While asexual reproduction in invertebrate species isn’t unheard of, it isn’t exceptionally common either. Instead, such behaviors are typically attributed to plants or a smaller list of animals, of which some have the capability to reproduce asexually or with a mate. Komodo dragons are one such example, as they often live in isolation, meaning that female dragons are known to occasionally reproduce asexually when they are unable to find a male.
This type of reproduction is referred to as parthenogenesis, defined by a female of a species reproducing without the need for male fertilization. The process involves an egg doubling, dividing and then combining its genetic material to create offspring. While the process can produce viable offspring, there have also been cases of developmental complications as a result of being formed via parthenogenesis.
Of course, there is also the case that some species have never been observed to reproduce asexually, meaning that the rarity of such behavior could be a result of various evolutionary implications. Furthermore, it would appear that parthenogenesis is not the only means by which a species can reproduce without a partner.
A study conducted by scientists at the University of East Anglia and others at the University of Gothenburg revealed that female cichlids also have the potential to produce offspring independently. LiveScience reports that the team was surprised to find one female cichlid in their lab tanks swimming around with fertilized eggs in her mouth during the study. The tank was filled with other female cichlids, so there was no chance of the eggs being fertilized.
While the team originally assumed that this was a case of parthenogenesis being observed in female cichlids, the theory was dismissed upon closer examination of the fish. Upon looking closer at the fish’s sex organs, the subject appeared to have both normal ovaries as well as spermatocytes, cells that make sperm. This meant the fish could make offspring without the help of a mate. They named the subject “Mary.”
While under observation, Mary produced 14 broods of young and of those, 46 offspring hatched, and 17 survived. Interestingly, none of the offspring shared their mother’s unique characteristics. This observation also demonstrates how what was observed is not a case of parthenogenesis. If it had been, the offspring would have been clones of Mary, but they were not. As such, the scientist referred to the unusual reproductive ability as “selfing” instead of cloning as the spawn differed, and Mary’s offspring underwent some form of fertilization.
While being able to reproduce without a mate might seem like an advantage, there are some known complications. In situations where mates are scarce, or the species is particularly isolated, the ability to reproduce independently can certainly serve its own function. However, researchers believe there are downsides to being able to self-fertilize.
Specifically, they point to a phenomenon called inbreeding depression, wherein there is such minimal genetic diversity that birth defects can occur in the generations that follow. The development of two different sexes, after all, came about very likely to ensure more diversity in genes, they say.
Researchers say that the risk of genetic defects may be why “selfing” is so exceptionally rare in nature. But they don’t rule out that it could have happened before in a lab environment—fish are rarely swimming around by themselves, they say, and scientists focusing on other questions could easily have missed them.