The Paper: Harmon, T. S., Kamerman, T. Y., Corwin, A. L., & Sellas, A. B. (2015). Consecutive parthenogenetic births in a spotted eagle ray Aetobatus narinari. Journal of Fish Biology, 741–745. http://doi.org/10.1111/jfb.12819
Our story begins about 3 years ago at Epcot’s aquarium, The Seas. Aquarists were thrilled when their female spotted eagle ray (Figure 1; Aetobatus narinari) gave birth to a healthy female baby (called a pup). But something was a little strange about this pregnancy. The only male in the eagle ray’s tank was presumed sexually immature (based on age and size measurements) and she had been separated from sexually mature males for at least a year. Suspicions grew when a year later the female (still separated from sexually mature males) gave birth to another healthy female pup. How could this female be producing viable offspring without any males present?
Aquarists came up with a few plausible options. 1) The female had stored sperm from one of the sexually mature males she had been housed with previously. 2) The male she was housed with was, in fact, sexually mature and had successfully fathered (or sired) the two female pups. 3) Parthenogenic births.
Parthenogenesis is a type of asexual reproduction. An embryo develops without the egg ever being fertilized. Parthenogenesis has been reported as a viable reproductive strategy in several vertebrates. You may have heard of the entirely female whiptail lizard species (Aspidoscelis spp.). A few years ago, parthenogenesis was reported in cartilaginous fishes (sharks, skates, stingrays, and chimaeras) and since has been reported in bonnet heads (Sphyrna tiburo), black tips (Carcharhinus limbatus), whitespotted bamboo sharks (Chiloscyllium plagiosum), zebra sharks (Stegostoma fasciatum), white-tip reef sharks (Triaenodon obesus) and in the small tooth saw fish (Pristis pectinata). Parthenogenisis appears to be more frequent in captive populations where individuals may not have access to mates but has recently been reported in wild populations (including species of sharks) as well.
But there was a bit of a twist: prior to being separated from sexually mature males and having the two female pups mentioned earlier, the female had had two more successful pregnancies. Could these also have resulted from parthenogenesis?
Before we go over the lines of evidence, let’s take a very brief genetics review. DNA is the blueprint for life. It is nicely packaged into units called chromosomes. Animals have two sets of chromosomes. In sexual reproduction, one of these sets comes from the mother and one comes from the father. Because each parent randomly gives the offspring one of its two sets of chromosomes, the offspring has a unique combination. In asexual reproduction, however, the parent gives the offspring both of its sets of chromosomes, essentially resulting in a clone of the parent (Figure 2). Now, on to how genetics was used to solve the case:
- The eagle ray’s first birth (while in the tank with two mature males) resulted in 3 male pups. Sharks’ and stingray’s sex chromosomes are much like humans in that a male has both an X and a Y chromosome while females have two X chromosomes. In order to give birth to male pups, the mother would have needed a Y chromosome to be handed down from the father.
Diagnosis: the first birth was the result of sexual reproduction.
- The second birth (also while in the tank with mature males) resulted in a single female pup.
Diagnosis: this female could have been the result of a parthenogenic birth.
Because parthenogenesis would result in pups with the same genetic make-up as the mother, aquarists could test their hypothesis by comparing the pups’ genomes to that of their mother. Scientists took tissue and blood samples and analyzed portions of the genome of each of the female pups and compared them to the mother’s genes.
- The female pup from the 2nd birth had genes which did not match the mother’s genome and must have come from paternal chromosomes.
Diagnosis: the second birth was the result of sexual reproduction.
- The female pups from the 3rd and 4th births shared their mother’s genes. The combination of genes that the mother and the offspring have is also relatively rare in wild populations of spotted eagle rays.
Diagnosis: both the 3rd and 4th pups are the result of parthenogenic births.
The mother had two parthenogenic pregnancies back to back – a rarity that has only been documented in a few birds, snakes, and a single shark species. This documentation helps scientists further understand this interesting asexual mating strategy.
This spotted eagle ray is also interesting because she had given birth via sexual reproduction twice and then switched to asexual reproduction. Stranger yet, the switch occurred within a year of being separated from sexually mature males. What led to the rapid change in reproductive strategy?
Answering this question could help scientists understand parthenogenesis and will likely inform conservation practices for this species and other cartilaginous fishes that are being threatened by population depletion. How? Well, by killing eagle rays and depleting population sizes, humans are effectively depriving females of possible mates in the wild. Females may turn to asexual reproduction, which causes lower genetic variation.
Genetic variation is good. Changes in the environment (new diseases breaking out, changing climate, etc.) may be detrimental to the species. If the population has high genetic diversity, maybe some of the individuals will survive to reproduce. If all of the individuals have the exact same genotype, there is a higher likelihood that the entire population will be threatened. Think about it this way: You and a group of friends enter a contest and have no idea what you will be tested on. Your first challenge relies on physical strength and your team is comprised of your smartest, but not so fit, friends. You are going to lose. If you had chosen a team with an athlete, an engineer, and an historian though, you may have a better shot at winning. Same for genes: the more variety, the stronger the chances are that a species will make it to the next round.
Overall, solving this mystery led to some interesting findings. But like most scientific discoveries, it may have led to more questions than answers.