Milton assembles large mitochondrial clusters, mitoballs, to sustain spermatogenesis.

Mitochondria are dynamic organelles that undergo frequent remodeling to accommodate developmental needs. Here, we describe a striking organization of mitochondria into a large ball-like structure adjacent to the nucleus in premeiotic Drosophila melanogaster spermatocytes, which we term "mitoball". Mitoballs are transient structures that colocalize with the endoplasmic reticulum, Golgi bodies, and the fusome. We observed similar premeiotic mitochondrial clusters in a wide range of insect species, including mosquitos and cockroaches. Through a genetic screen, we identified that Milton, an adaptor protein that links mitochondria to microtubule-based motors, mediates mitoball formation. Flies lacking a 54 amino acid region in the C terminus of Milton completely lacked mitoballs, had swollen mitochondria in their spermatocytes, and showed reduced male fertility. We suggest that the premeiotic mitochondrial clustering is a conserved feature of insect spermatogenesis that supports sperm development.

Mitochondrial heteroplasmy beyond the oocyte bottleneck.

Inheritance of the mitochondrial genome does not follow the rules of conventional Mendelian genetics. The mitochondrial DNA (mtDNA) is present in many copies per cell and is inherited through the maternal germline. In addition, mutations in the mtDNA will give rise to heteroplasmy, the coexistence of different mtDNA variants within a single cell, whose levels can vary considerably between cells, organs or organisms. The inheritance and subsequent accumulation of deleterious variants are the cause of severe progressive mitochondrial disorders and play a role in many other conditions, including aging, cancer and neurodegenerative disorders. Here, we discuss the processes that give rise to cell-to-cell variability in mtDNA composition, focussing on somatic mtDNA segregation and on less conventional sources of heteroplasmy: non-maternal inheritance and mtDNA recombination. Understanding how mtDNA variants and mutations emerge and evolve within an organism is of crucial importance to prevent and cure mitochondrial disease and can potentially impact more common aging-associated conditions.