The sea lamprey shows an important divergence in the proopiomelanocorticotropin (POMC) prohormone in that adrenocorticotropin (ACTH) and melanocyte stimulating hormone (MSH) are encoded on distinct genes, proopiocortin (POC) and proopiomelanotropin (POM), respectively. POC encodes nasohypophysial factor (NHF), ACTH, and Ξ²-endorphin while POM encodes Ξ±and Ξ²-MSH-like peptides (called MSH-B and MSH-A, respectively), and a second form of Ξ²-endorphin. The present study examined the variation in expression of these two genes during the life cycle of sea lampreys.
Individual Northern blot analysis and pooled data from several Northern blots showed significant differential expression of POC and POM during the lamprey life cycle. While POC expression remained low over larval and metamorphic stages, POM expression rose substantially in stage 4 and stage 6 of metamorphosis. Both POC and POM expression levels increased in immediately post-metamorphic animals. The highest levels of POM expression were observed in larger juvenile adults and pre-spawners, whereas the highest level of POC expression was observed at the prespawning period. Statistical analysis showed POM expression to be much higher than POC expression in immediately post-metamorphic animals prior to feeding. Northern analysis of total RNA isolated from various tissues in the pre-spawning lampreys, including brain, showed expression of POC and POM only in the pituitary. These observations indicate that the expression of POC and POM, which encode different sets of hormones, may be related to developmental and maturation events during the life cycle of the sea lamprey.