Sexual reproduction is a wonderful thing — scientifically speaking, of course. Through the combining of genetic material two individuals create a genetic offspring that is genetically similar, but not identical to the parents. However some species of fish, birds and lizards have developed an ability to reproduce asexually, and are challenging the common knowledge that sexual reproduction is superior.
Traditional sexual reproduction
In normal sexual reproduction, a male sperm cell and a female ovum meet and merge, each contributing an equal number of chromosomes to the new life they have just formed.
Most animals have something called a “diploid chromosome count,” which means that their genome consists of paired chromosomes. To put this into a human context, our genome contains 23 different kinds of chromosomes, and there are two copies of each kind, resulting in 46 chromosomes.
When cells divide, each is given a “haploid” number of chromosomes, which is a fancy way of saying that each cell gets one copy of each of the 46 chromosomes. When the sex cells meet, they combine, creating a new diploid cell, or zygote, which divides and divides, eventually developing into a young human.
In humans, and in most vertebrates, two of the chromosomes are special in that they determine the gender of the offspring. These are called the sex chromosomes. In females there are two copies of the X chromosome, where in males there is a single X chromosome paired with a single Y chromosome.
Asexual reproduction in sharks
Studying sharks in the wild is a difficult task at best, and to this day, even with all of our advanced research methods, many of their behaviors still remain a mystery.
Adding to the lack of knowledge is the fact that different species of shark undergo very different forms of reproduction. For example some species of sharks lay eggs, while others give birth to live young. Some female sharks, it appears, can reproduce independent of a partner, in a process dubbed “parthenogenesis.”
The phenomenon was first observed in 2001 at the Henry Doorly Zoo in Nebraska when a shark pup was born in a tank containing three female hammerhead sharks that had been captured before reaching sexual maturity — eliminating the possibility that they might have been pregnant when introduced to the aquarium.
It was originally assumed that the pup had been the result of one of the sexually mature females having a sexual encounter with a male in the wild, prior to capture, whose body stored the sperm until the female reached reproductive maturity — an ability present in some species of fish — however, analysis found that the pup was genetically identical to one of the females in the tank, dismissing the theory of stored sperm.
In 2002 a biologist who had heard about the apparent parthenogenesis decided to try an experiment with some white spotted bamboo sharks. Occasionally, the bamboo sharks in an aquarium would lay eggs, which were removed from the aquarium and discarded, as they were assumed to be unfertilized (like the chicken eggs bought at a grocery store). On this occasion the biologist decided to leave the eggs where they were, and see what happened. To the surprise of all, they hatched.
In this case parthenogenesis, while assumed to have taken place, was not the only possible explanation given for the seemingly “virgin birth.” It was postulated that the female bamboo shark could have been a fertile hermaphrodite (having both male and female reproductive organs) and “self-fertilized,” although this was never investigated.
More recently, a white tipped reef shark at the Nyiregyhaza Centre in Hungary, who was known to have been completely isolated from members of her own species since birth gave birth to a pup in 2008; leaving no other possible explanation than parthenogenesis.
The rash of cases of parthenogenesis in captive sharks has led some scientists, such as Mahmood Shivji of the Guy Harvey Research Institute, to propose that parthenogenesis might be an alternate method of reproduction in sharks, and quite natural. Although Shivji notes that this method of reproduction, which is convenient for an animal that cannot find a mate, would eventually lead to a genetically stagnant population if it were used exclusively — a situation, which over time, would lead to greater susceptibility to disease, and fail to eliminate genetic abnormalities.
New Mexico Whiptail lizards
While parthenogenesis in sharks might be an alternative pathway to traditional sexual reproduction, in Mexico and the southwestern U.S. there is a species of lizard that only reproduces asexually.
New Mexico whiptail lizards are small, typically growing to between 16-23 cm long, and are known for running on their hind legs. Oddly enough, the species is made up entirely of females. Like the sharks, they reproduce via parthenogenesis, however unlike the sharks, they cannot reproduce sexually if conditions permit.
Through genetic analysis it was found that the New Mexico whiptail arose through a cross breeding of the Western and little striped whiptail lizards, both of which reproduce sexually. While most cross breeds, such as the mule, which is a hybrid of a horse and donkey, are sterile, in the case of the New Mexico whiptail a strange thing happened. Instead of being a true hybrid, with some chromosomes from one species, and some from the other, the offspring of the original interspecies mating retained a complete set of both parents’ chromosomes which continually recombine in the descendents, giving them an unprecedented level of genetic diversity.
Indeed this genetic diversity might be the key to the New Mexico whiptail’s success, since without sexual reproduction, as mentioned by Shivji in the case of the sharks, a species runs the risk of becoming genetically stagnant, since the entire population is effectively clones of just a few individuals. With such a diverse genetic menu to choose from, the little lizards seem to have done just fine without sex thus far.
Advantages of asexual reproduction
These are just a handful of the more than 70 species that can reproduce by parthenogenesis, and while sex-advocates might claim that these species are at a great disadvantage, it has to be noted that, in terms of expansion into new areas, since it only takes one individual to start a new colony, parthenogenetic animals can spread easily. Perhaps the best example of this would be the Brahminy blind snake, another exclusively female, exclusively parthenogenetic species.
The Brahminy blind snake is found on every continent except for Antarctica, and are spread partially through the sale of tropical plants. The tiny snakes — which are sometimes mistaken for earthworms and live almost exclusively underground — often take up residence in the soil surrounding tropical plants, and are unwillingly transported to all corners of the globe when these plants are sold to greenhouses. Since they appear to reproduce exclusively via parthenogenesis (a male has never been found), even a single snake is enough to populate a new area — a testament to the adaptability of parthenogenetic species.
So, is sexual reproduction going out of style then? Not quite yet. Parthenogenesis has never been observed in mammals, and while it does appear to be benign — or even advantageous — nobody seems to be able to predict what the long-term consequences of this form of reproduction might be. However, for now, in a small number of species, males have a reason to be nervous.
