Sex-specific differential gene expression during stolonization in the branching syllid Ramisyllis kingghidorahi (Annelida, Syllidae).

Background: Ramisyllis kingghidorahi (Annelida, Syllidae) is one of few annelid species with a ramified body, one anterior end and hundreds of posterior ends. R. kingghidorahi belongs to the family Syllidae, whose members reproduce by forming stolons, small autonomous reproductive units, at the posterior end. Molecular mechanisms controlling sexual reproduction are still poorly understood, but previous studies support an important role of the anterior end and stolons. The roles of different body regions during sexual reproduction in a complex branched body where there is only one head but multiple posterior ends, which develop hundreds of simultaneous stolons, have never been investigated. Consequently, we aimed to research the transcriptomic basis of sexual maturation and stolonization in R. kingghidorahi by performing differential gene expression analyses.

Results: Transcriptomes were assembled from different body regions (anterior end, midbody, and stolons) of male, female, and non-reproductive individuals. Comparative analyses revealed that body region had a greater impact on gene expression profiles than sex, with the anterior end and stolons showing extensive gene upregulation. Across-sex comparisons revealed sex-specific processes in all body regions, with stolons exhibiting the most differences in differential expression, likely related to gametogenesis and external sexual dimorphism. Fewer genes than expected were differentially expressed in the anterior region, a result for which different possible explanations are discussed. Surprisingly, key genes typically associated with segmentation and metamorphosis, such as Wnt and Hox, showed little differential expression, aligning with recent findings that stolon segments lack a specific segment identity.

Conclusions: This study presents the first transcriptomic data for a branched annelid species and offers new insights into the complex genetic regulation of reproduction in R. kingghidorahi. Additionally, it provides the first glimpse into the mechanisms of sexual maturation in branched syllids, which must coordinate stolonization across multiple posterior ends. These findings enhance our understanding of annelid reproductive biology and highlight the need for further research to uncover the physiological and molecular pathways regulating sexual maturation and stolonization in syllids and other annelids.