Animal cloning

SIGNIFICANCE: Animal cloning is the process of generating a genetic duplicate of an animal starting with one of its differentiated cells. Sheep, mice, cattle, goats, pigs, cats, and dogs are among the animals that have been cloned. While an inefficient process that may pose risks to the clone, animal cloning offers the benefits of replicating valuable animals.

Clones and Cloning

Asexual reproduction occurs in numerous bacteria, fungi, and plants, as well as some animals, leading to genetically identical offspring or clones. In addition, humans can assist in such reproduction. For instance, cuttings from plants generate thousands of replicates. Dividing some animals, such as earthworms or flatworms, allows them to regenerate. However, most vertebrates, including all mammals, reproduce sexually, requiring fertilization of an ovum by sperm. In such species, clones occur, as in the case of identical twins, when an embryo splits completely early in development. This process can be instigated artificially using microsurgical techniques to divide a harvested early-stage embryo and reimplanting the halves into surrogate dams (mothers). While this can be considered animal cloning, the term should be reserved for cloning from nonembryonic cells.

Cloning Procedure

Animal cloning typically refers to mammals or other higher vertebrates and involves creating a duplicate animal starting from a differentiated cell. Although such a cell only has the ability to perform its specialized function, its nucleus retains all genetic information for the organism’s development. Animal cloning requires that such information be reprogrammed into an undifferentiated cell that can reinitiate the developmental process from embryo to birth and beyond.

In theory, the process, known as "somatic cell nuclear transfer (SCNT)" or "fusion cell cloning," is straightforward. It consists of taking a differentiated cell from an adult animal, inserting its nucleus into a donor ovum whose own nucleus has been removed, initiating embryonic development of this ovum, inserting the resultant embryonic mass into a receptive surrogate dam (in estrus) and allowing it to proceed to term. In practice, the technique is difficult and was thought to be impossible until 1997. It also appears fraught with species specificity. Various differentiated cells have been used as the starting source; mammary cells were used in the first case, while skin fibroblasts and cumulus cells are also often used. The preparation of the enucleated ovum is an important step. A limitation to cloning dogs appears to be the difficulty in determining when estrus will occur. The technique for inserting the nucleus is crucial, as is the conversion to the undifferentiated embryonic state. Transfer of the embryonic cells to a receptive surrogate dam is generally a well-developed technology, although more than three viable embryos are necessary to maintain pregnancy in pigs.

Furthermore, the genetic makeup of a putative clone must be verified to ensure that it is indeed a replica of its progenitor and not an unintended offspring of either the donor of the ovum or the surrogate dam. DNA fingerprinting via microsatellite analysis at a number of polymorphic sites is an unambiguous way to establish its genetic identity.

Identicalness

Such a clone is not absolutely identical, because of mitochondrial differences and environmental effects. While the nuclear genome must be identical to its progenitor, the mitochondrial genome of the clone will invariably be different, because it comes from the ovum used. While mitochondria make a minor contribution to the total genetic makeup, they can influence phenotypic expression. In addition, the prenatal environment can affect some traits. Coat color and color pattern are characteristics that can be developmentally influenced; the first cloned cat was not an exact duplicate of its progenitor in coloration. Some behavioral features are also impacted during intrauterine development.

Cloned Animals

The first cloned animal was a sheep named Dolly. While she was the only live offspring generated from 277 attempts, her birth showed that was possible. Shortly thereafter, mice and cattle were cloned. Reproducible cloning of mice is more difficult than imagined, whereas more cattle were cloned in the first five years after Dolly’s birth than any other species. Goats, pigs, cats, buffalo, gray wolves, and a camel were among the animals that were subsequently cloned.

Problems and Potential Benefits

Prominent among the problems with animal cloning is its inefficiency. Although this may not be surprising as the technology continues to evolve, SCNT still had a less than 10 percent success rate in the 2020s. In addition to SCNT's poor success rate, most cloned animals are larger than normal at birth, often requiring cesarean delivery, and some have increased morbidity and mortality. Some have had smaller telomeres and shorter lives. Dolly exhibited this trait and lived for only six years (although she was euthanized, she clearly would not have lived much longer)—half of the average life span. Conversely, some cloned mice do not exhibit shortened telomeres or premature aging, even through six consecutive cloned generations. Further research will establish whether these problems are inherent to cloning, are consequences of some aspect of the current procedure, or are attributable to the small numbers of cloned animals studied.

The benefits of animal cloning would involve duplicating particularly valuable animals. Livestock with highly valued production characteristics could be targets for cloning. However, the technique is likely to be most beneficial in connection with transgenesis, to replicate animals that yield a therapeutic agent in high quantities or organs suitable for transplantation into humans. Some researchers also hold out hope that cloning could one day help stabilize the populations of endangered species. For example, US researchers have achieved groundbreaking milestones in preserving the genetic diversity of the endangered black-footed ferret through SCNT. Using tissue samples from Willa, a black-footed ferret, who had not reproduced and who had died in 1988, scientists successfully produced Willa's first clone, Elizabeth Ann, in 2021. Although Elizabeth Ann was unable to reproduce, another clone, Antonia, gave birth to two healthy kits in 2024. Antonia and her descendants have the potential to increase the genetic diversity of the North American black-footed ferret population, which is descended from a handful of individuals.

While countries such as the United States and China have not found any difference between healthy clones and healthy animals from conventional breeding and have been cloning livestock for breeding purposes for some time, in September 2015 the European Parliament voted in favor of banning the cloning of all farm animals. In addition, the ban extended to the sale of cloned livestock, their offspring, and any products derived from them. Arguments in favor of the ban centered upon animal welfare and claims that the inefficiency of the cloning process causes more harm to animals than it is worth; Europeans have also harbored more distrust regarding the safety of food products and cloned livestock. Meanwhile, only months later, the chief executive of BoyaLife announced that the company planned to build the world's largest animal cloning factory in Tianjin, China.

In 2023, Chinese state media reported that scientists at China's Northwest University of Agricultural and Forestry Science and Technology had successfully cloned three cows capable of producing 18 tons of milk annually, or about 1.7 times the amount an average cow in the US produced in 2021. The state media touted the so-called super cows as a way for China to decrease its reliance on imported cattle.

Chinese scientists have also worked on cloning primates using SCNT. In 2018, a research team based in Beijing and Shanghai reported that they had successfully used SCNT to clone two cynomolgus monkeys, Zhong Zhong and Hua Hua, who were still alive in 2024. In January 2024, the team published a study about their success in using a modified version of SCNT to clone a rhesus monkey, Retro, who had been born in 2020 and was still thriving.

The cloning of nonhuman primates was the subject of debate among scientists and ethicists in the 2020s. Proponents maintained that cloning nonhuman primates could accelerate the pace of biomedical research, given that humans share more similarities with other primates than with laboratory mice. Other proponents argued that cloned monkeys could be genetically engineered in ways that could make disease modelling more effective and efficient. Animal welfare advocates argued that the use of cloned animals in research posed significant welfare and ethical concerns because the procedures involved in cloning could cause pain, distress, and high mortality rates.

Key Terms

• asexual reproductionreproduction not requiring fusion of haploid gametes as a first step
• clonea genetic replica of a biological organism
• differentiated cella somatic cell with a specialized function
• mitochondrial genomeDNA found in mitochondria, coding for forty genes in humans, involved in energy metabolism, and maternally inherited
• nuclear genomeDNA found in the nucleus, coding for 30,000 genes in higher organisms, half inherited from each parent
• telomerea specialized structure at the chromosome end, which shortens in somatic cells with age

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