Longer Sperm Swim More Slowly in the Canary Islands Chiffchaff

Abstract

Sperm swimming performance affects male fertilization success, particularly in species with high sperm competition. Understanding how sperm morphology impacts swimming performance is therefore important. Sperm swimming speed is hypothesized to increase with total sperm length, relative flagellum length (with the flagellum generating forward thrust), and relative midpiece length (as the midpiece contains the mitochondria). We tested these hypotheses and tested for divergence in sperm traits in five island populations of Canary Islands chiffchaff (Phylloscopus canariensis). We confirmed incipient mitochondrial DNA differentiation between Gran Canaria and the other islands. Sperm swimming speed correlated negatively with total sperm length, did not correlate with relative flagellum length, and correlated negatively with relative midpiece length (for Gran Canaria only). The proportion of motile cells increased with relative flagellum length on Gran Canaria only. Sperm morphology was similar across islands. We thus add to a growing number of studies on passerine birds that do not support sperm morphology–swimming speed hypotheses. We suggest that the swimming mechanics of passerine sperm are sufficiently different from mammalian sperm that predictions from mammalian hydrodynamic models should no longer be applied for this taxon. While both sperm morphology and sperm swimming speed are likely under selection in passerines, the relationship between them requires further elucidation.

Biophysical Determinants and Constraints on Sperm Swimming Velocity

“…In our analysis of we found two key outcomes. We did not find a positive relationship between total sperm length and VCL, again emphasizing the general the mix of studies where total sperm length and sperm velocity do [ 16 , 61 ] or do not relate to each other [ 19 , 20 , 21 ]. Sperm length in each of our clusters was driven almost entirely by flagellum length ( Supplementary Figure S1 ), which could suggest that this is a key component of sperm speed.…”

“…In turn, it is often assumed that selection for faster swimming speeds has driven an increase in length of sperm and many studies across [ 14 , 16 ] and within species [ 17 , 18 ] have shown a positive length-speed relationship. Nonetheless, within species the relationship between sperm length and velocity is not consistent (e.g., [ 19 , 20 , 21 ]) with several key factors thought to alter the relationship. Individual male quality [ 22 ] or reproductive tactics [ 23 ] can lead to changes in sperm speed without changes in morphology.…”

“…Despite theoretical and empirical efforts, how sperm competition affects sperm swimming speed remains controversial [ 19 , 41 ]. One key future aim would be to demonstrate general principles governing sperm length-swimming speed relationships that allow testable predictions across and within species [ 42 ].…”

Assisted Reproduction Techniques to Improve Reproduction in a Non-Model Species: The Case of the Arabian Bustard (Ardeotis arabs) Conservation Breeding Program

“…Intra-individual variations in sperm size have been presented as a potential consequence of reduced sperm competition, therefore, relaxing selection on optimal sperm length [ 49 , 50 ]. Although it is typically believed that longer sperm swim faster, a study on the Canary Islands’ chiffchaff ( Phylloscopus canariensis ) found that longer sperm were slower, therefore not supporting the sperm morphology–swimming speed hypotheses presented by other groups where longer sperm were faster swimmers [ 51 ]. Thus, the relationship between sperm morphology and sperm motility should be investigated further with the Arabian bustard.…”

A sex chromosome inversion is associated with copy number variation of mitochondrial DNA in zebra finch sperm

“…8: 211025 which is not required in birds [69]. (iii) With its helical shape, Passerine sperm is morphologically distinct from mammalian sperm, moves differently by spinning around its long axis with little bending of the tail [30] and thus probably follows different hydrodynamic models [70]. (iv) In birds, the females' sperm storage tubules provide an antioxidant environment that shields sperm from ROS-induced damage for prolonged periods of time (at least a week and up to several months), which is considerably longer than in mammals [33].…”