Artificial Insemination and Parthenogenesis in the Zebra Shark Stegostoma tigrinum

Abstract

Maintaining self-sustaining populations of zoo and aquarium collections can be challenged when natural reproduction fails within mixed-sex populations; however, reproductive success can sometimes be restored with the application of reproductive technologies. Among a population of three female and one male Zebra Sharks (Stegostoma tigrinum), production of young failed despite constant male presence with two of the females. To determine if assisted techniques could be used to rescue sexual reproduction, artificial insemination was performed in a singleton female twice over a three-year period using freshly collected semen. Hatching success for eggs laid by all three females was monitored to compare natural and artificial insemination modes. After the first insemination (December 15th, 2011), 143 yolked eggs resulted in no sexually produced offspring and four genetically-confirmed, parthenogenetic offspring. After the second insemination (September 24th, 2013), 62 yolked eggs resulted in two sexually produced offspring, 18 and 33 days after insemination, and three parthenogenetic offspring > 213 days post-insemination. For the two females housed with the male, no sexual offspring resulted. All females produced at least one hatched parthenote. This study successfully employed artificial insemination to circumvent barriers to natural reproduction in Zebra Sharks. With further development, artificial insemination represents a powerful tool that could be used for maintaining genetic diversity for animals housed in aquaria and conservation-based breeding programs for elasmobranchs.

The role of aquaria in the advancement of elasmobranch reproductive biology

“…Considering how often this phenomenon has been described simply as a byproduct of females laying fertile eggs in the absence of males, it raises questions about how often parthenogenesis occurs when it is explicitly studied. For instance, parthenogenesis has been documented to occur alongside sexual reproduction in the Whitespotted Bamboo Shark (Wyffels et al, 2021) and the Zebra Shark (Adams et al, 2022); however, it is likely that the only reason why parthenogenesis was detected at all in these studies is that every hatchling was genotyped to determine the success of artificial insemination techniques, not to determine rates of parthenogenesis itself. Thus, in general, the impetus for examining young born in aquaria for parthenogenesis usually only occurs when young are produced at a time when females are maintained as a single sex population and/or when they have not been housed with males for an extended period of time.…”

“…obs.). For example, continuous egg laying is observed year-round for some Zebra Shark females held in aquaria (Adams et al, 2022), a pattern that is likely atypical from a natural cycle where there would be a distinct reproductive period. Current research is being conducted to determine what effect (if any) the lack of environmental cues has on influencing reproductive cycling in this species.…”

“…Despite the limitations of field samples, fecundity is a fairly reliable metric that can be estimated for viviparous species because the number of young can directly be associated with an individual female, assuming she does not spontaneously abort prior to sampling (Wosnick et al, 2018). For oviparous species, point-in-time sampling to quantify fecundity is not possible since females lay clutches of eggs over a reproductive period that can last for months (Wyffels et al, 2021;Adams et al, 2022). Furthermore, variance in fecundity is possibly more variable in oviparous species than viviparous species because the former do not have a physical limitation on available body space to carry young.…”

“…Parthenogenesis has been documented across a variety of shark and ray species, and at times, independently in the same species at multiple aquaria. For example, parthenogenesis in Zebra Sharks has been observed in three different countries across the world (Australia, United Arab Emirates, and United States) and has been documented in animals producing parthenotes yearly (Robinson et al, 2011) and switching between sexual and unisexual reproduction (Dudgeon et al, 2017;Adams et al, 2022). Likewise, parthenogenesis has been observed on several, independent occasions in the Whitespotted Bamboo Shark (Chiloscyllium plagiosum; Feldheim et al, 2010;Straube et al, 2016;Wyffels et al, 2021).…”

“…Thus, in general, the impetus for examining young born in aquaria for parthenogenesis usually only occurs when young are produced at a time when females are maintained as a single sex population and/or when they have not been housed with males for an extended period of time. However, two independent artificial insemination studies (Wyffels et al, 2021; Adams et al, 2022) demonstrate that even in the presence of semen, females still reproduced parthenogenetically. Therefore, it is likely that the known instances of parthenogenesis represent only a small portion of its true occurrence for elasmobranchs in aquaria and in situ populations.…”