Autophagy regulates organelle reorganization during spermiogenesis in the liverwort Marchantia polymorpha

Sperm mitochondria generally exhibit distinctive and diverse morphologies in animals and plants. Bryophytes, a plant group consisting of liverworts, mosses, and hornworts, produce motile male gametes, called spermatozoids, that possess a fixed number of two mitochondria in their cell bodies. Electron microscopy observations have revealed the detailed morphological aspects of plant spermatozoids, including mitochondrial morphology; however, the mechanism by which mitochondria are reorganized during spermiogenesis in bryophytes remains largely unknown. Our recent study using the liverwort, Marchantia polymorpha, revealed that the mitochondrial number is reduced to one via mitochondrial fission and macroautophagic/autophagic degradation, which subsequently becomes two via asymmetric division to form large anterior and small posterior mitochondria. Other cytoplasmic components, such as peroxisomes, are also degraded via autophagy; however, mitochondria are degraded at a time distinct from other cytoplasmic components. We also found that some cytoplasmic components were degraded in the vacuole independent of autophagy. Our study highlights the dynamic reorganization of organelles via multiple degradation pathways during spermiogenesis in M. polymorpha.