Genes that act to destroy mitochondrial DNA in spermatids and enforce maternal only inheritance. Steven Z. DeLuca, Patrick H. O'Farrell. Dept. of Biochemistry, UCSF, San Francisco, CA.

   It is widely appreciated that mitochondrial mutations and the mitochondrial genome itself (mtDNA) show maternal inheritance in metazoans. It is, however, not known why inheritance is restricted in this way. By following the male mtDNA in crosses, we found that it disappeared from within mitochondria during spermatogenesis. We summarize this finding and present results showing that the endonuclease, EndoG, and the mitochondrial DNA polymerase have roles in the disappearance of mtDNA. We followed male mtDNA by a polymorphism that is distinguishable by and quantifiable by PCR. None was found in recently fertilized eggs or in the mature sperm resident in the sperm storage organ of mated females. Using both cytological staining and GFP-mtSSB, we visualized mtDNA as numerous bright nucleoids that abruptly vanished from the huge mitochondria of late elongating spermatids. We have identified the mitochondrial endonuclease, EndoG, as being partially required at the late elongation stage for mtDNA elimination. In EndoG mutants, nucleoids declined in number more slowly, and the individualization complex later swept the remaining nucleoids out of the sperm. Expression of two independent RNAis during spermatogenesis revealed that knockdown of Tamas, the large sub-unit of the mitochondrial DNA polymerase, greatly stabilized mtDNA so that some of it persisted to mature sperm. Tamas includes a nuclease domain involved in proofreading, which we hypothesize contributes importantly to elimination of mtDNA during spermatogenesis. Removal of both EndoG and Pol- function resulted in even greater persistence of mtDNA in the mitochondria of developing spermatids, but the combination rendered males sterile. These findings suggest that a sophisticated program eliminates mtDNA during spermatogenesis and that this may be required for fertility as well as ensuring female only inheritance of mtDNA.