While as.ᴇ xual reproduction is typically reserved for unicellular organisms and plants, there are several members of the animalia kingdom that reproduce as.ᴇ xually. Some can even combine or alternate between both s.ᴇ xual and as.ᴇ xual reproduction depending on the circumstances, a helpful tool to share advantages and disadvantages that come with the lack of ɢᴇɴᴇᴛɪᴄ diversity.
- Whiptail Lizards
Some lizards, like the New Mexico whiptail, are unique in that they can reproduce as.ᴇ xually but still maintain DNA changes from generation to generation. In 2011, researchers from the Stowers Institute for Medical Research in Kansas City found that, while it is not uncommon for as.ᴇ xual reptiles to develop eggs into ᴇᴍʙʀʏᴏs without fᴇʀᴛɪʟɪᴢation, the female whiptail’s cells gained twice the usual number of chromosomes during the process. That means that whiptail eggs get the same number of chromosomes and resulting ɢᴇɴᴇᴛɪᴄ variety as those of lizards that reproduce s.ᴇ xually.
- Amazon Molly Fish
A species of freshwater fish native to Mexico and Texas, Amazon molly fish are all female. As far as we know, they’ve always reproduced as.ᴇ xually, which normally would put a species in ᴅᴀɴɢᴇʀ of ᴇxᴛɪɴᴄᴛion due to gene ʟᴏss. In the case of this particular fish, however, as.ᴇ xual reproduction has worked out greatly in their favor. A 2018 study compared the genome of the Amazon molly to that of two similar species only to find that the mollies were not only surviving, but thriving. They concluded that the molly genome had high levels of diversity and showed no widespread signs of genomic decay, despite being entirely female.
A tiny bug that feeds off plant sap, aphids reproduce so rapidly that they can cause extensive ᴅᴀᴍᴀɢe to crops in large numbers. Aphids are literally born pregnant, developing ᴇᴍʙʀʏᴏs within the mother’s ᴏᴠᴀʀʏ one after another, with those developed ᴇᴍʙʀʏᴏs containing more ᴇᴍʙʀʏᴏs and on and on (think ᴀssembly line or nesting doll). Aphids can replace their as.ᴇ xual reproductive habits with s.ᴇ xual reproduction during certain times of the year, most notably during autumn in temperate regions, to maintain natural diversity in their population’s ɢᴇɴᴇᴛɪᴄ pool.
- Water Fleas
Typically found in shallow bodies of water such as ponds and lakes, water fleas are microscopic zooplankton organisms that measure about 0.2 to 3.0 millimeters in size. While they normally reproduce as.ᴇ xually, water fleas have a special trick reserved for difficult times. When a population is ᴛʜʀᴇᴀᴛened by conditions such as food shortages or heat waves, they ᴍᴀᴛᴇ and lay eggs that can remain dormant for dozens of years. These eggs contain fᴇʀᴛɪʟɪᴢᴇᴅ ᴇᴍʙʀʏᴏs that are ɢᴇɴᴇᴛɪᴄally varied, unlike the offspring produced as.ᴇ xually who are identical to the parent. Not only that, but the dormant eggs are extra durable to survive ʜᴀʀsʜ conditions.
Scientists are able to use these eggs to study water flea evolution amidst climate change by comparing older eggs to modern ones. These studies have revealed that the maximum temperature for water flea activity is half a degree more than it was 40 years ago, suggesting that these tiny organisms have the ability to ɢᴇɴᴇᴛɪᴄally adapt to climate change.
Some ants have the ability to reproduce both s.ᴇ xually and as.ᴇ xually. In the case of common black carpenter ants, fᴇʀᴛɪʟɪᴢᴇᴅ eggs will become female workers, while unfᴇʀᴛɪʟɪᴢᴇᴅ eggs become males. Mycocepurus smithii, a fungus harvesting species of ant that ranges throughout the Neotropical region, is believed to be fully as.ᴇ xual in a majority of its populations — which is pretty impressive considering it’s the most widely distributed and most populous of any fungus-growing ant. Before a 2011 study published in the Proceedings of the National Academy of Sciences, these ants were thought to be fully as.ᴇ xual. The study sampled 1,930 million smithii ants from 234 colonies collected in Latin America, finding that every ant was a female clone of the queen in 35 of the 39 populations examined. In the remaining four, all found along the Amazon River, ants had a mixture of genes that suggested s.ᴇ xual reproduction.
Wasps reproduce both s.ᴇ xually and as.ᴇ xually. In those that reproduce s.ᴇ xually, females are born from a fᴇʀᴛɪʟɪᴢᴇᴅ egg while males come from unfᴇʀᴛɪʟɪᴢᴇᴅ eggs. There are some populations of wasp that produce only females from unfᴇʀᴛɪʟɪᴢᴇᴅ eggs, essentially laying eggs fᴇʀᴛɪʟɪᴢᴇᴅ by their own personal DNA. Scientists have found that whether or not a wasp reproduces s.ᴇ xually or as.ᴇ xually is determined by a single gene. Using crossing experiments in aphid wasps, researchers at the University of Zurich were able to demonstrate that the trait is inherited recessively, and that exactly 12.5% of females in a specific generation reproduced as.ᴇ xually.
