Global transcriptome analysis identifies regulated transcripts and pathways activated during oogenesis and early embryogenesis in atlantic cod

The molecular mechanisms underlying oogenesis and maternally controlled embryogenesis in fish are not fully understood, especially in marine species. Our aim was to study the egg and embryo transcriptome during oogenesis and early embryogenesis in Atlantic cod. Follicles from oogenesis stages (pre-, early-, and late-vitellogenic), ovulated eggs, and two embryonic stages (blastula, gastrula) were collected from broodstock fish and fertilized eggs. Gene expression profiles were measured in a 44 K oligo microarray consisting of 23,000 cod genes. Hundreds of differentially expressed genes (DEGs) were identified in the follicle stages investigated, implicating a continuous accumulation and degradation of polyadenylated transcripts throughout oogenesis. Very few DEGs were identified from ovulated egg to blastula, showing a more stable maternal RNA pool in early embryonic stages. The highest induction of expression was observed between blastula and gastrula, signifying the onset of zygotic transcription. During early vitellogenesis, several of the most upregulated genes are linked to nervous system signaling, suggesting increasing requirements for ovarian synaptic signaling to stimulate the rapid growth of oocytes. Highly upregulated genes during late vitellogenesis are linked to protein processing, fat metabolism, osmoregulation, and arrested meiosis. One of the genes with the highest upregulation in the ovulated egg is involved in oxidative phosphorylation, reflecting increased energy requirements during fertilization and the first rapid cell divisions of early embryogenesis. In conclusion, this study provides a large-scale presentation of the Atlantic cod's maternally controlled transcriptome in ovarian follicles through oogenesis, ovulated eggs, and early embryos.