Urea-based osmoregulation in the developing embryo of oviparous cartilaginous fish (Callorhinchus milii): contribution of the extraembryonic yolk sac during the early developmental period

Abstract

Marine cartilaginous fish retain a high concentration of urea to maintain the plasma slightly hyperosmotic to the surrounding seawater. In adult fish, urea is produced by hepatic and extrahepatic ornithine urea cycles (OUCs). However, little is known about the urea retention mechanism in developing cartilaginous fish embryos. In order to address the question as to the mechanism of urea-based osmoregulation in developing embryos, the present study examined the gene expression profiles of OUC enzymes in oviparous holocephalan elephant fish (Callorhinchus milii) embryos. We found that the yolk sac membrane (YSM) makes an important contribution to the ureosmotic strategy of the early embryonic period. The expression of OUC enzyme genes was detectable in the embryonic body from at least stage 28, and increased markedly during development to hatching, which is most probably due to growth of the liver. During the early developmental period, however, the expression of OUC enzyme genes was not prominent in the embryonic body. Meanwhile, we found that the mRNA expression of OUC enzymes was detected in the extra-embryonic YSM; the mRNA expression of cmcpsIII in the YSM was much higher than that in the embryonic body during stages 28-31. Significant levels of enzyme activity and the existence of mitochondrial-type cmgs1 transcripts in the YSM supported the mRNA findings. We also found that the cmcpsIII transcript is localized in the vascularized inner layer of the YSM. Taken together, our findings demonstrate for the first time that the YSM is involved in urea-based osmoregulation during the early to mid phase of development in oviparous cartilaginous fish.

Multi-tissue RNA-seq and transcriptome characterisation of the spiny dogfish shark (Squalus acanthias) provides a molecular tool for biological research and reveals new genes involved in osmoregulation

“…The analysis of the spiny dogfish transcriptome reveals the presence of all the genes involved in the two main biosynthetic pathways for urea synthesis in cartilaginous fishes ( Table 5 ) and as expected, all of these genes are expressed in the liver, the main tissue involved in urea synthesis. However, in accordance with previous studies [ 6 – 8 , 58 ] some of the urea synthesis genes were identified in extra-hepatic tissues as well.…”

On the vagal cardiac nerves, with special reference to the early evolution of the head–trunk interface

“…The egg capsules were opened and embryos were removed from the yolk sacs. Mean weights and days post-egg laying at each developmental stage were described previously (Takagi et al, 2014). Embryos were staged according to Didier et al (1998) Redrawn from Kuratani (1997).…”

Discovery of conventional prolactin from the holocephalan elephant fish, Callorhinchus milii

“…The absolute quantification of target mRNAs was performed by qPCR with an ABI Prism 7900HT Sequence Detection System (Life Technologies, Carlsbad, CA, USA) and KAPA SYBR FAST ABI Prism qPCR Kit (Kapa Biosystems), as previously described (Takagi et al, 2014). The results were shown either as normalized by -actin or as the copy number of target mRNAs per gram of RNA.…”

“…A breakthrough in chondrichthyan biology was achieved by Venkatesh and colleagues, who initiated whole genome sequencing for the holocephalan elephant fish (or elephant shark, Callorhinchus milii) (http://esharkgenome.imcb.a-star.edu.sg/) (Venkatesh et al, 2007). We have focused on this species as a model for molecular endocrinological and physiological studies of cartilaginous fish (Hyodo et al, 2007;Kakumura et al, 2009;Yamaguchi et al, 2012;Takagi et al, 2014). Recently, a PRL-like gene was found from the elephant fish genome and designated PRL2 (Huang et al, 2009).…”