Sperm utilization and fertility of mitochondrial introgression genotypes in Drosophila. James A. Mossman, David M. Rand. Ecology and Evolutionary Biology, Brown University, Providence, RI.

   We test the hypothesis that mtDNA-Y chromosome interactions affect male fertility. Different mtDNAs have been associated with male infertility in humans and have been linked to variation in OXPHOS activity in sperm (Ruiz-Pesini et al. 2000). Even synonymous mutations in mtDNA have been associated with poor semen quality (Holyoake et al. 2001). Synonymous polymorphism in human and animal mtDNA is extensive and may account for a substantial proportion of variation in male fertility. Mutations in genes affecting spermatogenesis disrupt mitochondrial morphology (Hales and Fuller 1997). Since mitochondria provide the energy to power sperm motility, sperm function provides a stringent testing ground for mitochondrial performance. However, the statistical association between the mtDNA and the Y chromosome in all male offspring of a given mated pair of animals is 100%. Yet there is no means by which a Y and mtDNA can be transmitted to the same offspring (in mammals and insects, barring paternal leakage which is very rare). These rules of transmission dictate that modifier mutations arising in a population that suppress deleterious mtDNA effects cannot be transmitted by males, which may explain the higher incidence of mitochondrial disease in males (Frank and Hurst 1996). The lack of co-transmission between Y and mtDNA means that beneficial interactions are not promoted, and deleterious interactions can accumulate. The Y chromosome also carries important fertility factors in humans (Lahn and Page 1997) and Drosophila (Carvalho et al. 2000). While a single-gene study reported no DNA polymorphism on the Y (Zuroycova and Eanes 1999), a recent survey of other Y-linked genes (Carvalho et al. 2001) have uncovered polymorphism (A. Clark and B. Carvalho, Dros. Res. Conf. 2002). Given the important role that mtDNA and Y chromosomes play in fertility, it is surprising that no experimental manipulation of these markers has been conducted to dissect their relative contribution to animal fertility.