Gene Transfer (Endosymbiotic)

Gene transfer is the process by which genetic material moves from the symbiont's genome to the host's genome during endosymbiosis. Over hundreds of millions of years, most mitochondrial genes migrated to the host nucleus, where they are expressed under host regulatory control and their protein products are imported back into the mitochondrion. This transfer serves efficiency: nuclear genes are easier to regulate, better protected from oxidative damage, and replicated with higher fidelity. But transfer also deepens dependency. Each migrated gene is a function the symbiont can no longer perform independently, a loss of autonomy that binds it more tightly to the host. The transfer is incremental, mechanical, and effectively irreversible — restoring a transferred gene to the mitochondrial genome would require reverse migration and functional reintegration, events so improbable that they effectively never occur. Gene transfer is the molecular ratchet making endosymbiosis permanent.