The word clone comes from the Greek word klon, meaning twig or cutting.A modern definition:
Comment: Originally clone referred to the process of vegetative propagation, used by gardeners for many centuries to propagate grape vines, apple trees, rose bushes and the like. Vegetative propagation depends on taking cuttings (slips) of shoots and rooting them or grafting them to a rootstock. This produces a new plant (or shoot, if grafted on a rootstock) that has the same genetic characteristics as the cutting from which it was taken.
Figure right: Stages of propagation from a rose bush cutting (left to right), a. prepared cutting, b. first root starting, c. additional roots starting, d. ready for planting .
The process of asexually producing a group of independent organisms or cells, all genetically identical, from a single ancestor.Are individual clones identical?
Comment: In current usage a clone may be a bacterial cell line, a rose bush or a culture of mouse cells. They must all derive directly from a common ancestor without an intervening sexual cross or genetic manipulation.
Yes and no!Duckweed colonies are natural clones.
Comment: Clones are genetically identical. If they were not, then (by definition) they would not be clones. However, cloned organisms may differ in other ways. Consider two cloned rose bushes planted side by side. One bush may get more morning sun, but its roots may have their growth resisted more by stones. These differences will affect the metabolism and, therefore, the growth of the two plants. Even small differences will add up over time to affect their overall form. Their shoots may branch at different positions, their leaves face differently, their roots grow more or less deeply, their flowers bloom more or less often. Such individual differences are real, but not genetic. They are considered "accidents" or environmentally caused. Differences between clonal individuals will be greater in species with more complex patterns of development, since this complexity provides greater opportunity for environmental accidents.
Figure right: Propagation of roses on a commercial scale.
It would be the same with cloned animals (or even humans if that ever comes to pass). Cloned animals are no more alike than identical twins. They are individuals with real individual differences.
A colony is a group of one or more attached fronds. Since the daughter fronds in the colony grow from a single mother frond, all are genetically alike. They will be clones as long as they do not undergo genetic recombination via a sexual stage. So if you wish to isolate a duckweed clone with genetically identical plants, just start a culture from a single colony of attached fronds.
Natural populations of duckweeds and other vegetatively propagated species are often a mixture of just a few clones that descended from a small number of parents. These original parents are called founders. The founders were carried into the body of water by an animal or a the current of a stream and then multiplied to populate their niche. If these mixed clones never flower and interbreed, their progeny will remain distinct. The genetic differences between clonal varieties may not be visible to the eye, but can be studied using genetic markers, such as isoenzymes (isozymes) or DNA sequence patterns.
How are clones useful?
Clonal reproduction ensures that all individuals of a variety are genetically identical. In agriculture, genetic uniformity is very useful. The breeder can eliminate individuals with susceptibility to disease and pests, those with bad flavors or otherwise unattractive to consumers etc. Thus uniformity increases the value of a farmer's produce. It also reduces the farmer's costs, since with uniform ripening an entire crop can be picked in one pass through the field.
Clones of ornamentals are also valuable. They are usually selected for their appearance or size that makes them attractive. Some are chosen for their display of flowers, floral color, scent, time of flowering, leaf color, size at maturity, bushiness, and many other attributes that will make them attractive to gardeners. Others may be chosen for the absence of something, such as fruit, which is undesirable in ornamentals. Resistance to cold and frost is important to Northern gardeners, drought tolerance to Westerners, and resistance to fungi in humid locales.
Biological disadvantages of clones.
The characteristics of clones also can be biologically disadvantageous. When the environmental conditions change, a clonal population is at a disadvantage if its genes are unsuited to the new conditions. The chances of survival are much greater if the plants in a population have a mix of different genes with different environmental responses. Having such a mixture of genes is facilitated by sexual reproduction, which ensures that many combinations of genes will be present in the population.
Cloning duckweeds and other plants doesn't intrude on bioethical considerations, unless the clone already has been patented. Patents are issued on vegetatively propagated clones, like rosebushes, flower bulbs, shrubs, fruit trees and the like to compensate the breeders for their hard work. Those plant variety patents are generally non-controversial and very common.
However, cloned animals have provoked much discussion. There are two reasons for this:
[ 1 ] Everett, TH. "Rose Propagation"
New Illustrated Encyclopedia of Gardening. Greystone Press, New York
(1960) vol. 11, pp. 1931-1933.
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Last revised: December 17, 2011