Essential transmembrane GTPase, which mediates mitochondrial fusion during spermatogenesis (PubMed:9230308, PubMed:18799731). In early spermatocytes, fusion of mitochondria give rise to two organelles named Nebenkern and constitutes an important step in mitochondria morphology, which is balanced between fusion and fission (PubMed:9230308, PubMed:18799731). Essential for fertility (PubMed:9230308). GTP + H2O = GDP + H(+) + phosphate Specifically expressed in male germ cells, in spermatocytes and early spermatids. Not expressed in other tissues. Associated with mitochondria in spermatids during a narrow developmental window. Mitochondria align on the spindle equator throughout meiotic divisions, but it is not expressed on mitochondria until the last of meiosis II. In postmeiotic haploid spermatids, it is associated with aggregating mitochondria and highly expressed on onion stage Nebenkerns. Expressed at lower levels associated with early elongation stage mitochondrial derivatives. Not detected on more elongated mitochondria. Defective mitochondrial fusion in spermatids results in onion-staged nebenkerns with irregular borders that contain many small mitochondria (PubMed:18799731). At the leaf-blade stage nebenkerns contain many small mitochondria instead of the two mitochondria derivatives seen in wild-type spermatids (PubMed:18799731). Belongs to the TRAFAC class dynamin-like GTPase superfamily. Dynamin/Fzo/YdjA family. Mitofusin subfamily.