Flagellum tapering and midpiece volume in songbird spermatozoa

Sperm mtDNA Copy Number Is Not Associated With Midpiece Size Among Songbirds

Tremendous variation in sperm morphology is observed across the animal kingdom. Within avian taxa, the songbirds (infraorder Passerides) have the largest variation in sperm morphology. Their spermatozoa move by using energy generated in the midpiece, which is formed by multiple mitochondria fusing together during spermatogenesis. However, very little is known regarding the number of mitochondria required to form the songbird midpiece. Based on previous research showing an association of midpiece length and mitochondrial DNA (mtDNA) copy number in the zebra finch Taeniopygia guttata, we hypothesize that songbird species with longer sperm midpieces have more copies of mtDNA. We estimated the sperm mtDNA copy number in 19 species from 10 families within Passerides, covering a broad range of midpiece sizes. Mitochondrial and nuclear DNA abundance were determined using droplet digital PCR (ddPCR) and the ratio between mitochondrial and single-copy nuclear genes was used to estimate mtDNA copy number per spermatozoon. We found that species differ in their average mtDNA copy number, but the variation was small and not significantly related to midpiece length. A possible explanation is that mitochondrial genomes are eliminated in the spermatids during spermatogenesis.

Sperm morphology and performance in relation to postmating prezygotic isolation in two recently diverged passerine species

Divergence in sperm phenotype and female reproductive environment may be a common source of postmating prezygotic (PMPZ) isolation between species. However, compared to other reproductive barriers it has received much less attention. In this study, we examined sperm morphology and velocity in two hybridizing passerine species, the common nightingale (Luscinia megarhynchos) and thrush nightingale (L. luscinia). In addition, we for the first time characterized a passerine female reproductive tract fluid proteome. We demonstrate that spermatozoa of the common nightingale have significantly longer and wider midpiece (proximal part of the flagellum containing mitochondria) and longer tail compared to spermatozoa of thrush nightingale. On the other hand, they have significantly shorter and narrower acrosome. Importantly, these differences did not have any effect on sperm velocity. Furthermore, the fluid from the reproductive tract of common nightingale females did not differentially affect velocity of conspecific and heterospecific sperm. Our results indicate that the observed changes in the flagellum and acrosome size are unlikely to contribute to PMPZ isolation through differential sperm velocity of conspecific and heterospecific sperm in the female reproductive tract. However, they could affect other postcopulatory processes, which might be involved in PMPZ isolation, such as sperm storage, longevity or sperm-egg interaction.

Flagellum tapering and midpiece volume in songbird spermatozoa

In contrast to numerous studies on spermatozoa length, relatively little work focuses on the width of spermatozoa, and particularly the width of the midpiece and flagellum. In flagellated spermatozoa, the flagellum provides forward thrust while energy may be provided via mitochondria in the midpiece and/or through glycolysis along the flagellum itself. Longer flagella may be able to provide greater thrust but may also require stronger structural features and more or larger mitochondria to supply sufficient energy. Here, we use scanning electron microscopy to investigate the ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic families in the Passerides infraorder. Our data confirm the qualitative observation that the flagellum tapers along its length, and we show that longer flagella are wider at the neck. This pattern is similar to mammals, and likely reflects the need for longer cells to be stronger against shearing forces. We further estimate the volume of the mitochondrial helix and show that it correlates well with midpiece length, supporting the use of midpiece length as a proxy for mitochondrial volume, at least in between-species studies where midpiece length is highly variable. These results provide important context for understanding the evolutionary correlations among different sperm cell components and dimensions.