The prospect of extinct species once again roaming the earth has always been a science fiction fantasy. However, with the advance of genome editing technologies, such as CRISPR, this vision could soon become conceivable, provoking considerable attention amongst the scientific community.
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The success of this new, complex technology is not without its difficulties, but it could have important consequences for conservation projects and in improving biodiversity.
CRISPR gene-editing technology
CRISPR is a new gene-editing technology harnessed from the naturally occurring bacterial immune system. Short for ‘Clustered Regularly Interspaced Short Palindromic Repeats’, these DNA sequences are remnants of previous bacteriophage infections. The associated Cas 9 protein is an endonuclease that acts as the ‘molecular scissors’.
It cuts foreign viral DNA, inserting it into the bacterial genome. These resulting DNA arrays transcribe short RNAs known as crRNAs. These guide the Cas9 protein to matching foreign DNA, to cut and disable re-occurring viral infections.
This CRISPR-Cas9 system has been exploited for genetic engineering purposes due to how quickly, cheaply, and efficiently genetic modifications can be done compared with existing methods. CrRNAs can be programmed to guide Cas9 to cleave DNA at specific sites of interest. This allows scientists to manipulate the DNA during the repair process. It can be removed, added, or changed which has significance for biomedical research.
The technique has been used to study gene function and holds promise for the treatment and prevention of certain diseases, such as sickle cell disease. It has also been used to create novel phenotypes, for example, disease-resistant chickens and hornless cattle.
How can CRISPR be used in species de-extinction?
A new potential use for this technology has been investigated in species de-extinction. Adding genes from an extinct species to the genome of the most closely related living organism could in theory recreate these lost species. The technique has experimented on the passenger pigeon, woolly mammoths, and aurochs.
George Church’s group at Harvard Medical School has researched this area and has been the first to succeed in using CRISPR-Cas9 for this use. They edited the genomes of living Asian elephant cells at 14 loci, to contain particular genes from the woolly mammoth. Although they have not yet achieved their goal of re-creating a woolly mammoth, they are testing for altered phenotypes in CRISPR-Cas9 manipulated cells that have been transplanted into tissues.
The pitfalls of CRISPR for reviving an extinct species
Although this technique looks promising for de-extinction, research is ongoing and several barriers need to be overcome. The first hurdle in de-extinction is sequencing the extinct species’ genome. This is complicated when there are no longer any living cells from which to obtain DNA, and when DNA begins to decay immediately after death.
Complex technology is therefore required to recreate these ancient genomes from preserved remains. For recently extinct species this process is easier as cells and DNA have often been preserved before extinction.
The next difficulty is the large sum of nucleotide differences between the extinct genome and that of its most closely related living species. Genome-wide scans are used to detect these extensive discrepancies. There are 1.5 million nucleotide variances between the genomes of the woolly mammoth and that of the Asian elephant. This large number of edits can be reduced by changing large sequences of DNA in a single edit, or by focusing on manipulating phenotypically relevant traits. For the woolly mammoth, George Church’s group focused on editing genes that controlled ear size, hair, blood hemoglobin, and subcutaneous fat.
Cloning by nuclear transfer is the next barrier to resurrecting an extinct species. The genetically engineered genome needs to be implanted into the oocyte of its most closely related living species. However, the extent of evolutionary differences between the two species may affect the success of the interspecies nuclear transfer. For example, disparities in size or developmental conditions.
Cloning has been achieved in several mammalian species but not yet in elephants; it is estimated that the Asian elephant would be an appropriate surrogate for the woolly mammoth.
Alternative approaches are required in some species where a nuclear transfer is not possible, for example in birds. Ben Novak’s research on pioneer pigeons focuses on germline engineering their eggs with the Cas9 gene. Artificial wombs (ectogenesis) could be utilized in other species.
Nature vs nurture in species de-extinction
Current research is focussing on creating organisms with a limited selection of genes acquired from an extinct species, resulting in a hybrid species. This is where George Church’s research is initially heading towards; creating an Asian elephant with mammoth derived adaptations to cold climates.
Ultimately the goal is to create exact genetic copies with the full genomic sequence of the extinct species. However, nature vs nurture plays a large role in phenotype expression. Despite being exact genetic copies of the extinct species, differences in utero conditions and rearing will affect development.
Additionally, extinction is not only the loss of physical traits but of behaviors too. Behaviors fundamental to a species such as migration routes, mating rituals, hunting techniques, and communication methods are irretrievable. How will these species learn to behave like their kind if they have no others to model behavior from?
CRISPR as a new tool for conservation
Re-introducing an extinct species would be advantageous in conservation. In the long term, it has the potential to restore lost genetic diversity, ecological interactions, and niches. However, it seems that genetic manipulation techniques, such as CRISPR, could be more beneficial for the conservation of endangered species.
Introducing ‘extinct traits’ into species struggling to adapt to their rapidly changing environment, could help them to survive and avoid their extinction. With regards to the Asian elephant, woolly mammoth traits would help it to survive in much colder climates, increasing their range of habitats.
The ever-expanding number of endangered species as a result of climate change could see this technology increase in urgency!
References
- Chen, L., Tang, L., Xiang, H., Jin, L., Li, Q., Dong, Y., Wang, W., and Zhang, G., 2014. Advances in genome editing technology and its promising application in evolutionary and ecological studies. GigaScience, 3(1).
- Reference, G., 2020. What Are Genome Editing And CRISPR-Cas9?. [online] Genetics Home Reference. Available at: <https://ghr.nlm.nih.gov/primer/genomicresearch/genomeediting> [Accessed 14 September 2020].
- Richmond, D., Sinding, M., and Gilbert, M., 2016. The potential and pitfalls of de‐extinction. Zoologica Scripta, 45(S1), pp.22-36.
- Shapiro, B., 2015. Mammoth 2.0: will genome engineering resurrect extinct species?. Genome Biology, 16(1).
Further Reading