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
In focus: Peptides, the “little brothers and sisters” of proteins
12.11.2018 | Technische Universität Berlin
How to produce fluorescent nanoparticles for medical applications in a nuclear reactor
09.11.2018 | Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague)
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
Physicists at ETH Zurich demonstrate how errors that occur during the manipulation of quantum system can be monitored and corrected on the fly
The field of quantum computation has seen tremendous progress in recent years. Bit by bit, quantum devices start to challenge conventional computers, at least...
Scientists developed specially coated nanometer-sized vehicles that can be actively moved through dense tissue like the vitreous of the eye. So far, the transport of nano-vehicles has only been demonstrated in model systems or biological fluids, but not in real tissue. The work was published in the journal Science Advances and constitutes one step further towards nanorobots becoming minimally-invasive tools for precisely delivering medicine to where it is needed.
Researchers of the “Micro, Nano and Molecular Systems” Lab at the Max Planck Institute for Intelligent Systems in Stuttgart, together with an international...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
12.11.2018 | Life Sciences
12.11.2018 | Materials Sciences
12.11.2018 | Physics and Astronomy