…and this was true across all species, and in many of the major vertebrate groups. However, we did notice some variation across clades and between fertilization modes in our analyses, which likely due to complex species-specific selection.

Internal fertilizers also have faster evolving flagella lengths, while external fertilizers have faster evolving sperm heads. When we compared the rates of evolution of each sperm component against one another, we found that the midpiece was the fastest evolving component...

We ended up with 1103 species across vertebrates. Broadly, we found that fertilization mode shapes both the length of the sperm components, and the rate at which they evolve. Internal fertilizing species have longer sperm components than external fertilizers.

…where fertilization occurs may vary in the selective pressures on each component resulting in differences in how morphology evolves between fertilization modes. To test this hypothesis, we used the SpermTree database and focused on the vertebrates in the dataset.

Most sperm are made up of 3 basic parts of the cell: the head containing the nucleus, the midpiece containing the mitochondria, and a flagellum to propel the cell forward. Each part of the cell has a different function, therefore, we predicted that the environment…

Thread time! If you follow me, you probably know that sperm are amazingly diverse cells. This variation is not only in their length (which we’ve examined in a previous paper), but also in the lengths/proportions of the components of the cell.