Flagellum tapering and midpiece volume in songbird spermatozoa

In contrast to numerous studies on spermatozoa length, relatively little
work focuses on the width of spermatozoa, particularly the width of the
midpiece and flagellum. In flagellated spermatozoa, the flagellum provides
forward thrust while energy may be provided via mitochondria in the
midpiece and/or through glycolysis along the flagellum itself. Longer
flagella may be able to provide greater thrust but may also require
stronger structural features and more or larger mitochondria to supply
sufficient energy. Here we use SEM imaging to investigate the
ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic
families in the Passerides infraorder. Our data confirm the qualitative
observation that the flagellum tapers along its length, and we show that
longer flagella are wider at the neck. This pattern is similar to mammals,
and likely reflects the need for longer cells to be stronger against
shearing forces. We further estimate the volume of the mitochondrial helix
and show that it correlates well with midpiece length, supporting the use
of midpiece length as a proxy for mitochondrial volume, at least in
between-species studies where midpiece length is highly variable. These
results provide important context for understanding the evolutionary
correlations among different sperm cell components and dimensions.