IDENTIFICATION OF A RECEPTOR TYROSINE KINASE INVOLVED IN GERM-CELL DIFFERENTIATION IN PLANARIANS

To investigate external signals involved in germ cell differentiation from somatic stem cells, we have tried to identify protein kinases who se expression is regulated during the process of sexualization of asex ual-state planarians. It is known that in planarians germ cells differ entiate from totipotent somatic stem cells called ''neoblasts'' during sexualization. As a first step, we have isolated twelve protein kinas e genes from cDNAs of sexual-state planarians, including three non-rec eptor tyrosine kinases, three receptor-tyrosine kinases and three non- receptor serine/threonine kinases, and then analyzed their expression patterns during sexualization. One of them, the DjPTK1 gene, is specif ically expressed in germ cells of sexual-state planarians. DjPTK1-posi tive cells were also detected in the mesenchymal space during the proc ess of sexualization, and it appears that these cells migrate to the d orsal side and then differentiate into spermatogonia/spermatocytes in testis. Sequence analysis indicated that the DjPTK1 gene encodes a rec eptor protein tyrosine kinase belonging to the FGFR/PDGF family. These results suggest that a receptor tyrosine kinase system may be involve d both at an early stage of germ cell differentiation and in a step of germ cell maturation in planarians. (C) 1998 Academic Press.