Abstract
Few animals are known to lay eggs in the absence of ovulation or copulation, as it is presumably energetically wasteful and subjected to negative selection. Characterization of Smed-boule, a member of the DAZ family of germline RNA-binding proteins, revealed that egg capsule (or capsule) production and deposition occurs independently of the presence of gametes in the planarian flatworm Schmidtea mediterranea. Reduction of Smed-boule expression by RNA-interference (RNAi) causes ablation of spermatogonial stem cells and the inability of ovarian germline stem cells to undergo oogenesis. Although animals subjected to Smed-boule RNAi lose their gametes and become sterile, they continue to lay egg capsules. Production of sterile capsules is even observed in virgin Smed-boule(RNAi) and control planarians maintained in complete isolation, demonstrating that egg production in S. mediterranea occurs independently of ovulation, fertilization, or mating. Evidence suggests that this is a conserved feature amongst Platyhelminthes, and therefore relevant to the pathology and dissemination of parasitic flatworms. These findings demonstrate that Smed-boule functions at different stages during male and female germline stem cell development, and also demonstrate that egg capsule production by planarian flatworms occurs independently of signals produced by mating or ova.
Highlights
The characterization of developmental processes involved in sexual reproduction has important implications towards reproductive medicine, stockbreeding, farming, and for controlling the dissemination of infectious disease
Production of sterile capsules is even observed in virgin Smed-boule(RNAi) and control planarians maintained in complete isolation, demonstrating that egg production in S. mediterranea occurs independently of ovulation, fertilization, or mating. Evidence suggests that this is a conserved feature amongst Platyhelminthes, and relevant to the pathology and dissemination of parasitic flatworms. These findings demonstrate that Smed-boule functions at different stages during male and female germline stem cell development, and demonstrate that egg capsule production by planarian flatworms occurs independently of signals produced by mating or ova
Our work shows that production and deposition of egg capsules by planarian flatworms does not require fertilization, mating, ovulation, or even the existence of gametes
Summary
The characterization of developmental processes involved in sexual reproduction has important implications towards reproductive medicine, stockbreeding, farming, and for controlling the dissemination of infectious disease. Conserved molecular processes involved in metazoan germline development have been identified through decades of research using model organisms. Post-transcriptional regulation of gene expression by conserved germline-specific RNA-binding proteins is one of the conserved molecular processes that ensure development of gametes [1,2,3]. Some parasitic flatworms (i.e. schistosomes or blood flukes) have complex life cycles that involve dioecious and asexual reproductive phases during transitions between vertebrate and invertebrate hosts, respectively [13]. Since the complex life cycle of schistosomes complicates husbandry and experimentation in laboratory settings, researchers have begun to use planarian flatworms as a model to dissect the molecular mechanisms behind the extensive lifespan and reproduction of their parasitic cousins [14]. One aspect of particular interest is the continuous production of thousands of eggs that both facilitate dissemination and sustain the pathology of schistosomes by populating organs of their host [13,14]
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