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"Virgin births" discovered in zebra sharks

Although asexual reproduction – a process called “parthenogenesis” that allows some female animals to fertilize an egg with their own genetic material – is common for animals such as starfish, stick insects, or deep-sea worms, it is a rare phenomenon among vertebrates. Scientists have long assumed that female vertebrates which usually reproduce sexually turn to parthenogenesis as a last-ditch effort at reproduction when there aren’t enough males around. 

However, a team of researchers led by the Field Museum of Natural History in Chicago were surprised to discover recently that a female zebra shark in an aquarium reproduced via parthenogenesis, even though there were healthy, reproductive males in the same enclosure. These findings have implications not only for the continued care of zebra sharks in aquariums and zoos, but also for conservation efforts of their wild conspecifics.

“We’ve known for several years that parthenogenesis occurs in animals like sharks, but some aspects of it remain unknown, like why it occurs and what triggers it,” said study lead author Kevin Feldheim, a researcher at the Field Museum. “This latest paper is just another step in learning why these ‘virgin births’ happen.”

“As we started to see successful breeding of zebra sharks at Shedd in 2004, we also began genetic testing to confirm which of the sharks were the parents of the offspring,” added study senior author Lise Watson, the assistant director of Animal Operations and Habitats at the Shedd Aquarium. “By confirming the lineage of the offspring, we could more soundly make decisions on future breeding efforts to maintain maximum genetic diversity while supporting the Association of Zoos and Aquarium’s Species Survival Plan for zebra sharks.”

Unfortunately, shark pups born by this type of “virgin birth” tend to have a short life expectancy, since they are likelier to experience rare recessive genetic conditions. Yet, although the pups examined in this study survived just a few months, the fact that they were born at all challenges long-standing biological theories.

“This is only the second case that we know of where sharks have been born by parthenogenesis even when there were healthy mates available,” said Feldheim. “This discovery throws a wrench in what we thought we knew about how and why parthenogenesis happens, and it illustrates a key aspect of science: we’re continually learning.”

“This study is just the beginning of our understanding of the occurrence of this genetic phenomenon in zebra sharks. Zoos and aquariums like Shedd have a key role to play in the conservation of species like zebra sharks, which are nearly extinct in some parts of the world. Knowing more about parthenogenesis and confirming the genetic makeup of our populations in zoos and aquaria is crucial to making informed decisions that fuel this work,” Watson concluded.

The study is published in the Journal of Fish Biology

By Andrei Ionescu, Staff Writer

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