A hybridised female fish bred in the aquarium of a UK university recently shocked researchers when it was discovered she had developed a male sex organ, fertilised her own eggs and produced four offspring.
The tropical freshwater fish, known as a cichlid, gave birth to 42 more offspring over the subsequent year, a rare case of “selfing” in an otherwise sexually reproducing vertebrate (animal with a backbone and/or spinal column), according to a paper published in the journal Royal Society Open Science.
Selfing essentially refers to having sex with oneself and breeding.
The practice is rare in vertebrates. It was previously discovered among mangrove killifish, but for those fish, selfing is a primary mode of reproduction.
“In the mangrove killifish, selfing is an adaptation,” said lead author Assistant Professor Ola Svensson. “It is believed that it can be hard for them to find a mate, and selfing is better than not producing at all.”
Dr Svensson, a researcher from the University of Gothenburg’s Department of Biological and Environmental Sciences and colleagues crossed two different species of cichlids to produce what they thought was a normal, albeit hybrid, female fish.
Such hybridisations do occur in nature and have often happened before in laboratory settings.
Upon the fish’s death, the scientists determined that the individual had both ovaries and a male testis, so the fish was intersex.
Unusual case of sexual reproduction
Senior author Professor Cock van Oosterhout of the University of East Anglia said breeding took place in the fish’s mouth.
“These are mouth brooding fish and fertilisation takes place in the mouth. If some spermatozoa are released together with the eggs, they may be fertilised upon release, or in the mouth of the fish,” he explained.
Many cases of “virgin births” have been reported in various species ranging from sharks to scorpions. That type of reproduction, however, is referred to as parthenogenis, and involves no fertilisation.
What makes the recent event so different and unusual is that the individual was both mother and father to the offspring.
“It is a case of sexual reproduction,” said Dr Svensson.
Some of the offspring were male, while others were females. They were able to reproduce normally, and none of their offspring were capable of selfing.
Nevertheless, the progeny suffered from what the researchers called “inbreeding depression”, showing minimal genetic diversity evident among them.
Dr Svensson said many of their colleagues therefore wondered: “If this is such an unsuccessful form of reproduction, why might it have evolved in the first place?”
What is the point of selfing?
Evolutionary biologist Dr Lukas Schärer from the University of Basel and his team share the view of Dr Svensson’s group that some reproduction for certain species is better than none at all.
Dr Schärer and his team discovered selfing in the flatworm species Macrostomum hystrix. In that case, the process is doubly unusual because the intersex worms must inject sperm directly into their own heads in order to reproduce.
“To us this sounds traumatic, but to these flatworms it may be their best bet if they cannot find a mate but still want to reproduce,” said Steven Ramm of Bielefeld University, who studied the phenomenon.
So selfing is not always an evolutionary dead end, since it can result in some offspring that may reproduce. Time will tell if the process may occur in vertebrates other than certain fish.
Professor Van Oosterhout said scientists now have clues on what to look for in different species.
“Selfing would be advantageous in a species that has a low population size or excellent colonising potential because, under these circumstances, finding a mate may be most difficult,” he said.