Scientists were mystified as to how a fish called Mary seemingly fertilised its own eggs, but have now shown it was no miracle.
Three years ago, scientists from the University of Nottingham travelled to the Outer Hebrides in Scotland as part of an expedition to collect wild stickleback fish, a small fish common to both fresh and coastal waters in the northern hemisphere. However, one particular stickleback fish that the researchers named Mary did something that totally took them by surprise.
Publishing its findings to Scientific Reports, the team presented the first ever discovery of a ‘virgin’ fish achieving internal fertilisation and development of offspring without the need for a male fish.
Normally, male sticklebacks build a nest and try to attract females to lay eggs in it using a zig-zagging dance routine. Once the eggs are deposited, the male chases the female away, fertilises the eggs and then guards them for a period of two weeks until they hatch. Because it was totally unexpected, Mary’s bizarre pregnancy was only discovered when the researchers noticed she was close to death.
To save her young, the researchers put her to sleep and subsequently delivered 54 near-complete embryos through Caesarean section, 20 of which are still alive today.
“She looked like an ordinary egg-bound fish so we couldn’t believe it when we found she had almost completely developed embryos inside her ovaries,” said Dr Laura Dean, one of the researchers. “This is pretty much unheard of in an egg-laying species. The embryos were perfectly healthy, not deformed in any way, and most have gone on to live a normal adult lifespan.”
Freak incident or evolution?
This discovery is currently the only record of this kind of fertilisation and delivery of live offspring in any fish ever, and the researchers went to work to see if one of three abnormal types of reproduction was responsible.
The first possibility was parthenogenesis – which is where the fish clones herself – while the second was that she could be a hermaphrodite with both male and female sex organs.
These two were ruled out because Mary’s offspring were not identical, and if it was hermaphroditism they would have only had versions of genes that she had, with no genetic input from anywhere else.
“Our theory is that somehow sperm had got into the fish, fertilised the eggs and they developed into normal two-parent embryos,” Dean said.
“What has probably happened is that she has gone into a nest to lay her eggs where another female had already laid her eggs, which had been covered with sperm by a male stickleback. Somehow, some of the sperm in the nest must have got into Mary, presumably through her egg tube, and fertilised the eggs inside her but she never laid the eggs.”
The researchers are now back on the hunt for more sticklebacks that may display the same birth process to see if it was just a freak incident or an indication of an evolutionary or genetic change in the reproductive mechanism of the species.