The multi-scale architecture of mammalian sperm flagella and implications for ciliary motility

Motile cilia are molecular machines used by a myriad of eukaryotic cells to swim through fluid environments. However, available molecular structures represent only a handful of cell types, limiting our understanding of how cilia are modified to support motility in diverse media. Here, we use cryo focused ion beam milling-enabled cryo electron tomography to image sperm flagella from three mammalian species. We resolve in-cell structures of centrioles, axonemal doublets, central pair apparatus, and end piece singlets, revealing novel proto filament bridging micro tubule inner proteins throughout the flagellum. We present native structures of the flagellar base, which is crucial for shaping the flagellar beat.We show that outer dense fibers are directly coupled to micro-tubule doublets in the principal piece but not in the mid piece.Thus, mammalian sperm flagella are ornamented across scales,from proto filament bracing structures reinforcing micro tubules at the nano-scale to accessory structures that impose micron-scale asymmetries on the entire assembly. Our structures provide vital foundations for linking molecular structure to ciliary motility and evolution.