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

Propulsion of sperm cells via movement of the flagellum is of vital importance for successful fertilization. Presumably, the energy for this movement comes from the mitochondria in the sperm midpiece. Larger midpieces may contain more mitochondria, which should enhance the energetic capacity and hence promote mobility. Due to an inversion polymorphism on their sex chromosome TguZ , zebra finches ( Taeniopygia guttata castanotis ) exhibit large within-species variation in sperm midpiece length, and those sperm with the longest midpieces swim the fastest. Here, we test through quantitative real-time PCR in zebra finch ejaculates whether the inversion genotype has an effect on the copy number of mitochondrial DNA. Taking the inversion genotype as a proxy for midpiece length, we find that zebra finches with longer midpieces indeed have more copies of the mitochondrial DNA in their ejaculates than those with shorter midpieces, with potential downstream effects on the rate of ATP production and sperm swimming speed. This study sheds light on the proximate cause of a fitness-relevant genetic polymorphism, suggesting the involvement of central components of gamete energy metabolism. Data availability Supplementary data file