One of the first steps towards a successful pregnancy is fertilization, where sperm and oocyte come together to form a zygote. In mammals, this process can occur in vitro, allowing assisted reproductive technologies (ART) to alleviate certain infertilities. ART – which includes in vitro fertilization and gamete donation – accounted for 1.7% of all US births in the US in 2015 (Sunderam et al. 2018).
In mammals, fertilization increases the oocyte’s free calcium levels, initiating oocyte activation and allowing embryonic development. In contrast, Drosophila melanogaster oocytes activate prior to fertilization, in response to changes in pressure and hydration as oocytes travel down the oviduct (Heifetz et al. 2001; Horner and Wolfner 2008). A calcium rise also occurs in activating fly oocytes, and can be visualized by GCaMP3 sensors (Kaneuchi et al. 2015; York-Andersen et al. 2015); this is important because calcium initiates signaling cascades that lead to meiosis resumption. Here, we show this rise in frames taken ~5 seconds apart and false-colored to show calcium levels induced by changing osmotic pressure; calcium concentrations are high in red areas.
As shown here, in vitro the calcium rise begins at both poles of the fly oocyte, and travels in a wave toward the center. However, in vivo the wave almost always initiates only at the posterior pole of the egg (Kaneuchi et al. 2015; York-Andersen et al. 2015). These differences between what occurs in vivo and in vitro are a reminder that many unknowns remain regarding the impact of ART. As utilization of ART will likely continue to increase in developed countries, there is a significant need to understand how these in vitro technologies affect the offspring’s immediate and future health.
Acknowledgments
Funding: The authors would like to thank the NICHD and Burroughs Wellcome Fund for their generous support of the Frontiers in Reproduction Course. Additional support was provided by NIH/NICHD K12 HD065987 (EALE).
Abbreviations:
- ART
assisted reproductive technologies
Footnotes
Conflict of Interest: The authors have no conflicts of interest to disclose.
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