Asexual reproduction is common in animals like starfish, deep-sea worms, and stick insects, but in vertebrates it is a rarity. A process called parthenogenesis allows some females to fertilize an egg with their own genetic material, producing offspring by “virgin birth”. Scientists have speculated that vertebrates that habitually reproduce sexually turn to parthenogenesis as a “hail pass,” a last-ditch reproductive effort when there aren’t enough mates to go around.
However, a recent study revealed an example of a female zebra shark in an aquarium reproducing parthenogenetically, even though there were healthy breeding males in the same enclosure. This discovery has implications not only for the ongoing care of zebra sharks in zoos and aquariums, but also for conservation efforts focused on their wild counterparts.
“We have known for several years that parthenogenesis occurs in animals like sharks, but some aspects remain unknown, such as why it happens and what triggers it,” says Kevin Feldheim, a researcher at the Field Museum in Chicago and corresponding author of the study. “This latest article is just one more step in understanding why these ‘virgin births’ are happening.”
The study, published in the Fish Biology Journalfocuses on endangered zebra sharks at the Shedd Aquarium in Chicago.
“While we started to see successful breeding of zebra sharks at Shedd in 2004, we also started genetic testing to confirm which sharks were the parents of the offspring,” says Lise Watson, assistant director of animal operations and habitats at Shedd Aquarium and an author of the study. “By confirming the lineage of offspring, we could make more informed decisions about future breeding efforts to maintain maximum genetic diversity while supporting the Association of Zoos and Aquariums’ species survival plan for zebra sharks. .”
Shedd’s Wild Reef exhibit features a huge floor-to-ceiling habitat containing a variety of sharks, including zebra sharks. In 2008, Watson and his colleagues noticed a clutch of eggs; they moved them to a backstage nursery to hatch safely. When the baby sharks hatched, Jean Dubach of Loyola University analyzed the sharks’ DNA to determine their parentage. The results seemed impossible.
“These pups didn’t match any of the mature males that were in the pen. But they did match the female that laid the eggs,” Feldheim says.
In addition to having genetic markers in common with their mother but none of the potential fathers, the puppies had identical homozygous copies of certain alleles. Think back to the genetics unit in your high school biology class. You have probably created square Punnett grids with different types of genes represented by upper and lower case letters, such as B as the dominant allele for brown eyes and b as the recessive allele for blue eyes. You get one allele from each parent, and if they are the same (BB or bb), it’s called homozygous.
The genetic markers used in these sharks are more complicated than just B vs. b – they have many more alleles, to the point that it’s virtually impossible for them to look exactly the same in two different individuals. So when the baby sharks’ DNA results came back homozygous, it meant they had to get those two strands of DNA from their mother, rather than two different parents.
“Finding that these pups were parthenotes came as a surprise to Shedd’s team, given our previous success in encouraging breeding through sexual reproduction,” says Watson. “This news has underscored exactly why regular and ongoing genetic testing of offspring is important.”
Puppies born of this type of “virgin birth” unfortunately tend to have a short life expectancy, as they are more likely to have rare recessive genetic conditions. The baby sharks in this study only survived a few months. But the fact that they were born challenges long-held ideas in biology.
“This is only the second instance we know of where sharks were born parthenotically even when there were healthy mates available,” says Feldheim; the other example was at the Aquarium of the Pacific. “This discovery upends what we thought we knew about how and why parthenogenesis occurs, and it illustrates a key aspect of science: we are continually learning.”
“This study is only the beginning of our understanding of the occurrence of this genetic phenomenon in zebra sharks,” says Watson. “Zoos and aquariums like Shedd have a key role to play in conserving species like zebra sharks, which are nearly extinct in some parts of the world. Learning more about parthenogenesis and confirming the genetic makeup of our populations in zoos and aquariums is crucial to making informed decisions that feed into this work.”
Kevin A. Feldheim et al, Parthenogenesis in an elasmobranch in the presence of conspecific males, Fish Biology Journal (2022). DOI: 10.1111/jfb.15268
Quote: Study investigates “virgin birth” in aquarium sharks, even when potential mates are nearby (2022, December 15) Retrieved December 15, 2022 from https://phys.org/news/2022-12- virgin-birth-aquarium-sharks-potential.html
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