Reproduction takes two, right? Well, not always. Many arthropods and microscopic animals called rotifers reproduce clonally and although it is relatively rare in vertebrates, clonal reproduction has been confirmed in several species of fish, amphibians, birds and reptiles. Known as parthenogenesis, clonal reproduction in vertebrates can occur when an offspring develops from an unfertilised egg. Parthenogenesis has never been observed to occur naturally in mammals, although it is possible to induce it artificially in the lab.
An Issue of Numbers
During normal egg production, cells divide by meiosis, giving them half the number of chromosomes of an adult cell. This ensures that when they fuse with a sperm cell, the resulting embryo has the right number of chromosomes. Development from an unfertilised egg therefore necessitates this process be altered slightly, and there are a few different ways that this can be achieved. The obvious solution is to simply skip meosis, so that each egg cell carries the full genome of it’s parent (known as apomixis). Another option involves fusing two egg cells after meiosis, doubling the number of chromosomes again (known as automixis). There are some advantages to doing it this way – a process called recombination ‘shuffles’ genes during meiosis – fusing cells after this process means that there is more genetic variation in the offspring. Without this step, clonally-reproducing organisms could suffer from a slow degradation of their genes over generations, and face possible extinction.
Keeping meiosis also has another advantage – it allows for flexibility. Facultative parthenogenesis means that organisms can pick and choose – reproducing sexually when the opportunity arises, and switching to clonal reproduction when no mates are available. Obligate parthenogenesis has been documented in over 80 taxa of fish, amphibian and reptile, existing in all-female lineages. Pathenogenetic reproduction can only be achieved by females, because sperm cells lack the necessary machinery, such as mitochondria, to form a functioning embryo, this means obligate asexual reproduction forms all-female species.
Clonal Reproduction in Snakes
Parthenogenesis was first identified in snakes in 1997 and has now been confirmed genetically in reticulated pythons (Python reticulatus), copperhead vipers (Agkistrodon contortrix), cottonmouth vipers (Agkistrodon piscivorus), burmese pythons (Molurus bivittatus), boa contrictors (Boa constrictor imperator) and the Brahminy blind snake (Ramphotyphlops brahminus). Most cases are facultative, with females producing diploid eggs when kept isolated from males for extended periods of time. Some suspected cases of parthenogenesis have since been disproved by genetic testing that revealed these apparently clonal offspring were actually produced from sperm the female was keeping in long-term storage. This was the case for the eastern diamond-backed rattlesnake (Crotalus adamanteus), where females have produced viable offspring from sperm stored for .
Automixis, where meiosis is followed by fusion to produce diploid egg cells, occurs facultatively in the reticulated python and copperhead viper. Parthenogenesis by automixis has also been identified in hammerhead sharks (Sphyrna tiburo), Atlantic blacktip sharks (Carcharhinus limbatus) and zebra sharks (Stegostoma fasciatum). Apomixis is less common in snakes, but has been documented in the burmese python. Even more unusually, one obligate clonal species has been identified – the Brahminy blind snake. This snake is an all-female species with , and a beautiful example of gynogenesis.
Gynogenesis, and the related phenomenon pseudogamy, is a sneaky solution to another barrier to the evolution of clonal reproduction. In many species, sperm or seminal fluid is needed to trigger egg maturation, meaning that clonally-produced eggs would be inviable. Essentially, this trigger mechanism locks species in to sexual reproduction. Or does it? Species like the Brahminy blind snake get around the problem by ‘mating’ with males of a closely related species, using the sperm to trigger egg development without the male ever contributing any genetic information to the resulting offspring. This tactic has also evolved in some species of fish, in fact, my first ever blog post for Curious Meerkat looked at this phenomenon in Amazonian Mollies (Poecilia formosa). Gynogenesis is also common among clonal amphibians, like the unisexual mole salamander (Ambystoma sp), and the Japanese wrinkled frog (Rana rugosa).
Imprinting and Parthenogenesis
Although it was once considered to be an evolutionary fluke, isolated to just a few species, we now know that parthenogenesis is actually quite widespread in vertebrates. Just not mammals. Not any mammals at all. Why? Well the genomes of mammals are regulated heavily by genomic imprinting, whereby certain genes are expressed or suppressed depending on which parent they were inherited from. Because of this, clonal eggs lack male imprints and key genes are not expressed making the eggs inviable. This has posed a major barrier to scientists’ attempts to achieve clonal reproduction for stem cell research and reproductive solutions to enable same-sex couples to produce their own genetic children. However, researchers have begun to overcome these barriers in recent years as our understanding of genomic imprinting has grown.
Having spent decades trying to work out why on Earth animals would reproduce sexually (since it seems like it would be a lot simpler just to clone yourself instead of wasting time searching for a high quality mate), evolutionary biologists them came to the realisation that it’s actually rather odd that we see asexual species out there. Turns out sex has a lot of benefits, and in the 1980s scientists began to realise the difficult thing to explain was actually how asexuality could be maintained by evolution – statistical models and evolutionary theory suggested their genomes should slowly degrade. Scientists reasoned, therefore, that all the asexual species we see must have evolved only very recently. Parthenogenesis was seen as an evolutionary slip up that paved the inevitable road to extinction. Now we realise parthenogenesis is far more common than once thought, and mechanisms to prevent this slow genetic degradation may enable species to reap the benefits of cloning themselves without all the costs. Facultatively asexual species have the best of both worlds, since it actually only takes a little bit of sex to fix the problems caused by asexual reproduction. For species who frequently find themselves without a mate, it makes sense to have a back-up plan.
Want to Know More?
- Booth et al (2014) New insights on facultative parthenogenesis in pythonsBiological Journal of the Linnean Society
- Booth and Schuette (2011) Molecular genetic evidence for alternative reproductive strategies in North American pitvipers (Serpentes: Viperidae): long-term sperm storage and facultative parthenogenesis Biological Journal of the Linnean Society
- Booth et al (2011) Evidence for viable, non-clonal but fatherless Boa constrictors Biology Letters
- Neaves and Baumann (2011) Unisexual reproduction among vertebrates Trends in Genetics
- Chapman et al (2007) Virgin birth in a hammerhead shark Biology Letters/li>
- Groot, Bruins and Breeuwer (2003) Molecular genetic evidence for parthenogenesis in the Burmese python, Python molurus bivittatus Heredity