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.
Highlights
In 1970, Parker [1] introduced the idea of sperm competition, namely, that when multiple males copulate with a female in one reproductive bout, their sperm compete to achieve fertilization
If total sperm length or the flagellum:head ratio directly influenced sperm swimming speed through hydrodynamic effects, these morphological variables should be consistently correlated with swimming speed across species and studies
Additional insights may be obtained by correlating the speed and morphology of individual sperm cells [9], overall, empirical studies in passerines do not provide support for sperm morphology–motility relationships predicted from hydrodynamic models of mammalian-like sperm [15,17]
Summary
In 1970, Parker [1] introduced the idea of sperm competition, namely, that when multiple males copulate with a female in one reproductive bout, their sperm compete to achieve fertilization. Models suggest that swimming performance should be more directly impacted by the relative length of the flagellum (which produces forward thrust in the models by beating back-andforth or in a helical motion) compared to the head (which is thought to only produce drag, proportional to its surface area, [15]) This hypothesis is based on a large body of literature on hydrodynamic models that recognize that, at the scale and speed of sperm cells, viscosity has a vastly greater effect than inertia, such that a cell almost instantly stops making forward progress when it ceases actively swimming [15,16,17]. Several comparative [18,19] and intraspecific [20,21] studies support the hypothesis that a higher flagellum:head ratio increases swimming speed, contradictory patterns have been found [11,22]
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