Comparative sperm proteomics in selected passerine birds reflects sperm morphology and mitochondrial metabolism

Abstract. Spermatozoa are the most variable cells across animal taxa. Phylogeny, speciation and postcopulatory sexual selection are typical factors that explain the sperm morphology variation in animals, and now these differences can also be explored on the level of genomic and proteomic differentiation. However, in non-model organisms, it is often difficult to employ these techniques because genomes are not yet available for most animal species, particularly for free-living songbirds (Passeriformes). Here, we employed label-free proteomics to generate proteomes in the zebra finch, a songbird species with an annotated genome and five wild-living songbirds representing five families within the Passerida clade, all with poorly known genomes. The results show that protein mapping of the new passerine proteomes to the zebra finch genome was successful, thus yielding highly similar protein identifications and a sufficient number of unique peptides in all the studied proteomes. Interestingly, while passerine sperm proteomes only partially reflect phylogenetic relationships between passerine families, midpiece length correlates with at least 59 proteins enriched in mitochondrial metabolism. Similar sperm proteomes seem to have evolved convergently across passerine lineages, potentially due to varying levels of sperm competition and marked variation in sperm sizes.