A Tale of No Tail: Sperm Flagella Loss in Mormyrids

This project investigates genetic and evolutionary factors behind the loss of conserved traits in weakly electric fishes, specifically mormyrids. Mormyrids are a group of over 200 species known for unique traits, including the ability to generate electric signals and the complete absence of flagella in their sperm. The research team recently discovered a partial deletion mutation in a gene that is essential for sperm flagella formation in mormyrids. The project uses gene editing in zebrafish to test whether this single mutation results in the loss of flagella in fishes, and investigates a newly discovered unique morphological trait of mormyrid eggs that may facilitate fertilization when flagella are absent. In addition, several hypotheses about the evolutionary causes of flagella loss in mormyrid sperm will be tested, providing a link between genetic mutations and the evolutionary forces that operate on them. By revealing effects of the mutation on other functional systems dependent upon cilia and flagella, the research may also provide novel insight into human ciliary disease. The project promotes teaching and training by integrating research outcomes into an undergraduate course and through training of a post-doctoral fellow, two graduate students, and several undergraduate student researchers. The objective of the project is to understand the genetic basis of the loss of flagella in sperm in weakly electric mormyrid fishes, a group of over 200 species known for their numerous derived and novel traits, including the evolution of electrogenesis, electroreception, and extreme encephalization. Interestingly, mormyrids are also the only known vertebrate group to completely lack flagella on their sperm. The research focuses on a highly conserved cilia- and flagella-related protein which plays a crucial role in the formation of sperm flagella and ciliary motility; the gene coding for this protein was recently discovered to have a partial deletion within mormyrids. By using CRISPR/Cas9 gene editing to create mutant zebrafish, the investigators will determine whether this deletion leads to flagellar loss, and what pleiotropic consequences of this mutation result for other ciliated cell types. Additionally, the project will examine sperm-egg interaction in mormyrids, test for evidence of sperm competition, and assess the hypothesis that sperm flagellar loss is driven by selection for energetically efficient testes. The study of mormyrids will provide important insights into the evolution of a unique vertebrate mating system and may also have implications for novel therapeutic interventions for primary ciliary dyskinesia in humans. In addition to educational and training impacts, the project also supports continuation of the EFISH Genomics Portal, a resource for the scientific community studying electric fishes. This project was co-funded by the BIO-IOS Physiological Mechanisms and Biomechanics Program and the Animal Developmental Mechanisms Program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.