An international team of researchers from Queen’s University Belfast, the Guy Harvey Research Institute at Nova Southeastern University in Florida and the Henry Doorly Zoo in Nebraska has found evidence that sharks can reproduce asexually by an unusual method known as “parthenogenesis”. This is the first scientific report of asexual reproduction in sharks.
Head of the Queen’s research team and study co-author, Dr Paulo Prodöhl, from the School of Biological Sciences, said: “The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals.”
“The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate group where this form of reproduction has not been seen.”
The long-term study was prompted by the unexpected birth of a baby hammerhead shark in an aquarium at the Henry Doorly Zoo in December 2001. The astonishing thing about the birth was that none of the three candidate mother hammerheads in the tank, all of whom been caught in Florida waters as babies themselves, had been exposed to any male hammerhead sharks for the three years since their captivity.
Initial thinking was that the mother had mated with a male before capture, and then somehow stored the sperm for over three years before finally fertilizing her eggs in the aquarium, or alternatively, perhaps the hammerhead female had mated with a male shark of a different species that was in the tank.
By using DNA profiling techniques to examine the genetic makeup of the baby hammerhead and the three candidate mothers, the researchers were able to identify which of the three females was the actual mother. Surprisingly, the baby shark’s DNA only matched up with the mother’s – there was no DNA of male origin in the baby shark! This finding eliminated the possibilities of earlier mating with a male hammerhead followed by sperm storage, or hybridization with another shark species in the tank.
Females of only very few vertebrate species can give birth to fully formed young without requiring their eggs to be first fertilized by a male’s sperm. This unusual reproductive ability, known as “parthenogenesis”, is only very occasionally seen in some vertebrate groups such as birds, reptiles and amphibians. However, it has never before been seen in other major vertebrate lines such as mammals or sharks.
Co-author Dr Mahmood Shivji, who led the Guy Harvey Research Institute team, said: “We may have solved a general mystery about shark reproduction – our findings suggest that parthenogenesis is the likely explanation behind the anecdotal but increasing observations of other species of female sharks reproducing successfully in captivity despite not having contact with males.
“It now appears that at least some female sharks can switch from a sexual to a non-sexual mode of reproduction in the absence of males. Unfortunately, this occurrence is not benign because it results in reduced genetic diversity in the offspring since there is no new genetic variation introduced from the paternal side.”
The researchers found that the most likely form of asexual reproduction that had occurred was a specific type called “automictic parthenogenesis” that leads to less genetic diversity in the offspring compared to even the mother.
“During this process the unfertilized egg, which contains about half of the mother’s genetic diversity, is activated to behave as a normal fertilized egg by a small, genetically nearly-identical cell known as the sister polar body. The resulting baby shark therefore gets a double-dose of genetic disadvantage”, says lead author Dr Demian Chapman, who took part in the study while he was a graduate student at the Guy Harvey Research Institute.
The discovery raises concerns about the genetic and reproductive health of dwindling shark populations.
Now Head of Shark Research at the Pew institute for Ocean Science, Dr Chapman continued: “Not only does it experience reduced genetic diversity because it has no father, but around half of the genetic variation present in the mother is not passed on to the offspring.”
“Female sharks might reproduce like this more often when they have difficulty finding mates at low population densities. This could hasten the erosion of population genetic diversity and perpetuate the production of genetically disadvantaged offspring.”
Dr Prodöhl added: “The bottom line is that we have to include a whole new dimension to our thinking about shark reproduction and its influence on population health. If the ability of female sharks to switch from sexual to asexual reproduction is widespread under conditions of low encounter rates with males, incorporating this new information into our management and conservation efforts will be imperative to prevent further declines in genetic diversity for an intensely, and many instances over exploited, group of fishes.”
The research team’s paper will be published in the Royal Society journal, Biology Letters, on Wednesday 23 May 2007. It can be found at www.pubs.royalsoc.ac.uk/.
Sarah Williams | alfa
A Map of the Cell’s Power Station
18.08.2017 | Albert-Ludwigs-Universität Freiburg im Breisgau
On the way to developing a new active ingredient against chronic infections
18.08.2017 | Deutsches Zentrum für Infektionsforschung
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
18.08.2017 | Life Sciences
18.08.2017 | Physics and Astronomy
18.08.2017 | Materials Sciences