Highly unsaturated fatty acids (HUFAs) are a series of straight-chain fatty acids with three or more double bonds and 20 or more carbon atoms, which play important roles in maintaining cell membrane fluidity, regulating fat metabolism, enhancing immunity, and reducing inflammation (Zhang et al., 2019; Ri et al., 2022). Notably, eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are the most prominent HUFAs that play critical roles involved in promoting growth and development, and preventing cardiovascular and neurodevelopmental disorders (De Giuseppe et al., 2014; Zárate et al., 2017).

Usually, vertebrates possess the ability to biosynthesize HUFAs by using essential polyunsaturated fatty acids as precursors with two critical rate-limiting enzymes including elongases of very long chain fatty acids (Elovls) and fatty acid desaturases (Fads) (Oboh et al., 2017). The former can elongate the carbon chain of polyunsaturated fatty acids while the latter can introduce a double bond at a specific position on the carbon chain to produce desaturation (Nakamura and Nara, 2004; Guillou et al., 2010). In teleosts, it is commonly accepted that freshwater and migratory fishes have obvious capacity to biosynthesize HUFAs while seawater fishes usually have lower or no capacity to achieve this goal because they are surrounded by an environment with abundant HUFAs in diets (Li et al., 2010; Jaya-Ram et al., 2011). Meanwhile, it seems that food habitats also have effects on the HUFA biosynthesis in teleosts. For example, herbivorous or omnivorous freshwater fishes such as grass carp (Du et al., 2006) and zebrafish (Ishak et al., 2008) are proved to have the ability to synthesize arachidonic acid (AA), EPA, and DHA with linoleic acid (LA) and linolenic acid (LNA) as precursors. Differently, carnivorous fishes such as groupers (Li et al., 2016) and gilthead seabream (Ganga et al., 2005) lack this kind of capacity or have a weak capacity, and thus higher levels of dietary HUFAs are required to meet their requirements for growth, development, and reproduction. However, more studies are still required to investigate the exact mechanisms involved in HUFA biosynthesis in teleosts.

Thus far, seven members named ELOVL1–ELOVL7 in the ELOVL family were identified in mammals, and these members showed different substrate specificities for various fatty acid substrates (Jakobsson et al., 2006; Ohno et al., 2010). Overall, ELOVL1, ELOVL3, ELOVL6, and ELOVL7 primarily extend saturated fatty acids as well as monounsaturated fatty acids; ELOVL2, ELOVL4, and ELOVL5 mainly catalyze the elongation reactions of polyunsaturated fatty acids, while Elovl4 prolongs both very-long-chain saturated fatty acids and very-long-chain polyunsaturated fatty acids (Agbaga et al., 2008; Guillou et al., 2010). Recently, a novel teleost-specific member of the Elovl family, named Elovl8, had been identified in a marine rabbitfish (Siganus canaliculatus) (Li et al., 2020). Genetic synteny and gene phylogeny revealed that this new member commonly contains two isoforms, and it seemed that the elovl8a gene has been lost in some fish genomes (Li et al., 2020). Further functional experiments indicated that Elovl8b but not Elovl8a possesses the capacity to biosynthesize HUFAs (Li et al., 2020). Consistently, a single elovl8b gene was identified in hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂), and functional experiments revealed that diets full of HUFAs can inhibit elovl8b transcription in liver, suggesting that a negative feedback regulation of the HUFA synthetic pathway existed (Wu et al., 2022). However, the evolutionary and functional characteristics of this new member are still largely unclear, and much more studies are required to illustrate these issues.

Yellow catfish (Pelteobagrus fulvidraco) belongs to family Bagridae, order Siluriformes, and is an omnivorous freshwater fish that is widely cultured in China due to its rapid growth, good taste, and valuable nutrition values (Guo et al., 2023; Wei et al., 2023). Thus far, two key genes involved in HUFA biosynthesis including fad2 and elovl5 have been identified in yellow catfish (Song et al., 2015; Qin et al., 2017), but their exact roles have not been well investigated. In the present study, we identified a teleost-specific elovl8 gene from yellow catfish, and then its evolutionary and molecular characteristics, spatiotemporal expression patterns, and functional traits were determined for the first time. Our findings will provide not only a novel insight into mechanisms involved in HUFA biosynthesis in yellow catfish, but also valuable data for better understanding the evolutionary and functional traits of elovl8 in teleosts.

Elovls are key rate-limiting enzymes to extend the carbon chain of polyunsaturated fatty acids by catalyzing condensation reaction, which play important roles involved in the biosynthesis of HUFAs in animals (Nugteren, 1965). Thus far, elovl1–elovl7 are widely studied both in vertebrates and in invertebrates, but less is known about elovl8, the newly identified member of the elovl family (Ohno et al., 2010; Naganuma et al., 2011; Gregory and James, 2014; Yan et al., 2018). In this study, the elovl8 gene of the yellow catfish was identified and characterized; it contained a 795- bp-long ORF that encodes a protein of 264 amino acids. Similar results were also observed in rabbitfish (Siganus canaliculatus) (Li et al., 2020) and hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂) (Wu et al., 2022), suggesting that the elovl8 genes are conserved among different fishes.

Multiple protein sequence comparison showed that yellow catfish Elovl8 possessed all the characteristics of ELOVL protein family members, including four conserved regions (KXXEXXDT, QXXFLHXYHH, NXXXHXXMYXYY, and TXXQXXQ), endoplasmic reticulum (ER)-resident signals, six transmembrane regions, and a histidine cluster (HXXHH) that is involved in fatty acid elongation processes (Jakobsson et al., 2006). The secondary characteristics are similar to that of Elovl8b in the rabbitfish (Li et al., 2020) and hybrid grouper (Wu et al., 2022), implying the evolutionary and functional conservation of the Elovl8b among various teleosts.

