THE ECOLOGY OF DE-EXTINCTION Pathways to de-extinction: how close can we get to resurrection of an extinct species?

1. De-extinction, the idea that extinct species might soon be resurrected, receives considerable attention in both popular and scientific literature, in particular with regard to its potential ecological and ethical consequences. 2. Here, I review the three main pathways that are being considered at present for de-extinction: back-breeding, cloning via somatic cell nuclear transfer (SCNT) and genetic engineering. I present the state of the art in each pathway and discuss the limitations of each approach as a mechanism to resurrect extinct species. 3. Back-breeding aims to concentrate ancestral traits that persist within a population into a single individual using selective breeding. In back-breeding, ancestral phenotypes may be resurrected after many generations, but the genes that underlie these phenotypes may differ from those that were present in the extinct species. 4. Cloning aims to create genetically identical copies of an extinct species from preserved somatic cells. These somatic cells are fused with egg cells from a closely related and living donor species, which causes cellular reprogramming and embryogenesis, a scientific process known as SCNT. The developing embryo is then brought to term within a surrogate host. Because biological remains degrade post-mortem, cloning of long-dead organisms is not likely to be feasible. 5. Genetic engineering aims to edit the genome sequence within cells of living species so that these genome sequences closely resemble that of a closely related extinct species. This approach draws on recent advances in both ancient DNA and genome editing technologies and is a particularly promising approach to de-extinction. After the genome of a living cell is edited, that living cell can then be used for SCNT. 6. Because the phenotype of an organism is the consequence of the interaction between its genotype and the environment in which it develops and lives, even species with cloned nuclear genomes will not be exact copies of the extinct species on which they are modelled. We should therefore consider de-extinction as a means to create ecological proxies for extinct species.