A comparison between egg trancriptomes of cod and salmon reveals species specific traits in eggs for each species

Fish in use in aquaculture display large variation in gamete biology. To reach better understanding around this issue, this study aims at identifying if “egg life history traits” can be hidden in egg transcriptomes. To pursue this, salmon and cod eggs were selected due to their largely differencing phenotypes (size, robustness, fresh/marine). An oligo microarray analysis was performed on ovulated eggs from cod (~23 000 genes, n=8) and salmon (~44 000 genes, n=7). The arrays were normalized to a similar spectrum for both arrays. Both arrays were re-annotated based on official gene symbol to retrieve an orthologous KEGG annotation, in salmon and cod arrays this represented 14009 and 7437 genes respectively. The probe linked to the highest gene expression for that particular KEGG annotation was used to compare expression between species. Differential expression was calculated for genes that had an annotation with score > 300, resulting in a total of 2354 KEGG annotations (genes) being differently expressed between the species. The most differentially expressed genes in salmon and cod (FD≥2), were used to reveal pathways that were overrepresented in the eggs of each species. This analysis revealed that immune, signal transduction, and excretory related pathways were overrepresented in salmon compared to cod. The most overrepresented pathways in cod were related to regulation of genetic information processing and metabolism. To conclude this analysis clearly point at some distinct transcriptome repertoires for cod and salmon and that these differences may explain some of the species-specific biological features for salmon and cod eggs. 16 samples