Mitochondrial DNA content of mature spermatozoa and oocytes in the genetic model Drosophila

Although crucial to the success of fertilization and embryogenesis, little is known about the mitochondrial DNA (mtDNA) content of mature spermatozoa and oocytes across taxa and across different fertilization systems. Oocytes are assumed to hold a large population of mtDNAs that populate emerging cells during early embryogenesis, whereas spermatozoa harbor only a limited pool of mtDNAs that is believed to sustain functionality but fails to contribute paternal mtDNA to the zygote. Recent work suggests that mature sperm of the genetic model Drosophila melanogaster lack mtDNA, questioning the significance of zygotic mechanisms for the selective elimination of paternal mtDNA and their necessity for fertilization success. This finding further contradicts previous observations of the inheritance of paternal mtDNA in drosophilids. Using quantitative polymerase chain reaction, we estimate the mtDNA content of several laboratory strains of D. melanogaster and D. simulans to shed light on this discrepancy and to describe the mitochondrial/mtDNA load of gametes within this system. These measurements led to an average estimate of 22.91 +/- 4.61 mtDNA molecules/copies per spermatozoon across both species and to 1.07E+07 +/- 2.71E+06 molecules/copies per oocyte for D. simulans. As a consequence, the ratio of paternal and maternal mtDNA in the zygote was estimated at 1:4.65E+05.