Smalltooth sawfish are increasingly reproducing without males, according to new research. These “virgin births,” as they are dubbed, are better known as parthenogenesis, or reproduction without fertilization.
Parthenogenesis occurs as a result of a bastardized version of meiosis, where two female eggs combine genetic material instead of a sperm and an egg. In fish, the resulting embryo is diploid (there are two copies of each chromosome) but in parthenogenesis each chromosome is identical, or at least mixed up from an identical genome—so each offspring has roughly half the genetic material of a typical fertilized offspring. For the sawfish, this may be a sign of desperation.
Scientific interest in parthenogenesis has a long history. It has been observed in a wide range of species, including beetles, moths, flatworms, scorpions, snakes, and even komodo dragons. In aquariums, parthenogenesis has been observed in multiple shark species (close sawfish relatives), and many experts suspect that it might be common. The sawfish is the first example of widespread parthenogenesis in a wild fish population, providing some confirmation.
The origins of this unusual behavior have been debated for 100 years, but in the 1970s researchers zoomed in on an explanation dubbed the “cost of meiosis”. Basically, the main advantage of sexual reproduction is genetic diversity. However, if the “goal” of life is to pass on your genes, than reproducing through parthenogenesis passes twice as many of your genes to the next generation as sexual reproduction. An alternative point of view is that parthenogenic offspring will not share resources with offspring that are only half related—your genes get more out of the environment. Which, if any, of these scenarios is the true evolutionary motivation for parthenogenesis is still debated.
The explanation in the sawfish may be more straightforward— the population has dwindled so far that the females simply can’t find mates to fertilize their eggs. A 1980 paper theorized that when populations dwindled to a point where the probability of encountering a mate dropped too far, parthenogenesis was the likely response. In the sawfish, then, parthenogenesis is a last ditch effort to maintain the population, not a viable growth strategy.
Evolutionary explanations such as the cost of sex are less likely, since the sawfish produce fewer offspring through parthenogenesis than through sexual reproduction. Each offspring may contain more of the parent’s genes, but that’s a moot point if there are fewer than half as many offspring.
Smalltooth sawfish are critically endangered, and even with parthenogenesis their survival remains in doubt. As the saying goes, “life finds a way,” but it might not be enough.