Ontogenetic expression of bile and lipid metabolism genes in Atlantic cod (Gadus morhua L.) larvae in relation to hepatic lipid accumulation

Atlantic cod (Gadus morhua) stores excess dietary lipids as triacylglycerol (TAG) in the liver, butsubstantial accumulation emerges only during the late larval stage, characterized by an increase inhepatic vacuolization from around 4 to 35 % between 45 and 61 days post-hatching (dph). Toinvestigate mechanisms underlying this transition, we examined the ontogeny of bile and lipidmetabolic pathways in cod larvae (2-60 dph) using RNA sequencing of whole larvae and quantitativereal-time PCR of digestive tissues for key genes involved in dietary lipid processing. We furthertested transcriptomic effects of two dietary phospholipid (PL) levels (ca. 7 and 6 % dry matter) withor without bile salt (BS) supplementation (0.04 % dry matter) during co-feeding (17-34 dph) andthrough 60 dph. Ontogenetic expression patterns indicated that hepatic lipid accumulation in codlarvae is not restricted by their capacity for lipid digestion, synthesis or transport. Instead, the onsetof hepatic lipid deposition coincided with pronounced upregulation of dgat2 and plin2 in digestivetissues, with plin2 also increasing sharply in whole-body larvae, underscoring their roles in TAGsynthesis and lipid droplet stabilization. Dietary PL level and BS supplementation elicited onlymodest and inconsistent transcriptional responses, suggesting limited dietary modulation of theexamined pathways under the tested conditions. We propose dgat2 and plin2 as candidate markers ofliver development in cod and emphasize the need for studies linking tissue-specific transcriptomics toprotein function and hepatic lipid storage.