Ontogenetic development of metabolic and detoxification genes in Seriola rivoliana larvae.

The early larval stages in marine organisms have evolved to possess a genomic response ensuring survival and protection against environmental stressors, defined as the 'chemical defensome'. The larval fish defensome consists of genes transcribed and upregulated after hatching to form detoxification proteins, catalyzing enzymes, transporters, transcription factors, and antioxidant proteins. Therefore, we aimed to analyze the timing of transcription and upregulation of metabolic, protective and detoxification genes during the ontogenetic development in Seriola rivoliana larvae up to the onset of exogenous feeding. Larvae samples from each experimental replicate were taken after hatching (day zero), day one (24 h post-hatching), day two (48 h post-hatching), day three (72 h post-hatching), and day four (96 h post-hatching). We used RNA-seq followed by gene annotation to identify these genes and to evaluate differential gene expression. From day one after hatching, results showed the upregulation of cytochrome P450 and other genes that coded for enzymes capable of bio-transform xenobiotic compounds such as reductases, hydrolases, transferases and dehydrogenases. Additionally, a set of genes of the superfamily of ATP-binding cassette transporters became upregulated for cholesterol homeostasis and many others involved in detoxification and multidrug resistance of xenobiotics. We also analyzed genes involved in stress-responses (transcription factors) and genes that code for antioxidants proteins. However, none of them showed differential expression. Our study suggests an orchestrated and organized ontogenetic sequential upregulation of the defensome as a natural process to adapt to the environment, the regulation process of macronutrients, and a defensive metabolic response in case of encountering harmful xenobiotic compounds or toxins in food and water at the onset of exogenous feeding, which contributes to the understanding of the chemical defensome in early marine larval development.