Cloning is the process of creating a genetically-identical (or almost identical) copy of a living thing, either naturally or through the use of science and technology. The resultant copy of the original organism is called a clone; the term “lusus” is also occasionally used by botanists. In nature, many organisms including bacteria, plants, fungi, and some animals can clone themselves through asexual reproduction; in some species this is the main way they reproduce.
Scientists using cellular biotechnology can also create clones, even of species that cannot naturally clone themselves. In science, cloning is a useful way to create laboratory specimens, but in the modern age has also been considered as a way to replenish populations of endangered or extinct species. Artificial cloning is one major component of de-extinction, as famously performed by International Genetic Technologies. This use, along with most other scientific uses for artificial cloning, has a number of moral questions attached to it which are strongly intertwined with politics and culture.
Perhaps the simplest form of cloning is fission, which includes fragmentation. Many single-celled organisms reproduce by binary fission, or splitting in two. To accomplish this, cells will usually double in mass before arranging their organelles to be divided evenly as it cleaves in half. Prokaryotic cells almost exclusively reproduce this way, and some of the organelles within eukaryotes (such as mitochondria, which may have evolved from prokaryotic organisms) do this as well.
In addition to binary fission, some organisms reproduce by multiple fission, resulting in several new genetically-identical offspring being created. This is common in eukaryotic microorganisms, but can also occur in macroscopic creatures. When macroscopic, multicellular organisms split into multiple offspring, the process is often called fragmentation. Plants such as Eurasian water-milfoil and fungi such as yeast and lichens can break apart into fragments, each of which will grow into a wholly new mature organism. Some animals can reproduce this way too; many cnidarians such as corals and sea anemones can undergo fission or fragmentation. It is also known in sponges, annelid worms, and flatworms. Echinoderms such as sea stars are particularly famous for this, being capable of regenerating into two organisms if split in half. A detached sea star’s arm can even regenerate an entire new body. In echinoderms, fission reproduction is called fissiparity.
Humans often intentionally initiate fission in captive organisms to propagate them. This takes many forms, but a common one is to take a viable clipping of an agricultural or decorative plant and cultivate it into a new mature plant. Aquarium enthusiasts and environmental groups will also practice coral fragmentation, growing new coral colonies from pieces of larger ones. This practice has helped restore reefs that had been damaged by construction projects and ocean acidification.
Vegetative reproduction and budding
Similar to fission, some organisms can reproduce by spreading outward into clonal colonies. Many plants, including strawberries and birch trees, will grow vines or roots which give rise to new plants identical to the parent. These can grow to enormous sizes: a male quaking aspen in the United States named Pando has expanded to an area of 108 acres, making it one of the largest organisms in the world. An aquatic plant, the Mediterranean tapeweed, can also form enormous clonal colonies including a 5-mile (8-kilometer) colony near Ibiza. Fungi are also known to reproduce this way using the mycelium, a fungus’s equivalent of a root structure; the largest example may be a honey fungus growing in the Malheur National Forest, which covers an area of 2,240 acres. If it is indeed a single organism, it is the largest-known living thing on Earth at this time.
When organisms that reproduce by vegetation break apart, similar to fragmentation, each resultant organism is a clone of the original. Animals largely do not breed like this, but another process called budding is known in some animal species as well as various microorganisms. When an organism buds, it produces an outgrowth of its body which eventually splits off and becomes a genetically-identical offspring. Some cnidarians such as jellyfish and hydras can reproduce this way.
Budding can occur in microscopic animals and is very similar to fission. Yeast and mitochondria can bud, but are also considered to reproduce via binary fission.
Parthenogenesis and apomixis
Some animals can reproduce by parthenogenesis, a form of reproduction that does not involve a male gamete. In this process, an unfertilized egg develops into a viable individual. Over two thousand species of animals can reproduce this way, including many reptiles, amphibians, fish, and insects. In some parthenogenetic organisms, mating is still necessary to initiate the reproductive process, but fertilization does not occur. This is called gynogenesis. All parthenogenetic frogs are gynogenic, as is the Amazon molly. These animals must mate with a male from a related species, but do not incorporate his genes; the sperm from these males is merely a trigger that starts the development of an unfertilized egg into a new female.
When plants reproduce like this, it is called apomixis. In ferns and some flowering plants, a seed embryo forms and develops into the plant’s sporophyte form without fertilization. Depending on the exact mechanism, the process is called either gametophytic apomixis or nucellar embryony. This normally occurs in female plants, but on rare occasions some male plants can perform it: for example, in the Saharan cypress, the genetic material of apomotic offspring comes completely from pollen.
