IMMUNOCYTOCHEMICAL EVIDENCE FOR THE INVOLVEMENT OF AN FMRFAMIDE-RELATED PEPTIDE IN EGG-PRODUCTION IN THE FLATWORM PARASITE POLYSTOMA-NEARCTICUM

The monogenean flatworm Polystoma nearcticum exhibits reproductive syn chrony with its treefrog host, Hyla versicolor, and becomes reproducti vely active only during the short period of host sexual activity at sp awning. In this way, it provides a useful model system for exploring f actors that may influence egg production in flatworm parasites. One su ch factor is the peptidergic innervation of the egg chamber or ootype. By using immunocytochemical techniques, the occurrence and distributi on of GYIRFamide-like immunoreactivity, an authentic flatworm FMRFamid e-related peptide (FaRP), have been monitored in the cells and fibres innervating the reproductive apparatus of worms collected at different stages of host sexual activity. Serotonin (5-HT) immunoreactivity in the worm was mapped for comparison. Extensive immunostaining for the F aRP and 5-HT was obtained throughout both the central and the peripher al nervous systems of worms, which were recovered from reproductively active frogs. In contrast, the innervation of the ootype of worms that were determined to be sexually inactive, including those recovered fr om frogs postspawning, showed little or no immunoreactivity for the Fa RP; immunostaining for 5-HT in the ootype was unaffected by the reprod uctive state of the worm. These results indicate that FaRP expression in the neurons of the ootype innervation of P. nearcticum coincides wi th the parasite's brief period of egg production and, thus, provides e vidence that regulatory peptides may be involved in the egg-assembly m echanism in flatworm parasites. (C) 1997 Wiley-Liss, Inc.