Phylogenetic analysis showed that the elovl8 gene family was clustered into two branches of elovl8a and elovl8b subfamilies, which was consistent with the findings in previous studies, suggesting that teleost-specific elovl8 family may commonly contain two paralogs in teleost genomes, and this phenomenon may be caused by an additional teleost-specific genome duplication (TSGD) event (Kuraku and Meyer, 2009; Sun et al., 2021). Genetic synteny further supported teleost lineage, which usually contains elovl8a and elovl8b isoforms, such as fishes that belong to the orders Cyprinodontiformes, Salmoniformes, and Osmeriformes. However, it seemed that the elovl8a gene had been lost in Siluriformes teleosts, including C. gariepinus, P. hypophthalmus, I. punctatus, and P. fulvidraco, suggesting that elovl8b should be more conserved than elovl8a, and thus, the former but not the latter may play important roles in HUFA biosynthesis (Li et al., 2020). To our knowledge, the elovl8a gene lost in some teleost genomes may be caused by functional redundancy between elovl8a and elovl8b isoforms in the early period of evolution. In addition, the structure of the elovl8b gene commonly contained eight exons and seven introns in teleosts, suggesting that this gene was highly conserved and might play similar functions among various teleosts.

Tissue distribution experiments showed that elovl8b was widely expressed in various tissues with high expression levels in liver and intestine, which was different with rabbitfish and hybrid grouper (Li et al., 2020; Wu et al., 2022), indicating that the tissue distribution pattern of elovl8b is species-specific and it may play diverse roles in various teleosts. Indeed, this phenomenon may be caused by the different ability to synthesize HUFAs in different fishes (Gong et al., 2014). Notably, the elovl8b was highly expressed in liver in most teleosts, suggesting that this gene may also play similar roles in HUFA biosynthesis because liver is the major organ that is involved in HUFA production.

The gene expression pattern of the elovl8b gene during the early development stage of yellow catfish was also determined, and results showed that elovl8b was stably expressed within 12 h after fertilization, which was slightly different with other HUFA biosynthesis-related genes in rabbitfish (You et al., 2017), suggesting that HUFA biosynthesis is very important for the early development of embryo despite the fact that the pattern may be different among various species. However, the expression level of yellow catfish elovl8 was significantly decreased from 24 h to 96 h. It has been shown that during the early stage of embryonic development in yellow catfish, the PUFAs in yolk are involved in metabolism and catabolism to provide energy preparation for embryonic development, with C18:2n-6, C20:4n-3, and DHA dominating (Yao et al., 2009). Interestingly, the elovl8b gene was shown to have the ability to prolong C18 (18:2n-6, 18:3n-3, and 18:4n-3) and C20 (20:4n-6 and 20:5n-3) HUFAs to long-chain polyunsaturated fatty acids (LC-HUFAs) in rabbitfish (Li et al., 2020). This expression pattern suggests that yellow catfish elovl8 is also involved in the synthesis of long-chain polyunsaturated fatty acids and plays an important role in early embryonic development.

Previous studies have shown that diets full of HUFAs could inhibit the transcriptional expression in rabbitfish and hybrid grouper (Li et al., 2020; Wu et al., 2022). In the present study, functional experiments showed that the transcription of elovl8 was not significantly affected by feeding with A. salina, whereas it was inhibited after feeding with egg yolk in yellow catfish. It is shown that the lack of HUFAs in A. salina might explain why dietary A. salina slightly increased elovl8 gene expression (Tocher et al., 2003; Garcia et al., 2008). Meanwhile, dietary egg yolk induced but then inhibited elovl8 expression, which may be caused by the high content of phospholipid in egg yolk. Our findings further supported the idea that a compensatory mechanism also existed in yellow catfish, which was similar to previous reports and could demonstrate a key role for elovl8b in LC-PUFA biosynthesis (Cho et al., 1999; Seiliez et al., 2001).

In summary, an elovl8 gene was identified and characterized from yellow catfish for the first time. Multiple protein sequence alignment, genetic synteny, gene structure comparisons, and phylogeny revealed that elovl8a had been lost in Siluriformes, and elovl8b was highly conserved among various teleosts. Moreover, spatiotemporal expression analyses suggested that elovl8 was widely distributed, and it could play important roles in the early development stage in yellow catfish. Finally, functional experiments showed that a diet full of HUFAs may inhibit the transcription of elovl8 in yellow catfish. Our findings will help us to better understand the evolutionary and functional characteristics of elovl8 in teleosts, and lay a solid basis for investigating the regulation mechanism of HUFA biosynthesis.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material .

The animal study was approved by Neijiang Normal University Animal Care and Use Committee. The study was conducted in accordance with the local legislation and institutional requirements.

W-HZ: Formal Analysis, Investigation, Writing – original draft. X-YW: Writing – original draft, Formal Analysis, Investigation, Software. WQ: Writing – original draft, Investigation. C-JQ: Writing – review & editing. QS: Writing – review & editing. SG: Writing – review & editing, Formal Analysis, Software. PP: Software, Writing – review & editing. SZ: Writing – review & editing, Funding acquisition, Investigation. PF: Writing – review & editing, Formal Analysis, Investigation. WH: Methodology, Supervision, Writing – review & editing. H-WY: Supervision, Writing – review & editing. Z-YW: Funding acquisition, Methodology, Project administration, Supervision, Writing – review & editing.