Abstract
During fertilization, a series of reactions between the eggs and spermatozoa proceed predominantly in a species-specific manner. The molecules mediating these species-specific reactions remain unknown except in a few organisms. In this study, we focused on two species belonging to the phylum Nemertea, Kulikovia alborostrata and K. fulva, and explored molecules involved in species-specific interactions between gametes. Orthologs of molecules known to be involved in species-specific reactions were not expressed in the ovaries of these two species. In contrast, we identified six novel proteins, named NeZPL1-NeZPL6, containing a ZP module. Among these, we found that NeZPL6 is located on the surface of an unfertilized egg and is suggested to be involved in its interaction with spermatozoa. Furthermore, we found an indel of three amino acids in the EGF-like domain of NeZPL6, which possibly confers species specificity to this interaction. Our results suggested the existence of a novel system for species recognition in animal gametes.