by Brandon Le
What if you wanted to start a family, but without the whole partnering up thing? For us humans, well, that’s complicated – but for some other species it’s just natural. Some species of plants, bugs, fish, reptiles, and others can take on baby-making solo with a form of asexual reproduction called parthenogenesis – a mashup of the Greek words “Parthenos” for virgin, and “genesis” for creation. It’s also a mashup of the two copies of genetic material needed to make a new embryo, but unlike sexual reproduction, both of those copies come from the same parent.
Sexual reproduction requires a male sperm to fertilize a female egg. The egg and sperm, or gametes, are produced through a special kind of cell division called meiosis. Meiosis makes haploid gametes, meaning sperm and eggs get equipped with only half of the genetic material required by the rest of the body’s diploid cells. Parthenogenic reproduction has a workaround to make diploid cells without the need for sperm. Plants often get it done through a process called apomixis, where the other kind of cell division, mitosis, generates fully viable, diploid reproductive cells. In this case, any offspring produced are genetically identical clones of their single parent. Parthenogenic animals can take a different approach called automixis. Mothers produce haploid eggs through meiosis like we do, but with a twist. In automixis, the smaller nonviable cells, or polar bodies, step in for sperm and fertilize the egg with another copy of mom’s genome.
Some of the most established examples of parthenogenesis occur in the order Squamata, which includes lizards and snakes. Female Komodo dragons turn to automixis to lay self-fertilized eggs when male mating partners are absent. Some species of desert whiptail lizards have taken it to a deeper level. Aspidoscelis neomexicanus and Cnemidophorus uniparens, for example, have diverged from their sexually reproducing cousins to establish themselves as an all-female species who rely on parthenogenesis as their primary means of reproduction. But the sexual pasts of these all-female whiptails aren’t totally forgotten. Researchers have noted that even though the females can reproduce on their own, they still engage in mating rituals that improve fertility. Other rare examples of parthenogenesis have been documented in some birds and fish including sharks, though the viability of the offspring produced in these cases may be compromised.
Animal parthenogenesis seems to be either a temporary strategy to keep a species alive during tough times (like the Komodo dragon), or an evolutionary divergence from a sexual past (like the whiptail lizards). Having the flexibility to reproduce solo clearly benefits a species’ survival, so why don’t we see animals doing it more often? A big reason is that parthenogenesis, and asexual reproduction in general, tends to reduce genetic diversity, and in turn, limits the species’ chance of survival. Having novel combinations of genetics from two parents gives offspring a coin flip’s chance that genetic problems inherited from either mom or dad can be masked and overcome by the genes from the other parent. For example, conservationists strive to understand how to breed more Komodo dragons in captivity because of their endangered status. Research shows that female Komodos housed without males would lay viable eggs via parthenogenesis, but offspring produced this way are almost genetically identical to their mothers. Consequently, parthenogenesis for more than a few generations results in a dangerous loss of genetic diversity that species depend on to resist changing environments, natural disasters, and infections.
Dr. Malcom from Jurassic Park was right: life does “find a way”. Parthenogenesis represents one of many examples of how organisms meet basic life goals in surprising and novel ways. While it’s easier to observe the many ever-changing strategies animals use to get nutrients or defend themselves, parthenogenesis reminds us that, at least for some, there are alternative paths to reproduction, too.
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