Molecular cloning and characterisation of an isotocin paralogue ([V8] isotocin) in catfishes (superorder Ostariophysi): Origin traced likely to the fish-specific whole genome duplication

The present study reports the molecular cloning of a previously uncharacterised neurohypophyseal nonapeptide precursor cDNA in two catfish species: Heteropneustes fossilis and Clarias batrachus. The deduced nonapeptide is CYISNCPVG ([V8] isotocin), which has not been reported in any vertebrate till date. Phylogenetic and conserved synteny analyses showed the gene to have originated from the isotocin precursor (pro-it) gene by fish-specific whole genome duplication (3R). The two isotocin lineages have been designated as pro-ita (new gene) and pro-itb (conventional it gene). All teleost groups may not possess both pro-ita and pro-itb and the pattern of losses/retention was found to be lineage-specific. Quantitative reverse transcriptase-polymerase chain reaction studies showed the expression of the pro-ita gene in the brain and ovary of H. fossilis. In situ hybridisation studies localised the pro-ita transcripts in the nucleus preopticus of the hypothalamus and the follicular layer (theca-granulosa) of oocytes, comprising tissues in which pro-itb and vasotocin precursor (pro-vt) mRNA expression was previously reported. The transcript levels varied with the reproductive stage and a high abundance was found in both brain and ovary during the breeding phase. The substitution of valine in place of isoleucine at the eighth position in Ita may have modified the ligand-receptor interaction, leading to sub-functionalisation and the retention of the gene in catfishes.