Asymmetric male meiosis and its implications in heredity

Meiosis is the process where diploid cells reduce the ploidy to generate haploid gametes. Meiosis in males and females is often regarded as polar opposites: female meiosis produces a large cell (oocyte/egg) with all essential ingredients to start a new life in it, whereas male meiosis produces tiny sperm cells that have stripped down all cytoplasm and only contribute nuclear DNA to the next generation. Female meiosis is inherently asymmetric, producing one oocyte and three polar bodies, the latter of which do not participate in fertilization. In contrast, male meiosis is generally regarded as a symmetric process, where one diploid cell generates four sperm cells. This assumed dichotomy has influenced the framework of how we view the process of inheritance. However, interestingly, a considerable number of species exhibit asymmetric male meiosis, in which spermatogenesis generates fewer than four sperm from a single diploid germ cell, akin to oogenesis, where each germ cell produces only one oocyte. This review attempts to examine the possibility that male meiosis can be asymmetric, even outside the species that exhibit constitutive asymmetric male meiosis. We discuss the possibility that asymmetric male meiosis may provide a foundation for meiotic drive, the phenomenon that causes biased inheritance of alleles and violates Mendel's Law of Equal Segregation.