaeral rearrangement may be around 1 in 1000, the frequency
of mutation is around 1 in 100,000 to a million. So there is a differential
factor of 100 to 1000 and usually what is called a mutation is actually due to
recombination or chimaeral rearrangment.
Plants That Result From the Above Processes
A mutant is a plant that has a mutation in DNA in flower or leafbud that can
be transmitted to its offspring. The term "mutant hybrid" used in some books, is
A sport is part of a plant (an offset) that deviates from the rest of the
plant. Actually "sport" is a convenient term used by gardeners, but it is not a
genetic term. It is the end result of an earlier genetic process. In a sport the
change has taken place in a leafbud (mitose), not in the flower (meiose). Three
processes can be responsible for a sport to arise. In most cases it will be due
to somatic recombination or chimaeral rearrangements and rarely due to a (back)
mutation. Most sports can therefore only be propagated vegetatively. So a sport
can be a chimaera, but also a recombination or rarely a mutant. A seedling is by
definition never a sport.
A chimaera is a plant consisting of genetically different tissues. Two
processes can be responsible for a chimaera to arise. The origin can be a
mutation as in H. sieboldiana 'Elegans' giving the chimaera H.
'Frances Williams'. Much more often a new chimaera is the result of a somatic
(mitotic) recombination like the green centered H. 'Wagon Wheels' from
all gold H. 'Gold Nugget'.
When we encounter a plant with a deviating leaf, branch or crown there are
several explanations. These deviations can be divided in three parts, depending
on the way the plant is propagated.
- Vegetative Propagation (Cuttings, Offsets)
1. Phenotypic Variation
A plant can look different due to growing circumstances. This is called
phenotypic variation. An example is a plant growing in poor circumstances. It
will be much smaller then the same plant when growing under ideal conditions.
But this can be remedied by improving the circumstances, and cuttings
transplanted to an identical environment will ultimately not differ in
2. Variation due to somatic recombination. See above.
3. Variation due to chimaeral rearrangement. See above.
4. Variation due to (back)mutation. See above.
5. Variation due to chloroplast change. See above.
- Generative Propagation (Seeds)
In principle, during pollen and egg cell formation and in the resulting
seed the same processes can take place as during vegetative propagation.
Usually two parents are involved, but in self-fertile genera like Hosta
it can be just one. Moreover most plants don't come true from seed unlike
cuttings. The explanation for the observed variation therefore is a bit
Hybrids are always the result of a cross between two plants (unless
selfed). So there is always seed involved. Mutation has nothing to do with it.
Variation in the seedlings is often due to the fact that the parent(s) were
different in two or more genes. Only crossing two wild species will give more
or less uniform offspring in the first generation.
Chimaeral rearrangement can take place in the germline if the mutation
is in chloroplast in a single cell. A nuclear mutation can only lead to a
chimaera directly if happening at the stage that the organism already has two
or more cells.
Meiotic recombination occurs during formation of pollen and egg cells
with a high frequency. So, if the parents are not homozygous, i.e.,
have several genes in two different forms, this may lead to new combinations
in the offspring due to segregation. A plant with the dominant yellow mutation
will make pollen and egg cells with a gene for green or a gene for yellow.
Crossing with an all-green plant will then lead to 50% yellow
Mutation can also take place during seed formation but only with very
- Meristem Culture
Although meristem culture is only an artificial way of vegetive propagation
there are a few differences. In meristem culture the neat order of layers is
easily disturbed. This leads to chimaeral rearrangements with much higher
frequency than in a garden. (H. 'Frances Williams' gives a large
percentage of all gold explants). Also the mutation frequency might be higher as
the whole metabolism can be upset, but usually it will be a rare event. Lastly,
it seems that chloroplast loss/change takes place rather often if we see the
frequency with which cream to white edges are found.
Who is Afraid of Green, Yellow and White
There are basically two deviating colors in hostas: yellow or white. Yellow
plants are, as far as is known, nearly always due to a nuclear mutation. White
sectors can, in my opinion, be due to change both in the DNA of the nucleus and
the chloroplast. A yellow plant is a heterozygote with one gene for yellow and
one for green whereby the yellow is dominant. This dominance is peculiar and it
seems that the mutant protein upsets the organization in a chloroplast giving a
yellow color to it. Recombination in a yellow plant can result in a plant with a
green or a white edge. In a green-edged plant the gene for yellow has been
exchanged (recombined) for a gene for green so we have two genes for green. In a
white-edged plant the gene for green has been exchanged for a gene for yellow
and two genes for yellow may give rise to white. Usually only the green edge is
visible, but occasionally we see both green and white, as in H.
'Striptease'. Another explanation for the white edge in a yellow plant can be a
mutation in the DNA of a chloroplast but this seems less likely as mutations are
rare. However, in meristem culture cream to white edge (but not yellow ones) are
rather frequent. In The Hosta Journal, Vol. 17 No.1, page 23 there is a
list of sports found in tissue culture. In this list ten green plants all give a
cream to white edge and none arose with a yellow edge. Most likely the outer
layer had a change or loss of chloroplasts. This may be related to the fact that
meristem culture is sometimes started from flowerbuds.
There are yellows that become green, others that become white and greens that
turn yellow as the season progresses. These characters are usually stable in the
progeny. So, probably different mutations are involved in the same or in
different genes. Some stay yellow and might have yet another mutation or maybe
combined two mutations.
Other Color Deviations
Streaked plants have an irregular mixture of green, yellow and/or white in
the leaf and are unstable due to chimaeral rearrangements of the cells.
A different kind of instability is found in plants like H. 'Cynthia',
H. 'Wollcott' and H. 'Freckles'. They have unstable genes involved
in chloroplast functioning (K. Vaughn, AHJ, 16, 1985, p.71) H.
sieboldii has yet another kind of recessive mutation leading to a spotted
leaf (Yasui 1929). This mutation is however not often encountered in cultivars.
Lastly K. Vaughn mentions that in H. 'Northern Mist' the variegation is
transmitted 100 percent to its offspring even when crosses with a green plant.
So this is a dominant mutation. Plants differ in the broadness of the colored
edge or center. It may be a change in the number of cells in the meristem
contributing to the different layers.
Despite all of the above there are for sure other cases and exceptions, but
the above covers the most encountered ones.
Mutation is an event that changes DNA which its offspring inherits and this
can be in the flower and in a bud, and both in the nucleus or in the
Recombination is an event that exchanges part of chromosomes
leading to green or white in a yellow plant.
Chimaeral rearrangement is an event whereby cells are exchanged
between layers differing in color.
Sports are changes in a bud only and can be due to recombinations,
chimaeral rearrangements or rarely new mutations. Most of what we see as
variation in offsets is due to recombination or chimaeral rearrangements and
only rarely will mutations be involved. Notwithstanding that, both recombination
or chimaeral rearrangements can only take place if a mutation has taken place
earlier and so provided the ground to play on.
This article reprinted with permission from the American Hosta Society.
Information on joining the American Hosta Society and its regional and
local societies can be obtained from the AHS.
The American Hosta