Asexual reproduction in animals
Reproducing asexually results in genetically identical offspring and requires only one parent animal; below are examples of asexual animals and the type of asexual reproduction carried out by each animal. The parent organism does not need to find a mate and therefore no combination of genetic information from another organism is involved.
This kind of reproduction in animals can increase population rapidly; the downside to reproducing asexually is that the population of the organism is only best adapted for a specific habitat. Thus all members of the population are prone to the same predators and diseases.
Even though asexual reproduction is typically common among unicellular organisms and plants, there are several cases of asexual reproduction in animals. In the animal kingdom, there are various animals that reproduce asexually. In some cases, some of them combine or alternate between reproducing sexually or asexually due to some circumstances. Let’s look at some of the animals that reproduce asexually.
Examples of Asexual Animals
- Komodo dragons
- Whiptail lizards
- Python snakes
- Marbled crayfish
- Amazon Molly Fish
- Water Fleas
One of the animals that reproduce asexually is the shark. An asexual reproduction type known as parthenogenesis whereby unfertilized eggs develop into an embryo has been observed in female animals that are separated from the males for long periods due to captivity. A captive hammerhead shark in 2001 was the first evidence recorder of parthenogenesis in cartilaginous fishes. Even though the wild-caught shark hadn’t been exposed to a male in at least 3 years, it still reproduced and gave birth to offspring and studies showed no evidence of any genetic contribution from paternal sources.
Also, the first evidence of any species of sharks switching from sexual reproduction to parthenogenesis was exhibited in a zebra shark named Leonie in Australia. In 2017, this shark gave birth to 3 baby sharks even after being without a mate for 5 years. Genetic analysis and testing were carried out on the offspring, the mother shark, and the suspected father shark but there was no trace of paternal genetic material as the offspring only inherited DNA from their mother.
The Komodo dragon has been discovered recently as one of the asexual animals. These animals exhibit mating behavior during the mating season where they fight with each other aggressively. After mating, some of the males of these organisms can be seen staying with the female species for days to ensure she doesn’t mate with another male.
These reptiles like the sharks were not known to be asexual animals until recently. The observation of asexual reproduction in the komodo dragon was specifically at England’s Chester zoo in 2006. The Komodo dragon in captivity had no contact with a male in her entire life but was able to lay 11 eggs that under testing showed only inheritance of her DNA. Since komodo dragons are listed by the IUCN as vulnerable species, their ability to reproduce asexually can help conserve them from adding to the number of extinct animals.
The starfish is one of those animals that reproduce asexually. However, they are also capable of reproducing sexually. In sea stars (starfish), reproducing asexually is done via binary fission or fragmentation (autonomy of the arms).
During the process of fission, the central disc of the starfish breaks into two and regenerates by growing the missing parts to form two complete organisms. Out of the 1800 extant sea star species, only about 24 species reproduce asexually via binary fission.
The starfish in the process of autonomy sheds an arm that has a part of the central disc attached to it which further lives independently as a comet and grows a new set of arms eventually. Almost all starfishes can regenerate their arms. However, only a few selected species of starfish can reproduce via autonomy.
The whiptail lizards are one of the asexual animals which are unique and can maintain DNA changes from one generation to another. It is common in asexual reptiles for embryos to form without fertilization from unfertilized eggs but the female whiptail’s cells in this process gained twice the usual number of chromosomes that other asexual reptiles get.
Initially, they produce 8 copies of each chromosome and give rise to 4 daughter cells after two rounds of cell division. Thus, each daughter cell gets two sets of chromosomes instead of just one set. This was discovered by researchers in 2011 from the Stowers Institute for medical research in Kansas City. Therefore, the eggs of the whiptail lizards possess the same number of chromosomes as well as genetic variety as the sexual lizards.
These lizards still exhibit mating behavior even though they don’t need a male mate. They display a pseudocopulation behavior where a female whiptail mounts another female whiptail which seems to promote ovulation in these organisms. Unlike other asexual animals that reproduce genetic clones, these lizards still have the ability to produce genetically diverse offspring. This is because they exhibit facultative parthenogenesis, thus can reproduce by another means of reproduction aside from parthenogenesis. The new Mexico whiptails even have a hybridization event that involves the female mating with the males of another species.
In 2012, the world’s longest snake; the Burmese python had a virgin birth which was recorded in Kentucky at the Louisville zoological gardens. The 11-year-old snake was named Thelma, which weighs about 91 kg and was 20 feet long (6 meters). It lived with another female snake the entire time and despite not being with a male in 2 years still produced a clutch of eggs. These eggs were a combination of unhealthy and healthy embryos that eventually gave rise to 6 healthy baby snakes. Scientists from the Biological Journal of the Linnean Society analyzed the DNA of the offspring and confirmed Thelma as the sole parent.
It was said that virgin births have been seen in other python and snake species like boas and garter snakes. Thelma’s ability to carry out virgin birth was attributed to optimal conditions. it is assumed that Thelma living in ideal conditions and feeding well may have triggered the parthenogenesis that resulted in the virgin birth. There are hypotheses that captivity and the geographic isolation of the snake from males may have been a contributing factor to the parthenogenesis in the snake.
In 1995, the marbled crayfish was discovered by a german aquarium owner to have cloned itself. The offsprings produced were all females which suggested that the marbled crayfish might be the only decapod crustacean that is asexual among other crustaceans like lobsters, shrimp, and crabs. Since this finding, the marbled crayfish species have formed wild populations in freshwater habitats. They are common in Africa and Europe and as invasive species cause havoc. It was estimated that between 2007 and 2017, the wild range of the invasive marbled crayfish increased a hundredfold.
