Regulation of Male and Female Gametogenesis by Two Distinct CPEB Paralogs in Planaria

The planarian Schmidtea mediterranea reproduces asexually by transverse fission and sexually by hermaphroditic cross-fertilization.  Both modes of reproduction depend on neoblasts, a population of adult pluripotent stem cells distributed throughout the planarian body.  Neoblast differentiate to give rise to missing somatic tissues after amputation or fission, and also give rise to the germline, which makes planarians an excellent model for the study of stem cell and reproductive biology.  In planarians, as in other organisms, germline formation requires a cascade of post-transcriptional regulation events that ultimately result in the formation of eggs and sperm.  An important regulator of mRNA activity, the Cytoplasmic Polyadenylation Element Binding protein (CPEB), has two homologs in S. mediterranea: SmedCPEB-1 and SmedCPEB-2.  SmedCPEB-1 expression is detected by in situ hybridization in ovaries, whereas SmedCPEB-2 is detected in the brain and testes. Disruption of expression by RNAi of either CPEB paralog leads to infertility.  Analysis by confocal microscopy revealed that SmedCPEB-1(RNAi) animals fail to complete oogenesis.  By contrast, SmedCPEB-2 RNAi results in a block in spermatogenesis, leading to the accumulation of arrested spermatogonia. The separate oogenesis and spermatogenesis defects of CPEB paralogs were used to identify hundreds of genes potentially involved in sex-specific gamete development and function. These results further our knowledge of the molecular mechanisms that take place to ensure proper gamete development and function, as well as promote medical efforts towards the treatment of infertility and development of contraceptives.