Introduction
The epithelial sodium channel (ENaC) plays a key role in salt and water homeostasis in tetrapod vertebrates and is also suggested as a candidate receptor for salt taste sensation in rodents. There are four ENaC subunits (α, β, γ, δ) which form heterotrimeric αβγ- or δβγ‑ENaCs. Whereas αβγ-ENaC mediates appetitive salt taste in mice, the SCNN1D gene, coding for δ-ENaC, is absent in Myomorpha (mice, rats and hamsters) but δ-ENaC appears to be present in human taste buds. The ancestors of modern cetaceans (whales/dolphins) transitioned from a terrestrial to a marine environment 40-50 million years ago. The transition to a marine environment was associated with extensive loss of chemoreceptor genes, including genes coding for olfactory receptors and the receptors mediating bitter and sweet taste. The genes coding for αβγ-ENaC are intact in cetaceans, thereby leading to the wide-spread assumption that cetaceans can only taste salt.

Methods
Using genome information in the National Center for Biotechnology Information (NCBI) database, we analysed the genes coding for the four ENaC subunits, SCNN1A (α-ENaC), SCNN1B (β-ENaC), SCNN1G (γ-ENaC) and SCNN1D (δ-ENaC) in 21 cetacean species, belonging to 12 families and 23 sister species of the Artiodactyla (even-toed ungulates), belonging to 9 families. 

Results
Whereas SCNN1A, SCNN1B and SCNN1G are intact in species of all investigated cetacean and artiodactylan families, the SCNN1D gene displayed multiple insertions/deletions or loss of splice donor-acceptor sites, causing disruption of SCNN1D open reading frames across all investigated cetacean families. SCNN1D is intact in the investigated artiodactylan families. Pseudogenisation of SCNN1D is evident in toothed whales (Odontoceti) and baleen whales (Mysticeti), suggesting that loss of the SCNN1D gene occurred early in cetacean evolution. Interestingly, like the previously reported exon 11/12 fusion in the rodent infraorder Hystricognathi (Gettings et al. 2021), a fusion of exons 11 and 12 of the SCNN1D gene was observed in the Bovidae (cloven-hoofed, ruminants), with exception of the subfamily Bovinae (bison, buffalos, cattle, and relatives).

Conclusion
These data indicate that there is no functional δ-ENaC in cetaceans, excluding a role for this ENaC isoform in cetacean salt taste. Given the importance of αβγ-ENaC in mammalian renal and lung physiology, the presence of this ion channel does not necessarily point to a role in salt taste physiology and the ability of cetaceans to taste salt.