Gerbera flower showing a mutation to a secondary flower color. Photo: slowlyretarding, reddit.com
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 noticeable 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 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 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 lycopersicum) by accident: it was carefully manipulated by selection until it reached its present form.
Mutations in Your Own Backyard
The observant gardener observer will occasionally find visible changes in the plants they cultivate: a flower on a double plant with single petals, 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.
Quite often, the contrary occurs: a plant with double or semi-double flowers will produce a flower with fewer petals, even a single flower. In that case, the mutation would be considered a reversion (see below).
• Peloric flower: a mutation towards radial symmetry in a flower that normally exhibits bilateral symmetry, that is, that has two symmetrical sides—left and right—while its upper side is different from its lower side. This is essentially a reversion, a return to the ancestral form of the flower. The term peloric comes from the Greek pelôros for monster.
• 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. The entire flower can be a different color or only part.
• Seedless fruits: such a mutation leads to a plant produces fruits, but ones that don’t contain viable seeds. The seeds do begin to form, but abort before they can mature. Such plants are highly prized in agriculture and include such everyday fruits as bananas, seedless grapes and Navel oranges. However, in the garden, seedless fruits are a rare mutation, often linked to polyploidy (see below).
• Variegation: bicolor foliage, one part being normal chlorophyll-producing tissue (and therefore green), the other without chlorophyll and thus albino, which reveals the leaf’s secondary pigments, usually white, cream or yellow, but sometimes pink or other colors. Sometimes flowers also show a similar two-tone effect. Variegated 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. This is 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.
However, some albinistic plants are saved by grafting them onto a related green plant that can carry out photosynthesis in their place. The famous red ball cactus (Gymnocalycium mihanovicii friedrichii ‘Rubra’, also called Hibotan) is an albino cactus (unusually, with red pigmentation rather than white) that is able to survive because it is grafted onto a green cactus that can carry out photosynthesis.
• 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”) or a stem mutates to such a coloration.
• 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 true mutation.
• Witches’ Broom: A dense mass of abnormally short shoots begins to develop from a single point, resulting in a structure that resembles a broom or a bird’s nest. Conifers are particularly subject to witches’ brooms.
A witches’ broom is not always a mutation, though. Sometimes it can be a symptom of a disease caused by different organisms: mites, bacteria, fungi, dwarf mistletoes, insects, phytoplasmas, viruses and others.
When it truly is a mutation, on the other hand, it is often possible to take cuttings and thus reproduce the witches’ broom which will continue to live on as a dwarf plant. The majority of the many dwarf conifers, for example, originally come from witches’ brooms.
• Polyploidy: not too visible except 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.
Transmissible 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 sometimes 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 the end of the season.
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. 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 needs 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 produce reversions (returns to the ancestral form), at least on occasion. The harlequin 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 green 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 original plant, sometimes to the point where the desired trait (variegated foliage in the case of the harlequin maple) gradually disappears. Plants with colored foliage (bronze, golden, variegated) seem to be particularly prone to reversals.
One of the most striking and common reversions are seen when a dwarf conifer, with a dense, shrublike habit, suddenly produces a normal-sized branch with normal node spacing. If the reversion is not removed, the dwarf shrub will soon turn into a full-sized tree!
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.