Isolation, characterization, and evolutionary aspects of a cDNA clone encoding multiple neuropeptides involved in the stereotyped egg-laying behavior of the freshwater snail Lymnaea stagnalis

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Abstract
The cerebral neurosecretory caudodorsal cells (CDCs) of the freshwater pulmonate snail Lymnaea stagnalis control egg laying, an event that involves a pattern of stereotyped behaviors. The CDCs synthesize and release multiple peptides, among which is the ovulation hormone (CDCH). It is thought that each peptide controls a specific aspect of the processes involved in egg laying. We isolated and characterized a CDC- specific cDNA clone that encodes the ovulation hormone (CDCH). RNA blot analysis and in situ hybridization experiments demonstrated that the CDCs are the major cell groups in the cerebral ganglia that transcribe the CDCH gene. In addition to CDCH, the 259-amino acid-long CDCH preprohormone contains 11 other predicted peptides. The overall homology of the CDCH preprohormone with the egg-laying hormone (ELH) preprohormones of the marine opisthobranch snails Aplysia californica and A. parvula is very low (29 and 26%, respectively). However, a more detailed comparison revealed a highly differential pattern of conservation of peptide regions. Significant homology was found between the regions containing (1) CDCH and ELH, (2) repeated pentapeptides, (3) alpha-caudodorsal cell peptide and alpha-bag cell peptide, and (4) 2 regions representing as yet unidentified peptides. Insignificant homology was found when comparing regions containing the other predicted peptides. The conserved peptides probably control similar aspects of the egg-laying fixed action patterns in these distantly related gastropod species. The pentapeptide region exhibits the highest level of homology (75%); in addition, an extra pentapeptide has been generated on the CDCH precursor. This indicates a vital function of these peptides in Aplysia, as well as in Lymnaea species.