By Anna Thomas | Contributor
This unusual occurrence is thought to be an example of parthenogenesis, or virgin birth, a phenomenon whereby females can reproduce despite the absence of male gametes.
Whilst some species exclusively reproduce by parthenogenesis, smoothhound sharks can switch between asexual and sexual reproduction according to the presence or absence of a male shark in their environment – An aspect that the Italian Aquarium has dubbed as a ‘miracle’ birth.
The science behind the ‘miracle’ shark birth
There are two types of parthenogenesis, automixis and apomixis. To produce an egg ready for fertilisation, cells undergo several steps known as oogenesis. During this process, polar bodies – an excess of genetic material – are formed which typically degenerate over time.
In automixis, polar bodies instead combine with the egg forming an embryo. Reproduction by this process results in offspring which are genetically similar to the mother but not identical. Alternatively, apomixis produces genetic clones of the mother, but this route is seen more commonly in plants than animals.
In addition to various invertebrates and plants, more than 80 species of vertebrates have been observed to be capable of reproducing in this way. However, since it is challenging to track occurrences in the wild, this data is mostly reliant on incidences recorded amongst animals in captivity, making it probable that this is an underestimate of its prevalence.
Why don’t all organisms do this?
Self-fertilisation results in a lack of genetic diversity through generations. Consequently, progeny may carry the negative genetic mutations present in the mother, as well as being predisposed to the same diseases. This absence of variation has the potential to make populations more vulnerable than those reproducing sexually.
Despite being genetically less advantageous, asexual reproduction is a necessity for females estranged from the males of their species. Whilst in the past, fluctuations in male populations could be attributed to natural selection pressures, presently human interference such as fishing and climate change are putting increasing stresses on shark populations. A recent study in Nature found there has been a 71% global decline of oceanic sharks and rays in only half a century. Drastic population changes such as these may lead to an increase in the necessity of asexual reproduction amongst sharks in the future.
The aquarium has reportedly sent the DNA of the new pup, Ispera, for laboratory testing to confirm her status as a parthenogenetic birth. This may well result in the addition of smoothhounds to the growing list of animals capable of self-impregnating.Science and Technology