‘Mini-Me’s in the Animal Kingdom
Parthenogenesis: from the Greek παρθένος, parthenos, meaning “virgin”, and γένεσις, genesis, meaning “birth”. Cutting through all the science-y jargon I couldn’t explain to you anyway, having never studied Biology as separate subject, parthenogenesis is essentially the production of offspring (by a female, of course) without fertilisation from a male. The progeny are either complete clones of their mother or ‘half’ clones, and are usually only female. It’s fairly common in invertebrates (Lampert, 2008) and occurs in approximately 80 unisex taxa of amphibians, reptiles and fish (Neaves & Baumann, 2011).
As clonally reproducing (including parthenogenetic) species lack the ability to recombine and generate new combinations of traits, they are often referred to as ‘dead-ends’ – they are thought to succumb rapidly to parasites, diseases, and predation because of their negligible ability to adapt in a changing environment. However, the recognition that several asexual taxa have thrived for long periods of time raises questions about the generality of this assumption (Neaves & Baumann, 2011)
I’d (ignorantly and erroneously) always considered parthenogenesis to be purely theoretical – until I read about a captive female adult hammerhead shark (specifically a bonnethead, Sphyrna tiburo) that had given birth to a live pup – in the absence of another male (Chapman et. al., 2007), the first recorded instance of such in a cartilaginous fish (Class: Chondrichthyes) (Chapman et. al., 2007).
More astonishingly, every year since since 2007, a captive female zebra shark (Stegostoma fasciatum) named ‘Zebedee’ (Gallatin, 2011) has produced 15 pups, also definitively via parthenogenesis (Robinson et. al., 2011). More recently, and perhaps rather surprisingly, a captive female common northern boa (Boa constrictor imperator), housed with up to four conspecific males (Booth et. al., 2010), parthenogenetically produced two litters of a total of 22 young (BBC – Earth News, 2010).
Parthenogenesis in captive females kept separate from males is generally viewed as an evolutionary novelty, albeit one that made sense from a reproductive standpoint (National Geographic News, 2012a). What is astounding, however, is that last year 1 out of 22 and 1 out of 37 wild-captured pregnant female copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus) pit vipers respectively, taken from fields where males were present, were found to have given birth parthenogenetically (Booth et. al., 2012 & National Geographic News, 2012b).
Although mammals are most likely committed to propagating the ‘usual’ way (Morgan, 2011), parthenogenesis still remains to me at least, a fascinating and captivating mystery – one that we may be a long time in figuring out.
Booth, W., C. F. Smith, P. H. Eskridge, S. K. Hoss, J. R. Mendelson, G. W. Schuett, 2012. Facultative parthenogenesis discovered in wild vertebrates. Biology Letters, 8(6): 983–985.
Booth, W., D. H. Johnson, S. Moore, C. Schal, E. L. Vargo, 2010. Evidence for viable, non-clonal but fatherless Boa constrictors. Biology Letters, 7(2): 253–256.
Chapman, D. D., B. Firchau, M. S. Shivji, 2008. Parthenogenesis in a large-bodied requiem shark, the blacktip Carcharhinus limbatus. Journal of Fish Biology, 73(6): 1473–1477.
Chapman, D. D., M. S. Shivji, E. Louis, J. Sommer, H. Fletcher, P. A. Prodöhl, 2007. Virgin birth in a hammerhead shark. Biology Letters, 3(4): 425–427.
Lampert, K.P., 2008. Facultative Parthenogenesis in Vertebrates: Reproductive Error or Chance?. Sexual Development, 2(6): 290–301.
Neaves, W. B., P. Baumann, 2011. Unisexual reproduction among vertebrates. Trends in Genetics, 27(3): 81–88.
Robinson, D. P. , W. Baverstock, A. Al-Jaru, K. Hyland, K. A. Khazanehdari, 2011. Annually recurring parthenogenesis in a zebra shark Stegostoma fasciatum. Journal of Fish Biology, 79(5): 1376–1382.
Ryan, M., 2011. Unauthorized Reproduction Not Prohibited. American Scientist, 99(1): 30.
BBC Earth News (2012) BBC – Earth News – Snake gives ‘virgin birth’ to extraordinary babies. [online] Available at: http://news.bbc.co.uk/earth/hi/earth_news/newsid_9139000/9139971.stm [Accessed: 3 Apr 2013].
Gallatin, A. (2011) Dubai Shark Wins “Most Virgin Births”. [online] Available at: http://www.globalanimal.org/2012/01/12/dubai-shark-wins-most-virgin-births/62758/ [Accessed: 10 Apr 2013].
National Geographic News (2012a) “Virgin Birth” Record Broken by Hotel Shark. [online] Available at: http://news.nationalgeographic.com/news/2012/01/120106-virgin-birth-shark-dubai-science/ [Accessed: 3 Apr 2013].
National Geographic News (2012b) “Virgin Birth” Seen in Wild Snakes, Even When Males Are Available. [online] Available at: http://news.nationalgeographic.com/news/2012/09/120914-virgin-birth-parthenogenesis-snakes-science-biology-letters/ [Accessed: 3 Apr 2013].