- Marbled Crayfish
The marbled crayfish made headlines in 1995 when a German aquarium owner found a previously undiscovered species of crayfish that appeared to have cloned itself. The offspring were all females, suggesting that this new crayfish could be the only decapod crustacean (which includes crabs, lobsters, and shrimp) with the ability to reproduce as.ᴇ xually. Since then, the unique species of marbled crayfish has formed wild populations throughout freshwater habitats in Europe and Africa, wreaking ʜᴀᴠᴏᴄ as an invasive species.
It wasn’t until recently, in 2018, when scientists were able to sequence the DNA of marbled crayfish both from the German pet store where it originated and wild individuals caught in Madagascar. They were able to confirm that all of the crayfish were indeed clones descended from a single organism through the parthenogenesis form of as.ᴇ xual reproduction. The species had very little ɢᴇɴᴇᴛɪᴄ diversity and was evolutionarily young, a rarity among as.ᴇ xual reproducing animals, and the timing was congruent with the original discovery in Germany. They also estimated that the wild range of the invasive marbled crayfish increased 100 fold between 2007 and 2017.
Parthenogenesis, a form of as.ᴇ xual reproduction where ᴇᴍʙʀʏᴏs develop from unfᴇʀᴛɪʟɪᴢᴇᴅ eggs, has been observed in captive female animals that are separated from males for extended periods of time. The first recorded evidence of parthenogenesis in a cartilaginous fish (which includes sharks, rays, and skates) occurred in 2001 with a captive hammerhead shark. The wild-caught shark hadn’t been exposed to a male in at least three years but still gave birth to a normally developed, live female. Studies found no evidence of a paternal ɢᴇɴᴇᴛɪᴄ contribution.
In 2017, a zebra shark named Leonie in Australia gave birth to three baby sharks after being separated from her ᴍᴀᴛᴇ for five years. Gᴇɴᴇᴛɪᴄ testing of tissue samples from the mother shark, the suspected father shark, and the offspring showed that the babies only carried DNA from their mother. This was also the first demonstration of an individual switch from s.ᴇ xual to parthenoɢᴇɴᴇᴛɪᴄ reproduction in any species of shark.
Hydras, a kind of small, freshwater organism native to temperate and tropical regions, are known for their as.ᴇ xual “budding.” The hydra develops buds on their cylindrical bodies that eventually elongate, develop tentacles, and pinch off to become new individuals. They produce buds every few days depending on their surroundings, and as far as scientists can tell, do not age. Zoologists believe that hydras first developed about 200 million years ago during Pangea, so they were around about the same time as dinosaurs.
- Python Snakes
The first “virgin birth” by a Burmese python, the world’s longest snake, was recorded in 2012 at the Louisville Zoological Gardens in Kentucky. A 20-foot, 11-year-old python named Thelma who lived full time with another female snake (appropriately named Louise) produced a clutch of 61 eggs despite having had no exposure to a male in two years. The eggs contained a mixture of healthy and unhealthy ᴇᴍʙʀʏᴏs, eventually resulting in the birth of six healthy female babies. Their DNA has since been analyzed by scientists from the Biological Journal of the Linnean Society, who confirmed Thelma to be the sole parent.
Sea stars have the ability to reproduce both s.ᴇ xually and as.ᴇ xually, but with an interesting twist. As.ᴇ xual reproduction in some starfish is achieved through fission, meaning the animal actually splits in two and produces two complete organisms. In some cases, starfish will voluntarily break off one of their arms and then regenerate the missing piece while the broken part grows into a whole other starfish. Of the approximate 1,800 extant species of starfish, just 24 species are known to reproduce as.ᴇ xually through fission.
- Komodo Dragons
Typically, Komodo dragon males engage in ᴀɢɢʀᴇssɪᴠᴇ combat with each other during ᴍᴀᴛɪɴɢ season. Some males will even stay with the female for several days after ᴍᴀᴛɪɴɢ to ensure that she doesn’t ᴍᴀᴛᴇ with anyone else.
Similar to sharks, Komodo dragons were not thought to have the ability to reproduce as.ᴇ xually until recently, specifically in 2006 at England’s Chester Zoo. A Komodo dragon who had never had contact with a male in her life laid 11 eggs that tested for her DNA only. Seeing as Komodo dragons are listed as “Vᴜʟɴᴇʀᴀʙʟᴇ” by the IUCN, the ability to reproduce without ᴍᴀᴛɪɴɢ could come in handy for the conservation of the species.