Sperm structure and sperm motility of the African and Rockhopper penguins with special reference to multiple axonemes of the flagellum

Abstract

This study evaluated the semen of two penguin species from separate genera with reference to unique features in sperm structure using light microscopy and transmission electron microscopy. Ejaculates from African penguin (n = 51) and Rockhopper penguin (n = 9) contained on average more than 60% motile spermatozoa and a sperm concentration of 3274 × 106/ml and 1423 × 106/ml, respectively. The percentage progressive motility was similar for the two species as well as all the kinematics parameters. The sperm morphology of these two penguin species is almost identical and largely resembles that of non-passerine birds in terms of the filiform head, small acrosome and mid-piece containing 13 spherical mitochondria, arranged around the proximal and distal centrioles in a single helix. Apart from a shorter mid-piece, penguin sperm morphometrics were similar to other non-passerine birds. The ultrastructure of the sperm principal piece revealed the typical 9 + 2 microtubular arrangement without any outer dense fibres. An unusual feature in both African and Rockhopper penguin spermatozoa was the occurrence of multiple axonemes contained in one plasmalemma in 4% of spermatozoa. These double, triple and quadruple axonemal arrangements have not been described previously albeit multiple tails were reported in other bird species. It is unclear whether such a unique structural feature will be of any advantage for sperm motility and might rather be a result of the absence of sperm competition. Multiple axonemes found in penguin flagella could be an apomorphism that distinguish them from other bird spermatozoa.