How is the delivery of functional mitochondria across generations achieved? In Drosophila, mitochondria driven by microtubules reach an evolutionarily conserved structure of the egg, the Balbiani body (Bb), which supplies mitochondria to the primordial germ cells (PGCs) of the new individual. In zebrafish, a selective accumulation of mitochondria with high inner membrane potential (Δψm) in the Bb was recently documented: the presence of high Δψm would indicate mitochondrial genome integrity and allow Bb mitochondria to be preferentially transported through the microtubule network and inherited. These are examples of what happens when mitochondria are transmitted through the egg. Mitochondria carried by sperm are commonly not inherited, but spermatozoa have high energy demand for swimming support, and numerous studies on different organisms report that sperm mitochondria have high Δψm.
So, how are mitochondrial activity and segregation linked? Based on what observed across multiple taxa, I propose that Δψm determines which mitochondria reach the PGCs, and how: the more ATP produced, the higher chance to be transported. In animals with an early germ line specification (preformation), the material determining the cell fate is sequestered early on into gonadic presumptive blastomeres along with the most active mitochondria, and it can happen them being sperm mitochondria. This hypothesis is supported by observations in some bivalve species, in which mitochondria are stably transmitted through sperm because of their early delivery into PGCs during embryonic divisions through microtubule dynamics. Instead, when germ line specification happens at a later stage of development (epigenesis), spermatozoon mitochondria would have been already degraded when germ cell precursors form.
In summary, Δψm can be a simple and effective system allowing the most active mitochondria to reach specific locations, but the different timing of action of germ line specification influences the outcome of the segregation mechanism.