Why and how paternal mitochondrial DNA gets cut out of the inheritance

Mitochondrial DNA (mtDNA) is inherited maternally across animals, yet the evolutionary rationale behind this unusual mode of inheritance remains a longstanding mystery. Understanding the processes that prevent the transmission of paternal mtDNA and thus ensure maternal-only inheritance is crucial to uncovering the evolutionary significance of this widespread phenomenon. Historically, research has focused on mechanisms that act within eggs to destroy sperm mitochondria via autophagy and the ubiquitin-proteasome degradation system. However, recent discoveries across multiple animal species, including humans, reveal a surprising twist: paternal mtDNA is actively degraded within mitochondria independently of and prior to the complete breakdown of the organelle itself, often even prior to fertilization. Only a few studies have begun to illuminate the molecular machinery responsible for this early mtDNA elimination. In this review, we explore the emerging landscape of paternal mtDNA elimination mechanisms across species, highlighting newly discovered pathways, evolutionary implications, and open questions that are furthering our understanding of mitochondrial inheritance.