In the case of cartilaginous fishes, which include species such as rays, sharks, and skates, as many as 70% of these species provide birth to live young. To shed light on the functions of VTG and its receptor in these species, the authors compared tissue-by-tissue transcriptome data from one egg-laying shark (the cloudy catshark) and 2 viviparous sharks. The frilled shark is a viviparous species that supplies no maternal nutrients to the establishing embryo, while the spotless smooth-hound has a placenta. In contrast, in the two viviparous sharks, VTG was revealed not only in the liver but also in the uterus. “It also led to the launch and advancement of the Squalomix consortium,” an effort launched in 2020 to promote genomic and molecular techniques particularly targeting shark and ray species.
According to Kuraku, who now works as a Professor of Molecular Life History Laboratory at the National Institute of Genetics in Mishima, detectives have long desired to find out more about the advancement of viviparity in sharks and their loved ones. “Reproduction is one of the most remarkable features of cartilaginous fishes because they show a broad spectrum of reproductive modes.”
Among viviparous types, this includes a series of mechanisms for providing nutrients to the establishing embryo, from relying entirely on nutrients present in the embryos yolk sac, to feeding the embryo unfertilized eggs, secreting nutrients from the uterus (” uterine milk”), or transferring nutrients through a placenta.
To better understand these numerous mechanisms, the authors browsed genomic and transcriptomic information from 12 cartilaginous fishes for homologs of vitellogenin (VTG), a major egg yolk protein synthesized in the female liver in egg-laying species. Despite their reproductive mode, all cartilaginous fish types had at least two copies of VTG, while all copies of VTG have actually been lost from mammals (although the authors did identify a copy in the Tasmanian devil, a marsupial, which was not previously known to harbor a VTG gene).
Next, the authors browsed for homologs of the VTG receptor; while mammals maintain a single copy of this receptor, Kuraku and his associates determined 2 ancient tandem duplications offering rise to 3 copies of the receptor in cartilaginous fishes. The authors note that this finding was unexpected.
” We forecasted the retention of egg yolk protein genes in the shark genomes since live-bearing sharks rely partly on nutrition supply from the egg yolk,” states Kuraku. “What surprised us the most was that cartilaginous fish consisting of sharks have more copies of the egg yolk protein receptor genes.”
This recommended that these proteins may offer an unique function in this viviparous lineage.
To shed light on the functions of VTG and its receptor in these types, the authors compared tissue-by-tissue transcriptome information from one egg-laying shark (the cloudy catshark) and two viviparous sharks. The frilled shark is a viviparous species that supplies no maternal nutrients to the developing embryo, while the pristine smooth-hound has a placenta. In the egg-laying cloudy catshark, VTG is mainly expressed in the liver, and its receptors are primarily expressed in the ovary.
On the other hand, in the two viviparous sharks, VTG was expressed not only in the liver however likewise in the uterus. Surprisingly, the VTG receptor was also expressed in the uterus in these species. This recommends that VTG proteins might not only operate as yolk nutrients but may also be carried into the uterus, where they may contribute in offering maternal-based nutrition in some cartilaginous fishes.
They also hope to expand this analysis to a genome-wide survey of elements associated with the various reproductive modes of cartilaginous fishes. Such experiments are difficult in these types offered the difficulty in getting biological samples.
” This study was made it possible for by networking amongst people with different types of know-how who acknowledge the biological potential of cartilaginous fishes,” states Kuraku. “It also caused the launch and development of the Squalomix consortium,” an initiative released in 2020 to promote molecular and genomic techniques particularly targeting shark and ray types. The consortium aims to make its resources publicly readily available, consisting of a cell culture technique that might help make it possible for practical assays of particles, assisting in future research into the reproductive modes of these fascinating and evasive creatures.
Referral: “Egg Yolk Protein Homologs Identified in Live-Bearing Sharks: Co-Opted in the Lecithotrophy-to-Matrotrophy Shift?” by Yuta Ohishi, Shogo Arimura, Koya Shimoyama, Kazuyuki Yamada, Shinya Yamauchi, Taku Horie, Susumu Hyodo and Shigehiro Kuraku, 20 February 2023, Genome Biology and Evolution.DOI: 10.1093/ gbe/evad028.
The frilled shark has a special mode of live-bearing and is believed to exhibit a long gestation time of no less than 3 years. Credit: Frilled Shark Research Project
Viviparity, or the capability to give birth to live young, is frequently linked to mammals. This mode of recreation has actually progressed multiple times throughout numerous vertebrates, with over 150 separate incidents. This consists of over 100 instances in reptiles, 13 in bony fishes, 9 in cartilaginous fishes, 8 in amphibians, and 1 in mammals.
To comprehend the development of viviparity, it is needed to investigate the characteristic across different evolutionary family trees. In the case of cartilaginous fishes, that include species such as sharks, skates, and rays, as lots of as 70% of these species bring to life live young. Despite this, the viviparity in these animals stays improperly comprehended due to their elusive nature, low reproductive output, and big and repeated genomes.
In a current short article published in Genome Biology and Evolution, a team of researchers led by Shigehiro Kuraku, formerly Team Leader at the Laboratory for Phyloinformatics at RIKEN Center for Biosystems Dynamics Research in Japan, set out to address this space. Their study identified egg yolk proteins that were lost in mammals after the switch to viviparity however kept in viviparous sharks and rays. Their outcomes recommend that these proteins might have progressed a brand-new function in offering nutrition to the establishing embryo in cartilaginous fishes.
