Why are bird sperm screw-shaped?
Hanna Nyborg Støstad, Natural History Museum
The helical, screw-shaped sperm cell of passerine birds is an example of a cell which appears to have evolved rather unique adaptations. Interestingly, the distinctive helical shape varies substantially even within this group, but this variation has rarely been studied in detail.
Using high-resolution scanning electron microscopy (SEM) we explore sperm shape diversity in 36 passerine species, and test whether sperm head morphology is associated with sperm swimming speed or with proportion of sperm with morphological damage. We find that species whose sperm heads have a long acrosome, a short nucleus, and a wide helical membrane (i.e. that look more like a screw or an ice drill bit) have a higher sperm swimming speed than those with a short acrosome, a long nucleus and a less prominent helical membrane (i.e. look more streamlined). Furthermore, we find that the sperm cells that are more screw-shaped and fast-swimming tend to be more fragile (have a higher rate of cell damage) than the straighter, slower-swimming sperm cells. We therefore suggest that the diversity in sperm cell morphology within passerine birds could, at least in part, be related to an evolutionary trade-off between swimming speed and structural integrity.
Our results provide an example of how such evolutionary trade-offs may influence cell morphology, and contributes to improving the understanding of sperm cell evolution in a broader sense.