But there are many living things that reproduce asexually, meaning they require no partners in order to produce offspring. Generally speaking, organisms create identical (or at least extremely similar) copies of themselves without the contribution of genetic material from another individual. Budding, binary fission, and self-fertilization (in hermaphrodites) are a few ways to get the job done. And many bacteria, animals, plants and fungi do it rather well. There is some overlap though. Bacteria, for example, divide asexually, but can also exchange genetic material with other bacteria by the process of conjugation. In fact, it's rather common to find asexual organisms that can reproduce sexually.
Not as common, or at least not thought to be as common, are sexually reproducing organisms that can also reproduce asexually. Especially within vertebrates. But more and more these days, headlines are buzzing with announcements of "virgin births" in all kinds of critters. Back in 2001, a bonnethead shark (Sphyrna tiburo) was born at the Henry Doorly Zoo in Omaha, Nebraska, in a tank that contained three females. It was presumed that one of these wild caught females must have been storing sperm from a previous encounter with a male, but that encounter would have taken place three years prior to the birth. As it turned out, this was not the case. It was a "virgin birth" (Chapman et al. 2007). This form of reproduction, known as parthenogenesis, involves only the female of the species, and is known in many invertebrates. But among vertebrates is considered an evolutionary novelty.
|Carcharhinus limbatus, modified from an original by Albert Kok,
from Wikimedia Commons.
Which made it all the more interesting when, in 2008, parthenogenesis was reported in a blacktip shark (Carcharhinus limbatus). Pretty cool to come across two different lineages of parthenogenic chondrichthyans. But is this strictly a chondrichthyan trait? What about osteichthyan fish, the group that includes the amniotes, like us?
Well, in 2006, Flora, a Komodo dragon (Varanus komodoensis) living at the Chester Zoo in England laid a clutch of 11 eggs. Like chickens, many reptiles often lay unfertilized eggs, so this wasn't that strange. But then seven of them hatched. This was a bit off, as Flora had never been in physical contact with a male Komodo dragon over the course of her life. When tests were conducted, it was found that all of the eggs contained genetic material from only one parent, Flora. This caused researchers at the London Zoo to go back and test eggs from their Komodo dragon, Sungai, who had laid a similar clutch the previous year. Same story, different dragon (Watts et al. 2006). Then in 2008, it happened again at the Sedgwick County Zoo in Wichita Kansas. This was the first time parthenogenesis in Komodo dragons was documented from start to finish. These cases showed that, contrary to popular belief, not all female vertebrates needed a male to produce offspring. This was bad news for guys everywhere. More interesting, it was in a completely different group of vertebrates. That's some wild stuff.
|One of the baby Komodo dragons from the Chester Zoo , modified from an original by Neil, fromWikimedia Commons.|
A little under two weeks ago, another paper came out in Biology Letters describing parthenogenesis in another group of squamates, the boid snakes. In their paper, Booth et al. (2010) talk about a female Boa constrictor (Boa constrictor imperator) who gave birth to 22 offspring over the last two years. Like the X and Y chromosomes in humans, boas have W and Z. Females have a ZW genotype and males have a ZZ genotype, but all of this boas offspring were WW - all female, all caramel-colored (a recessive color trait found in the mother). This was proof positive that all of the snakelets lacked any genetic material from a male boa. However, in this instance, the female in question lived with a bunch of males. In fact, she had bred with them before. So why would she suddenly, on more than one occasion, start to produce parthenogenic clutches of eggs? This method of reproduction, enigmatic as it is, is thought to be a sort of "last ditch effort" to produce offspring in the absence of a male. But with no shortage of males in this particular case, it makes one wonder how often this type of asexual reproduction take place in the wild, in species that would traditionally reproduce via sexual means.
Coincidentally, there are accounts of parthenogenesis in wild populations. Parthenogenesis is commonly known in another squamate, the New Mexico whiptail (Cnemidophorun neomexicanus). In fact, it's known in several whiptail species. But they've taken not needing a man around to an entire different level. The entire population of this little lizard is female. There are no males present, whatsoever. Generation after generation, the females produce offspring that share their genetic material, and go about it rather happily. I've also heard rumor of parthenogenic members of Amphibia, Aves, and Actinopterygii (try saying that ten times fast), meaning that "virgin births" are known in all major extant vertebrate groups, excluding Mammalia. Which means that, at least for the time being, human males across the globe still serve an integral role in the propagation of our species (and that the Madonna lyrics used in the title remain accurate per their original, unedited use).
For more on the various "ins and outs" (no pun intended... well, maybe a little) of animal reproduction, check out the notes from one of my favorite ELT lectures, "Raw Animal Sexuality", available online, courtesy of Dr. John Merck. I'm proud to say that a Google search for the above keywords still yield that site as the top result!
Chapman, D. D., Shivji, M. S., Louis, E., Sommer, J., Fletcher, H., & Prodohl, P. A. 2007. Virgin birth in a hammerhead shark. Biology Letters, 3, 425-427.
Chapman, D. D., Firchau. B., & Shivji, M. S. 2008. Parthenogenesis in a large bodies requiem shark, the blacktip Carcharhinus limbatus. Journal of Fish Biology, 73, 1473-1477.
Watts, P. C., Buley, K. R., Sanderson, S., Boardman, W. Ciofi, C., & Gibson, R. 2006. Parthenogenesis in Komodo dragons. Nature, 444, 1021-1022.
Booth, W., Johnson, D. H., Moore, S., Schal, C., & Vargo, E. L. 2010. Evidence for viable, non-clonal but fatherless Boa constrictors. Biology Letters, 1-4 DOI: 10.1098/rsbl.2010.0793