By Santiago Campo
Extinction is a natural phenomenon which has happened since the beginning of time due to environmental factors (as it happened with the dinosaurs), or due to a species’ evolutionary struggles to adapt to the development of other species and the world, following the current neo darwinism evolutionary theory based in natural selection (the survival of the most capable species). However, the exponential growth in human population that has occurred in the last century, accompanied by multiple industrial and technological revolutions, has resulted in overexploitation of the world’s resources and therefore, the destruction of many ecosystems. Collectively, together with the climate change and pollution, these changes have presented many species with the challenge of finding a new suitable ecosystem in which to live, or to adapting to new ones.
The truth is scientists have yet to discover all of the species of plants, fungi and different organisms that inhabit the world. However, there is one thing we know: species such as the rhinoceros and the Tasmanian tiger are among the creatures whose population at some point disappeared, and it is possible extinction is happening thousand times more quickly because of humans (National Geographic, 2019).
Although Dolly was not the first animal to be cloned, it was actually the first animal to be cloned from an adult cell. This is why there has been such an increase in interest from the scientific community in relation to cloning, and how it could enable de-extinction and protection of endangered species.
Cloning is based on the simple biological mechanism of reproduction and cell specialization. The zygote gives rise to a multiplicity of cells that are increasingly specialized. Cloning has provided insights into nuclear differentiation, nuclear reprogramming, cellular aging and genomic imprinting (Robert G. McKinnell, 1999). There are different types of cloning depending on the subject’s class (Amphibia, Mammalian, etc.); however, all of them are based on the same principle.
Reproductive cloning is the type of cloning that would be applied in the preservation of species. To achieve this, the strategy used is somatic cell nuclear transfer (SCNT). This involves taking a somatic cell nucleus and DNA and placing it in an oocyte’s cytoplasm. Then, the changes in chromatin structure that govern differentiation can be reversed, and the nucleus can be made to control development to term. Adding the DNA from the somatic cell to the empty egg can be achieved in two different ways. In the first method, the DNA-containing nucleus is removed from the somatic cell with a needle and injected into the empty egg. In the second approach, an electrical current is used to fuse the entire somatic cell with the empty egg. The embryo is then developed in-vitro in the early stages and finally implanted in the womb of an adult female (NIH, 2020).
Taking into account the mechanisms of reproductive cloning, we could state that adopting a strategy of developing in vitro cloned embryos of endangered species and reproducing a controlled amount of them could help give some time for scientists to recreate suitable ecosystems for them. On the other hand, there are projects of cloning and reproducing already-extinct species like mammoths and passenger pigeons. To date, there has been little consistency in descriptions of de-extinction technologies and purposes. In 2016, a special committee of the International Union for the Conservation of Nature (IUCN) established the first detailed description of de-extinction, yet incoherencies in published literature persist, which even extend to the definition established by the IUCN (Ben Jacob Novak, 2018).
There are two main approaches to achieving de-extinction through cloning. Firstly, adult SCNT was used to preserve the last surviving cow of the Enderby Island cattle breed. Since a few straws of its semen remain, progeny can be obtained from the cloned female to recover the breed through sexual reproduction. To avoid the loss of this genetic pool, samples of somatic tissue from each breed should be stored in liquid nitrogen. On the other hand, in the cases where this is not possible due to the already complete extinction of the species, cross-species cloning is a valid approach to recovering endangered species (Lawrence C. Smith, 2000). An example of this could be the project of the mammoth. In this case a frozen individual of this species was found trapped in Siberian ice. This enabled scientists to extract quality DNA from its cells. The idea in this case would be to implant this DNA in an elephant oocyte due to its genetic similarity: Asian elephants share 95.8% of their mitochondrial DNA with the woolly mammoth.
Nevertheless, there is an ethical problem that arises instantaneously from de-extinction and using cloning to protect endangered species. It is true that humanity has increasingly worsened the situation of many species. However, we could not differentiate between the species that were meant to disappear anyways despite the action of humans in this world. This situation could therefore result in the survival of the weak species and interfere with evolution, which could cause a reversion in progress and the development of species. In addition, it could generate problems for some extinct species to adapt to a new world that has changed and progressed a lot since their extinction.
Moreover, the current method used in reproductive cloning (SCNT) is very inefficient because of the extraordinary demands placed on the oocyte to reprogram the somatic nucleus rather than a sperm nucleus. This causes inappropriate expression of many genes, which can result in embryonic loss, fetal, perinatal and neonatal loss, as well as production of abnormal offspring. Not all of these problems are caused only by SCNT. An example of this would be an experiment carried out in sheep in which one third of the confirmed pregnancies due to cloning ended up in fetus loss (Nature, 2002). However, there have been recent advancements in cloning through SCNT. Since the first report on cloning using this strategy (Dolly) in 1997, many technical improvements in SCNT have been made through diverse epigenetic approaches, including enhancement of the levels of histone acetylation in the chromatin of the reconstructed embryos. This prevention of epigenetic errors during nuclear reprogramming is expected to improve the success rate of animal cloning (Atsuo Ogura, 2013).
National Geographic (2019). Extinct species, explained.
Robert G. Mc Kinnell (1999). The Biology of cloning: History and rationale. BioScience, Volume 49, Issue 11 (875-885)
National Human Genome Research Institute (NIH) (2020). Cloning Fact Sheet
Atsuo Ogura (2013). Recent advancement in cloning by somatic cell nuclear transfer. Philosophical Transactions of The Royal Society: Mammalian epigenetics in biology and medicine. Volume 368. Issue 1609.
Ben Jacob Novak (2018). De-extinction. MDPI: Conservation Genetics and Genomics. Genes (Basel) 9(11):548.
I. Wilmut (2002). Somatic Cell Nuclear Transfer. Nature 419 (583-587).
L C Smith (2000). Benefits and problems with cloning animals. Can Vet J. 41(12):912-924.