Recently, in 2018 scientists were able to analyze the DNA of the marbled crayfish and confirmed that all the crayfish were all clones from a single parent. The DNA of the marbled crayfish gotten from the wild individuals caught in Madagascar and the german pet store where it originated was both analyzed. They were all descendants of a single organism via parthenogenesis and had very little genetic diversity.
Amazon Molly Fish
The Amazon molly fish are all females and are native to Texas and Mexico. These freshwater fishes are asexual animals. Reproducing asexually can lead to gene loss which may put an organism at risk of being an endangered species.
However, when it comes to the Amazon molly fish this is not the case as reproducing asexually has worked in their favor. This was confirmed in a study carried out on the genome of the amazon molly in 2018 that was compared with two similar species. It was discovered that the amazon molly was thriving and not just surviving. It was concluded that the genome of these mollies had high levels of diversity. Also, despite them being all females, they showed no widespread genomic decay.
The Amazon molly may mate with a male fish of a related species but the males’ DNA is never incorporated in the offspring. Rather it has been observed that mating with the male-only triggers the replication of the maternal genome. Therefore, the Amazon molly fish clone themselves and don’t lay eggs. They give birth to large broods of live offspring.
One of the organisms that reproduce asexually is the wasps. However, they can reproduce sexually too. The wasps that reproduce sexually give birth to females formed from unfertilized eggs whereas, the males are formed from fertilized eggs. However, there are some wasps that lay eggs fertilized by their own personal DNA and reproduce only females formed from unfertilized eggs.
Scientists discovered that wasps are determined by a single gene where they reproduce sexually or asexually. Some researchers used cross experiments in aphid wasps to show that the trait is excessively inherited. From the experiment, exactly 12.5% of female wasps in a generation asexually reproduce.
There are some ants that can reproduce both sexually and asexually. For instance, the female workers of the common carpenter ants are from fertilized eggs whereas, the unfertilized eggs become males.
The queen begins an ant colony and the colony centers on her as her main purpose is to reproduce. During her brief period of mating, she may mate with many males and then doesn’t mate again. The queen stores sperm in the spermatheca (an internal pouch) which is near her abdomen’s tip. The sperm remains immobile in this pouch until she opens a valve for the sperm to enter her reproductive tract to be able to fertilize her eggs.
The queen has the ability to determine the sex of her offspring as the fertilized eggs produce females and the unfertilized eggs give rise to winged males. The females produced are usually wingless and work while the males exist to fertilize the queen and do not work. Usually, the females are seldom capable of reproduction as the queen secrets a chemical that retards ovary development and wing growth in the female larvae. The queen produces myriads of female workers and virgin queens are only produced when there are enough workers available for the colony to expand.
A fungus-growing ant species, Mycocepurus smithii which is native to Latin America is said to be fully asexual in a majority of its populations. It being asexual as one of the most populous and widely distributed fungus-growing ant is quite impressive. These ants were assumed to be fully asexual before a 2011 study published in the Proceedings of the National Academy of Sciences. Millions of smithii ants were sampled from 234 colonies that were collected in Latin America. The findings from the study showed that every ant in 35 of the 39 populations examined was a female clone of the queen. The remaining four ants found along the amazon river were a combination of genes that suggested sexual reproduction.
The aphids reproduce asexually but at a certain season of the year, they replace their asexuality with sexual reproduction. This happens mostly in temperate regions during autumn in order to maintain natural diversity in the genetic pool of their population.
Aphids reproduce very quickly and feed on plant sap. This is why they can cause substantial damage to crops when in a large population. These bugs are literally born pregnant and develop embryos in the ovary of the mother one after the other. The developed embryos contain more embryos and that’s how it goes on.
Most aphid species give birth to males and females when autum draws near. The male offspring mate later with the female to produce fertilized eggs. The eggs endure and survive the cold winter and then hatch in spring.
Interestingly, the newly hatched aphids do not mate or lay eggs as their parents did. Rather, they give birth to live offspring via a means of asexual reproduction (parthenogenesis). Strangely, the unborn offspring gestate and develop their own embryos in the mother’s womb. The developing embryos also contain developing embryos within them.
Hence, the majority of aphids are born pregnant and give birth to females without males. This is due to modified meiosis that skips the reduction division. The parthenogenetic oocytes form maintaining diploidy and heterozygosity.
One of the organisms that reproduce asexually is the hydras. These organisms are native to tropical and temperate regions. The freshwater hydra reproduces via budding as it develops buds on its cylindrical body. These buds eventually elongate and develop tentacles. They later pinch off to become new individuals and these buds depending on their surroundings are produced every few days. According to scientists, these hydrae do not age. It is believed that these organisms were around the same time as dinosaurs about 200 million years ago.
The water fleas are microscopic zooplanktons that are found in shallow water bodies like lakes and ponds. Normally, these organisms reproduce asexually and clone themselves under normal circumstances. However, when living conditions become unfavorable, they switch to sexual reproduction. Unfavorable conditions like heat and shortage of food can trigger them to mate and lay dormant eggs that can stay dormant for dozens of years.
These eggs are durable enough to survive harsh conditions and are different from the offspring produced asexually. The eggs comprise fertilized embryos that vary genetically whereas the asexual offsprings are identical to the parent. Scientists compare older eggs of water fleas to modern ones to assess their evolution during climate change. This study showed that 40 years ago, the maximum temperature for water flea activity is half a degree less than what it is now. Therefore, this suggests that the water flea are capable of genetically adapting to climate change.