Maternal transfer and transcriptional onset of immune genes during ontogenesis in Atlantic cod

The immune system in teleosts is not completely developed during embryonic and larval stages and immune competence is assumed to be restricted. This study is the first to address whether immune transcripts are maternally transferred to offspring and when immune genes are transcriptionally active in Atlantic cod (Gadus morhua). In unfertilised eggs, transcripts encoding lysozyme and cathelicidin were found indicating maternal transfer of antibacterial transcripts. Lysozyme activity was also present at this stage suggesting the presence of a functional protein. Transcripts of two other putative antibacterial genes (hepcidin and pentraxin) and antiviral genes (ISG15 and LGP2) were absent in unfertilised eggs. The transcriptional onset of these genes occurred during the gastrula period. Transcripts of the heavy chain constant regions of the immunoglobulin (Ig) D, membrane-associated and secreted form of IgM were absent in unfertilised eggs. Transcription of the heavy chain locus commenced at low levels during the segmentation period indicating the onset of B-cell development. Most innate immune genes showed an increase in transcription around hatch and first feeding, indicating a preparation for increased pathogen exposure at this time. Prior to and during metamorphosis all genes showed a pronounced elevation in transcript levels indicating a further maturation of the immune system during this period.