COMPARATIVE ASPECTS OF FMRFAMIDE GENE ORGANIZATION IN MOLLUSKS

In pulmonate molluscs two precursors for FMRFamide-related peptides (F aRPs) are generated from a single gene by alternative splicing. One pr ecursor gives rise to FMRFamide and FLRFamide and a few variants: the tetra-FaRPs. A second precursor gives rise to heptapeptides of the for m XDPFLRFamide and variants: the hepta-FaRPs. The tetra-FaRPs and hept a-FaRPs have mutually exclusive cellular localizations and have some d iffering actions on neurons and muscles. Hepta-FaRPs have been found i n all pulmonates examined, even the most primitive species, but only i n pulmonates and not other gastropods. A decapeptide FaRP isolated fro m the mussel Mytilus-ALAGDHFFRFamide-is one of the more hepta-FaRP-lik e peptides isolated from a non-pulmonate, and thus is a candidate homo log of the hepta-FaRPs. But we show that this decapeptide is encoded o n the same exon as FMRFamide in the mussel Geukensia, and thus is not a real homolog of the hepta-FaRPs. Since the one and only FaRP precurs or known from the opisthobranch Aplysia has a splice junction at a loc ation similar to that of the tetra-FaRP precursor of pulmonates, we sp eculated that an alternatively-spliced form of the FMRFamide gene exis ts in this species too. One candidate was the precursor for a group of peptides ending in LFRFamide, but we found that this precursor has no significant sequence in common with the FMRFamide precursor. Thus, th ere remains a dichotomy between pulmonates and all other molluscs with regard to their FMRFamide genes and FMRFamide-related peptides.