A female stickleback fish, nick-named ‘Mary’, has produced offspring from eggs that appear to have been fertilised while they were still inside her, according to scientists at the University of Nottingham.

The team of researchers from the School of Life Sciences collected Mary on an expedition to the Outer Hebrides to gather wild sticklebacks which are fully genome-sequenced models for a wide range of scientific research. 

In a paper published in Scientific Reports they present the first ever discovery of internal fertilisation and development of babies inside a normally egg-laying species, and their successful delivery. 

The three-spined stickleback, Gasterosteus aculeatus, is a small fish that is common to both fresh and coastal waters in the Northern Hemisphere. In normal reproduction, the male stickleback builds a nest and, by performing a zig-zagging dance routine, lures a female to lay her eggs inside it. The male then chases the female away and fertilises the cluster. He then guards and takes care of the eggs by fanning them with his fins to aerate them for around 2 weeks until they hatch. 

Mary’s unusual pregnancy meant that the ‘virgin’ stickleback was egg-bound and close to death when her predicament was noticed so the decision was taken to save the lives of her young, by putting her to sleep and delivering the near-complete embryos by Caesarean section. 54 embryos were successfully delivered, hatched into fry and grew to adulthood in the aquaria in Nottingham where around 20 still survive nearly three years on. The team has also successfully bred from Mary’s offspring in normal aquarium conditions. 

Dr Laura Dean, from the School of Life Sciences, said: “We were astounded at what we found when we examined Mary in our lab in the Hebrides. 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. 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.”

This discovery is currently the only record of this kind of fertilisation and delivery of live offspring in any fish ever so the research team was very keen to investigate how this might have happened. There are three known mechanisms by which abnormal type of reproduction in fish can occur. The first is parthenogenesis which is where the fish clones herself, the second is that she could be hermaphrodite with both male and female sex organs.


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