The word “mutation” often evokes images of evil, slimy creatures slithering out of nuclear waste ponds in one of those B-class science fiction or horror movies, but in fact, most mutations are simply small changes in the DNA (genetic structure) of a living being and actually have no visible effect on its appearance or health. Sadly, some mutations are harmful (most cancers are mutations, for example), but others can be beneficial. In fact, one way of seeing the living world around us is as the results of millions of years of good mutations giving rise to all that this planet’s varied inhabitants.
And just about everything we grow in our gardens, from vegetables and fruits to ornamental plants, came about, to begin with, by natural selection from spontaneous mutations in the wild, but from then on, through human intervention: people making further selections from mutations, provoked or accidental, that occurred in culture. That big, fat, juicy Beefsteak tomato didn’t arise from the tiny 1/2-inch (1.5 cm) fruit of the wild tomato (Solanum lyocpersicum) by accident: it was carefully manipulated by selection until it reached its present form .
Mutations in the Garden
The observant gardener observer will occasionally find visible changes in the plants he cultivates: a flower on a double plant with single flowers, spontaneous variegated foliage on a plant that was originally all green, a distinctly different color on one single flower, etc.
Among the mutations most commonly observed on plants in a garden setting are:
• Double or semi-double flowers: an increase in the number of petals or sepals. Or sometimes anthers and pistils mutate into simili-petals (petaloids), giving a double bloom.
• Flowers of a new color: a stem suddenly produces flowers of different color from the others. Sometimes this is a reversion to an ancestral form, but often this is an entirely new color.
• Variegation: bicolor foliage, one part being normal chlorophyllous tissue (and therefore green), the other without chlorophyll and thus albino, which reveals leaf’s secondary pigments, usually white, cream or yellow, but sometimes pink or other colors. Sometimes flowers also show a similar two-tone effect. These plants are generally “chimeric”: that is, they have two types of cells in the same plant, growing side by side.
• Albinism: complete absence of chlorophyll. Mostly seen in young seedlings (trying sowing Citrus and you should see some), albinism is usually rapidly fatal. The pale white to cream seedling lives for a while on reserves contained in the seed, but as soon as they are depleted, it dies because it can’t perform photosynthesis and feed itself.
• Colored Foliage: green is the basic color of most foliage, but sometimes seedlings are produced with an unusually dark purple to reddish or chocolate foliage (called “bronze” in horticulture) or lime green to chartreuse yellow leaves (referred to as “golden”).
• Fasciation: the stem or flower is abnormally flattened, often in a “cockscomb” pattern, thus growing in width rather than lengthening normally. Sometimes fasciation is a genetic mutation and can even be transmitted by seed (cockscomb celosia, Celosia argentea cristata, for example), but it can also result from a disease transmitted by an insect that causes abnormal growth, in which case it would not be considered a mutation.
• Polyploidy: especially visible to those who know the plant well, because the difference is often subtle: stronger stems, thicker leaves or petals, longer-lived plant or flower, etc. The plant can go from being diploid, with two pairs of chromosomes, which is the normal condition for most plants, to triploid (3 pairs), tetraploid (4 pairs), hexaploid (6 pairs) and even further. Once such mutations, which often give extra robust plants, occur, they are often widely used in hybridizing.
Transmissable or End of Line?
Most mutations occur in somatic (non-reproductive cells) and, in animals, will therefore die with the animal that produced it. Many plants can however be propagated asexually, through cuttings or grafts, for example, and, if so, the mutation can be propagated.
If variegated foliage appears on a shrub, for example, it might be possible to produce an entirely variegated plant, as shrubs can usually be propagated by cuttings. Mutations on annual plants, on the other hand, will likely die at the end of the growing season, as true annuals can only be propagated by seed (sexual propagation). So if the same variegated foliage appeared on a marigold (Tagetes), an annual plant that reproduces only by seed, the mutation would die with the plant at end of the season.
It was discussing a non-transmissible mutation that lead me to write this piece. A reader, Lise Ouellet, sent me a photo of an Oriental poppy (Papaver orientale) that produced, last year, one mauve pink flower while all the others on the plant were salmon pink. The following year, no mauve flowers were present, all were salmon again. In this case, the mutation likely took place on a single stem of the plant rather in its crown. Oriental poppies are perennials, but their flowering stems are annual organs. Therefore the mutation was lost when the stem died back. It might have been possible, under laboratory conditions, to take a piece of stem from the mutated part of the poppy and multiply it in vitro to preserve the mutation… or maybe not. (Mutations are not always stable: more about that later.)
A mutation in the reproductive cells, on the other hand, is generally transmissible… unless it is a lethal gene, of course. That’s how many of the complex flowers we grow as ornamentals were developed, for example. Double roses (Rosa) and double columbines (Aquilegia) are usually fertile, with functional anthers and pistils, and furthermore the trait is a dominant one: only one gene need to be present for the flower to be at least somewhat double. When crossed with single-flowered relatives, then this will usually give a good share of semi-double to fully double flowers in the following generation. Hybridizers can easily use genetically transmissible traits like this to develop improved plants.
But not all double flower genes can be transmitted by pollination. Double tulips (Tulipa), for example, are always sterile: their anthers and pistils have all been converted into petaloids, leaving nothing that can be used for sexual propagation. So you simply can’t hybridize double tulips. Instead, they only come from spontaneous mutations, when a mutant seedling occurs or when a normally single tulip spontaneously mutates to a double form. Once a double tulip appears spontaneously, it can be propagated by dividing its bulbs and sold commercially… if it is stable, that is.
And many mutations are not stable. They tend to return (mutate back) to their ancestral form. That’s why hybridizers are normally required to multiply their latest introductions over 3 generations (mother, daughter and granddaughter have to be identical) before releasing them to the market in order to prove they are true to type.
Despite this, many plants on the market do on occasion produce reversions (returns to the ancestral form), at least on occasion. The harlequin Norway maple (Acer platanoides ‘Drummondii’), for example, grown for its variegated leaves, green edged with creamy white, almost always produces a branch or two with the original entirely foliage over its very long life. Reversions are the most common mutations found in home gardens.
Gardeners should remove reversions, otherwise they tend to increase in size and come to dominate the plant, as they are usually more vigorous than the mutation, sometimes to the point where the desired trait (here variegated foliage) gradually disappears. Plants with colored foliage (bronze, golden, variegated) seem to be particularly prone to reversals.
Can Humans Provoke Mutations?
Most mutations are spontaneous: they happen quite by accident, but human beings have long tried to provoke them. We know in particular that X-rays can cause mutations. These are usually harmful mutations, but sometimes interesting traits show up. Thus, many plants now currently widely grown, including many fruits and grains, were produced by bombarding their parent plant with X-rays.
Some chemicals have similar effects. Colchicine, for example, a drug derived from the autumn crocus, Colchicum, a pretty flowering bulb, is well known for its capacity to stimulate the duplication of chromosomes (polyploidy).
However, these treatments (and others) remain largely tools for scientific experimentation. The home hybridizer tends to work with naturally occurring mutations, crossing and backcrossing them to try and bring out a superior cultivar, rather than trying to provoke new ones.
And there you go! A fairly simple explanation on mutations in our plants. Keep your eyes open: you may find a very original and possibly even valuable mutation among the plants you grow.