The two mitofusin genes and the canonical fusion machinery are essential for the complete fusion of the Drosophila spermatid mitochondria

Mitochondria constantly undergo fission and fusion. Mitochondrial fusion involves fusion of both inner and outer membranes, mediated by two conserved GTPases (Mitofusin for the outer membrane fusion and Opa1 for the inner). Whereas mitochondrial fusion usually forms a tubular network in most cell types, in the Drosophila round spermatids, all the cell mitochondria aggregate and fuse to form a giant oval mitochondrion called the Nebenkern. Both the significance of this mitochondrial formation for sperm development and function, and the molecular mechanisms underlying this phenomenon are still unknown. Although the first mitochondrial fusion gene identified in any organism was identified in this system (male germ cell-specific Mitofusin Fuzzy onions (Fzo)), we still don’t know how two Mitofusins promotes such distinct mitochondrial formations in different cell types.

To address this question, we carried out a genetic screen for genes involved in Nebenkern formation. We systematically knocked down genes encoding mitochondrially targeted proteins and cytoskeleton-related proteins and identified 4 genes essential for Nebenkern formation: 2 genes homologous to mammalian proteins involved in Opa1 processing, and the other 2 members of the Heat Shock protein family.

Our screen and analysis of known components in the mitochondrial fusion machinery, revealed that the canonical fusion machinery is essential for Nebenkern formation. Thus, distinctions between the two morphological modes of mitochondrial fusions in the soma and the testis may lay in other factors, such as factors responsible for the aggregation of the mitochondria and/or differences in the distribution of the fusion proteins on the mitochondrial membrane surface.