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.

Keywords: Macaronesia; chiffchaff species complex; sperm morphology; sperm motility; sperm velocity.

Figure 1

Relationship between sperm morphology and swimming performance for Canary Islands chiffchaffs. (A) Total sperm length (µm) versus sperm swimming speed, (B) flagellum:head ratio versus sperm swimming speed, (C) midpiece:total sperm length ratio versus swimming speed, (D) flagellum:head ratio versus the proportion of motile cells. Average values per male are shown, though statistics used curvilinear velocity (VCL, µm/s) and motility data from individual cells. Islands of sampling: El Hierro: solid line, n = 21 males. Gran Canaria: dashed line, n = 13 males. La Gomera: dotted, n = 9 males. La Palma: dot, single dash, n = 22 males. Tenerife: dot, several dashes, n = 8 males.

Figure 2

Minimum spanning network of 22 COI haplotypes from 49 individual Canary Islands chiffchaffs sampled in this study, with color indicating the island where the individual was captured. The sizes of the circles are proportional to the haplotype frequencies (smallest circle = 1 individual). Numbers of mutational steps are indicated along the lines connecting haplotypes.