Recently, Andrew Field, a Ph.D. candidate scientists working at Stony Brook University’s School for Marine and Atmospheric Studies, was doing routine genetic testing on a population of sawfish in southwest Florida when he discovered something abnormal. Many of the individuals in the sawfish population were extremely genetically similar to one another. In other words, the sawfish population was full of clones.
Cloning animals in the lab is an incredibly arduous process that doesn’t always yield satisfactory results. However, clones can occur naturally in animals via virgin birth.
Virgin birth, or parthenogenesis, is a phenomenon in which females of a particular species reproduce asexually. To a certain degree, this isn’t that weird. After all, botanical species such as fungi reproduce asexually, as do bacteria and other prokaryotes. What is weird is the fact that this instance of parthenogenesis was found in the animal kingdom, where creatures typically reproduce via sexual intercourse. Field was shocked because while scientists believed that wild parthenogenesis was possible, viable offspring from the asexual reproduction of females had never been discovered in the wild before.
Most animals reproduce sexually, but some species are able to switch to asexual reproduction under certain circumstances, a type of reproduction called facultative parthenogenesis. The reasons facultative parthenogenesis exists are not fully understood yet. However, many scientists believe that females resort to facultative parthenogenesis in the prolonged absence of a mate, which can occur due to population pressures and interruptions in migration patterns. Scientists believe that other environmental pressures that put selective evolutionary pressure on animals can also lead to parthenogenesis. Upon further examination, Field and others realized that they were actually looking at several cases of wild, facultative parthenogenesis. In other words, the sawfish population usually reproduced sexually, but had converted to asexual reproduction in response to environmental pressures.
Parthenogenesis occurs when an unfertilized ovum somehow produces a fully-fledged adult. Usually during meiosis, a female sex cell, called a gamete, replicates its DNA to create two sets of DNA and then divides into two pieces. These two cells are called “diploid” cells because they contain two copies of each chromosome, which makes a full set of DNA. The products of this division themselves divide, producing four cells called oocytes. Oocytes are haploid cells because they contain only one copy of each chromosome, or half of a full set of DNA.
Since most adult organisms are diploid, the oocyte needs to be fertilized by another haploid cell, usually a male sperm cell, before developing into a fetus. Thus, for parthenogenesis to occur, the oocyte somehow has to acquire a full set of DNA. This occurs by multiple mechanisms. In some situations, the oocyte merges with another haploid cell produced during meiosis to form a diploid cell. The oocyte can also duplicate its own DNA and then start to divide like a normal fertilized cell.
One of the most fascinating things about parthenogenesis is that the baby parthenogen is a clone of the mother. This occurs because the parthenogen is the product of just the mother’s DNA. However, in certain methods of parthenogenesis, the parthenogen isn’t a full clone, but rather a partial clone. Normal diploid offspring from sexual reproduction contain two of each chromosome, one from the mother and one from the father. Parthenogens only have one source of DNA: the mother. Thus, while the mother of a parthenogen might have two different copies for a gene, the baby will just inherit one of the copies twice. The sawfish population in Field’s study contained many of these partial clones.
For some creatures, parthenogenesis is a completely natural and normal mode of reproduction. Cyclically parthenogenetic organisms alternate between sexual and asexual modes of reproduction. For other creatures, it’s not as common. Facultative parthenogenetic organisms typically reproduce sexually, so while it’s known that they exist, it’s hard to observe facultative parthenogenesis in the wild.
Field’s study was monumental for two reasons. The first is that even though scientists have known about the existence of parthenogenesis for decades, parthenogens living in the wild have never been observed before. The second reason is that it signals a crisis for the smalltooth sawfish. The species is endangered. Switching from sexual to asexual reproduction won’t help the species replenish its numbers. For starters, it signals that there might be a lack of male mates, forcing the females to reproduce asexually. The bigger issue, however, is that parthenogenesis eliminates the genetic variance in a population. In a population of little genetic variance there are no safeguards against extermination; none of the individuals have a special genes that would allow them to survive a natural disaster or a selective pressure. If, for example, the sawfish population were hit by an oil spill, there would be no sawfish that are genetically “stronger” and more likely to survive the spill, and the population is more likely to be wiped out. Parthenogenesis is by no means a desirable form of reproduction, but rather a last resort, which is why it’s so hard to observe in the wild.
Finding clones in the wild is definitely pretty cool, as Andrew Field probably thought. But these clones are indicative of a larger environmental problem threatening an entire species. So while parthenogenesis might be exciting, maybe it’s better off being left undiscovered.