Exclusion of dysfunctional mitochondria from Balbiani body during early oogenesis of Thermobia.

Oocytes of many invertebrate and vertebrate species contain a characteristic organelle complex known as the Balbiani body (Bb). Until now, three principal functions have been ascribed to this complex: delivery of germ cell determinants and localized RNAs to the vegetal cortex/posterior pole of the oocyte, transport of the mitochondria towards the germ plasm, and participation in the formation of lipid droplets. Here, we present the results of a computer-aided 3D reconstruction of the Bb in the growing oocytes of an insect, Thermobia domestica. Our analyses have shown that, in Thermobia, the central part of each fully developed Bb comprises a single intricate mitochondrial network. This "core" network is surrounded by several isolated bean-shaped mitochondrial units that display lowered membrane potential and clear signs of degeneration. In light of the above results and recent theoretical models of mitochondrial quality control, the role of the Bb is discussed. We suggest that, in addition to the aforementioned functions, the Bb is implicated in the selective elimination of dysfunctional mitochondria during oogenesis.