Reproduction normally requires sperm to fertilize the egg, but there are cases when nature has outsmarted this system. Evolutionary adaptations have allowed some creatures to produce offspring without sex in order to cope with living in captivity or lack of suitable mates. There are plenty of animals that reproduce asexually, while there are also those that experience parthenogenesis.
Also known as virgin birth, parthenogenesis is the process where a female animal creates an embryo from an unfertilized egg. As a result, the body needs to compensate for the missing genes not provided by the absent sperm.
Parthenogenesis happens in various types of invertebrates like insects and nematodes. It may also occur in some vertebrates like reptiles, fish, amphibians, and a few bird species. Here are some of the animal species which are capable of reproducing through parthenogenesis.
1. Komodo Dragon (Varanus komodoensis)
It was only in 2006 when scientists first discovered that the Komodo dragon can have virgin births. At the time, only two of these reptiles lived in Europe, both were sexually mature females which were bred in captivity. One of these, named Flora from Chester Zoo in the UK, produced a clutch of 25 eggs despite not being kept with a male. Eleven of the eggs seemed viable, but three of them collapsed and instead provided embryonic material for genetic fingerprinting.
Both males and females carry out meiosis, a process where cells divide to form the sperm or egg. In females, meiosis produces four egg-progenitor cells, one of which turns to egg while the other three are reabsorbed by the female's body. For Flora, one of the extra cells served as a surrogate sperm and fertilized the egg cell, according to Kevin Buley of Chester Zoo.
2. Zebra Shark (Stegostoma fasciatum)
In 2022, a female zebra shark sharing tank with two healthy males gave birth by fertilizing her own egg cells. This is the second reported case where sharks were born by parthenogenesis despite the availability of males that can otherwise participate in breeding.
This strange event was discovered when Kevin Feldheim and his colleagues performed routine genetic testing on two shark pups at the Chicago's Shedd Aquarium. DNA analysis revealed that the pups did not match any of the male sharks in the tank. Several gene pairs were identical copies of the mother's genes, an indication of a virgin birth.
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3. American Crocodile (Crocodylus acutus)
The first known case of a virgin crocodile birth was described in 2023. A female American crocodile produced a clutch of 14 eggs after spending 16 years away from other males. This animal arrived at Costa Rica's Parque Reptilandia in 2002 and has since spent all her time alone in her enclosure.
4. California condors (Gymnogyps californianus)
This vulture can choose to have babies on their own even when housed with males. This happened at San Diego Zoo when researchers housed female condors with a fertile male in hopes of encouraging them to breed. Apparently, the male was not needed when the female produced two male chicks. DNA sampling of the chicks revealed that they carry two identical copies of their mother's DNA.
5. Cape Honey Bee (Apis mellifera capensis)
This insect is a subspecies of the European honey bee and is native to South Africa. In 2020, researchers from University of Sydney have identified the single gene which determines how female Cape honey bees reproduce without having sex. One gene, GB45239 on chromosome 11, was found to be responsible for virgin births.
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