It happens more often than you think

Plants can do this, just like aphids. Now the female crocodile has also been bred without the participation of the male. How and causes of asexual reproduction.

Seven fertilized eggs lie among the tropical greenery of a crocodile enclosure at Costa Rica’s Parque Reptilandia, a zoo specializing in reptiles. And that was a surprise. Only one animal was housed in the cage, an 18-year-old female crocodile (Crocodylus acutus) who had spent almost her entire life in isolation.

The seven eggs entered the incubator, and after three months, a full-fledged crocodile embryo was formed in one of them. Researchers from Costa Rica and the United States performed an autopsy on the fetus (the baby was unable to survive) and compared the animal’s genome with genetic material from the animal’s hair.

The result was recently published in the scientific journal Biology Letters: Although the genes have been rearranged a bit here and there, it turns out that the DNA of the fetus is 99.9 percent identical to that of the mother. This means that the crocodile has reproduced asexually, without the intervention of a male animal. Although the crocodile calf was not viable, the discovery is the first known case of asexual reproduction, known as parthenogenesis, in crocodiles.

Asexual reproduction occurs regularly in nature. Bacteria reproduce themselves, and there are all kinds of fungi, plants, and organisms, from water bears to aphids, that do the trick, too. But this phenomenon is rare in vertebrates. It is known that about eighty species of vertebrates (out of a total of about 70,000 species described) are capable of reproducing without a partner in certain cases. This mainly concerns reptiles (snakes, lizards) and some cartilaginous fish (sharks, sawfish, rays).

Two simple questions can be asked about this phenomenon: how and why? Obviously, some species that naturally reproduce sexually also have a mechanism for achieving asexual reproduction, but how does that work? And above all: in what conditions is this mechanism used?

DNA magic trick

In the claw of a mother crocodile in Costa Rica, the embryo developed via what is called autogenesis with terminal fusion, according to the researchers. Researcher Matthias Bettenbinder of the Naturalis Center for Biodiversity (not involved in the crocodile research) explains that the germ cell normally divides before forming an egg cell, and then divides again into a group of four cells – one egg cell and three so-called polar bodies.

“The egg cell contains a copy of half of the mother’s genetic material. To this is usually added one half of the father’s DNA. In the case of self-adhesion with terminal fusion, one of the polar bodies fuses with the egg, and as a result two halves of the mother’s DNA join together to form a complete set of genes.

It is difficult to say the exact reason why animals go into parthenogenesis. Although asexual reproduction may have advantages for the animal—for example, finding a mate takes energy—there are also disadvantages. “Genetic diversity is important for the survival of a species, and that’s why you need partner animals,” says Gert Flick, professor emeritus of animal physiology at Radboud University.

He suggests that asexual reproduction in crocodilians may be an escape mechanism to protect the survival of the species when there is a temporary shortage of males. “In crocodiles, the temperature of the clutch determines whether the fetus is male or female. If the nest temperature during the breeding period is about 28 degrees, females will be born. At a consistently higher temperature of 32 degrees, they become males.

This means that a simple variable environmental factor can lead to a perturbed ratio between the number of males and females. Parthenogenesis can provide the opportunity in such circumstances to extend the time, so to speak, for the preservation of the species.

This seems to contradict the fact that the crocodile embryo was not viable in the study in Costa Rica, but other species (including the royal python and the white-spotted bamboo shark) are known to be able to produce second-generation offspring through parthenogenesis.

Bettenbinder is cautious about explaining this phenomenon. It is possible that parthenogenesis protects the survival of the species under certain conditions, but it is also possible that there is no real cause for this phenomenon. He doesn’t want to call it an evolutionary industrial accident, but says: “Sometimes things happen in nature. Obviously, the binding of an egg cell’s genes to DNA from a polar particle does happen sometimes. There doesn’t have to be a reason for that.”

A crocodile embryo found in the Costa Rican Zoo is the first known case of crocodile parthenogenesis. This raises an interesting question. Crocodiles share a common ancestor with dinosaurs and birds. Now that it is known that some beaked birds and crocodiles are capable of parthenogenesis, it is possible that (some) dinosaurs could also reproduce without the involvement of a partner, the US researchers wrote.

Bettenbender and Flick react cautiously to this assumption. “I certainly don’t rule it out, but I don’t know how to prove it,” says Bettenbender. The main problem is that dinosaur DNA is necessary for definitive evidence, which is not available (unlike in Hollywood movies). “It is, I say carefully, conceivable. There is no harm in speculating about it. Theories do exist.”