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

Abstract

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 <i>Drosophila melanogaster</i> 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.