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 variegation.
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 plant.
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 misnomer.
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.
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 size.
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.
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 different.
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 offspring.
Mutation can also take place during seed formation but only with very low frequency.
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 chloroplast.
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.
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