Viviparity, or the ability to give birth to live young, is commonly linked to mammals. However, this mode of reproduction has evolved multiple times across various vertebrates, with over 150 separate occurrences. This includes over 100 instances in reptiles, 13 in bony fishes, 9 in cartilaginous fis
The frilled shark has a unique mode of live-bearing and is thought to exhibit a long gestation time of no less than three years. Credit: Frilled Shark Research Project
In a recent article published in Genome Biology and Evolution, a team of researchers led by Shigehiro Kuraku, previously Team Leader at the Laboratory for Phyloinformatics at RIKEN Center for Biosystems Dynamics Research in Japan, set out to address this gap. Their study identified egg yolk proteins that were lost in mammals after the switch to viviparity but retained in viviparous sharks and rays.
To better understand these various mechanisms, the authors searched genomic and transcriptomic data from 12 cartilaginous fishes for homologs of vitellogenin , a major egg yolk protein synthesized in the female liver in egg-laying species. Regardless of their reproductive mode, all cartilaginous fish species had at least two copies of VTG, while all copies of VTG have been lost from mammals .
In contrast, in the two viviparous sharks, VTG was expressed not only in the liver but also in the uterus. Interestingly, the VTG receptor was also expressed in the uterus in these species. This suggests that VTG proteins may not only function as yolk nutrients but may also be transported into the uterus, where they may play a role in providing maternal-based nutrition in some cartilaginous fishes.
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