Last year, I visited the London Aquarium. I really enjoyed seeing all the sealife and was impressed at the emphasis they had put on conservation. All around the walls were ‘fun facts’ and one of these caught my attention. It said that sharks can asexually reproduce and immediately I wondered how this was possible.
Female sharks are able to reproduce without the need for a male shark through the process of parthenogenesis. Parthenogenesis is the process of a female gamete becoming an individual without fertilisation taking place. It is known to happen in wasps, bees and ants who do not have sex chromosomes as well as some species of reptile, fish and very rarely, birds.
Normal gametes are produced by meiosis and have half the number of chromosomes than their parent cell. This means that they are haploid. However, some offspring of parthenogenesis can be diploid. These are called full clones, whilst those which only have half the mother’s alleles are called half clones.
Full clones can only normally be formed without meiosis taking place and instead, the embryo is produced by apomictic parthenogenesis in which mature egg cells are formed by mitosis and then develop into embryos.
Half clones are formed by meiosis and therefore can be haploid. However, as haploid individuals are generally not viable, there are various mechanisms through which diploidy can be restored. These are called automictic parthenogenesis. Methods of automictic parthenogenesis include the duplication of DNA without cell division, before or after meiosis, as well as the fusion of two blastomeres.
This process of parthenogenesis is amazing and I find it incredible the number of species which can do it. Sharks were first found to asexually reproduce in 2001 with a bonnethead shark. After research, it was shown that this offspring contained only half of her mother’s DNA and therefore, automictic parthenogenesis must have taken place. Since then, several other recordings have been made in aquariums and zoos of parthenogenesis happening. Although this process of nature is a fantastic invention for overcoming the problem of females finding males, it prevents the formation of genetic diversity which is essential for species to stay alive and evolve in the changing world. All shark half clones produced are females so if parthenogenesis continues to happen, the male shark population will become depleted and asexual reproduction will happen even more, leading the shark population into a vicious cycle.
However, at present this has not been recorded as a problem so let us hope that nature’s phenomenon can remain just that for centuries to come.