Examples of parthenogenesis in de-extinct animals are known chiefly from S/F canon, including Troodon and Scorpios rex. The latter is believed to breed in a similar manner to parthenogenic frogs, meaning it would need to mate with a male theropod in order to initiate its asexual reproductive process. Whether this is also the case in Troodon is unknown. The inclusion of black-throated monitor lizard DNA into the genome of the Velociraptor Blue makes her capable of parthenogenesis, but this has yet to be observed.
A common form of laboratory cloning involves only molecular DNA, normally used to amplify fragments containing desired genes or promoters. To clone DNA, a segment of adequate size first needs to be isolated, so the parent strand must be broken apart. Then, a ligation procedure needs to be used to glue together the pieces of DNA in the desired sequence and incubated with DNA ligase enzymes. Once the pieces of DNA are formed, they are transfected into cells. There are numerous ways to do this, but the end result is the same; the successfully affected cells must be identified and cultured. Each cell in the lineage will now contain the desired DNA, meaning the molecule has been cloned.
To clone a unicellular organism scientists need only provide them with a proper growth medium; their natural reproductive processes will take care of the rest. On the other hand, cloning cells from multicellular organisms is more difficult and time-consuming. These cells have evolved to grow as a part of an organism and will not multiply readily in the same culture media used for bacteria and other unicellular life.
A helpful tool for accomplishing this is the cloning ring. A suspension of cells is highly diluted, causing them to develop into separate colonies each descended from an original single cell. At this stage, the cloning ring comes into play; this tool is a sterile polystyrene ring dipped in grease. This is used to collect cloned cells from the colonies using small quantities of trypsin. Now, the cloned cells can be obtained and used for further culturing.
One popular method of cellular cloning is somatic-cell nuclear transfer (SCNT), which is often used in medical research or stem cell therapy. The process begins with the removal of the nucleus from an egg cell, or ovum. The nucleus from an adult somatic cell is then inserted in its place. Using the instructions from this nucleus, the ovum begins developing into an embryo, genetically identical to the parent. This process can be used to insert or remove genes before the clone is created. It has been successfully used on livestock and has potential applications in the cloning of endangered or extinct species.
Clones produced through SCNT are not perfect copies of the parent, though. Mitochondria are not transferred along with the nucleus, so the mitochondrial DNA comes from the egg donor rather than the somatic cell donor. Stresses placed on both the ovum and nucleus can hinder success, and in the early days of SCNT research there were many failures. However, the end result is to produce a blastocyst-stage embryo, which yields stem cells that can develop into any type of cell. At the moment, therapeutic cloning (creating embryos for their stem cells or organ development) is much more common than reproductive cloning, but both are distinctly controversial in many cultures.
Multicellular organism cloning
To clone a whole multicellular organism, scientists and horticulturalists can either take advantage of naturally-occurring cloning mechanisms or use SCNT and related methods. Some life forms, such as many plants, unicellular organisms, fungi, and some animals, are capable of vegetative reproduction, apomixis, budding, or fission as described in above sections. In horticulture, propagation and grafting are common ways to reproduce plants without the use of intensive laboratory techniques.
Cloning an organism with the intent of creating a viable adult is called reproductive cloning, as opposed to therapeutic cloning. SCNT is the most common way to do this; the only difference is that the embryo is kept alive to mature instead of being destroyed in the course of research. The earliest use of SCNT was in 1924, performed by the embryologist Hans Spemann and his student Hilde Mangold using frog embryos. The nucleus of a somatic cell from the parent is transferred into the denucleated ovum or blastocyst of a host, and if normal cell division is observed, the embryo is transferred into a surrogate mother. Technological equivalents such as “artificial womb” devices are also a possibility, but few such technologies are currently available. The first successful clone to be grown to adulthood was a Finn-Dorset ewe named Dolly, who was created by taking a somatic cell from the udder of an adult sheep and transferring the nucleus into a sheep ovum. After 434 attempts, success was had, and she was born in 1996. Since then, many more animal species have been cloned.
De-extinct animals, when first brought back to life, have no same-species surrogate mothers in which to incubate, so suitable alternatives must be found. These are usually close relatives. There can be complications here; since the cytoplasm and mitochondria are derived from the ovum rather than the nucleus of the biological mother, incompatibilities can lead to death. International Genetic Technologies, Inc. has had success implanting the embryos of extinct archosaurs into the eggs of modern ratites such as ostriches and emus, though they utilized artificial ova for the fertilization process itself. This avoided issues of mitochondrial incompatibility. The more recently-extinct Pyrenean ibex was cloned in 2003 by Advanced Cell Technology, Inc. using other varieties of ibex and ibex-goat hybrids as surrogate mothers, but none survived. De-extinct plant life also exists, most likely created through the use of a surrogate seed which would then have to be cultivated.