Evolution of apolar sporocytes in marchantialean liverworts: implications from molecular phylogeny

In meiosis of basal land plants, meiotic division planes are typically predicted by quadri-lobing of the cytoplasm and/or quadri-partitioning of plastids prior to nuclear divisions. However, sporocytes of several marchantialean liverworts display no indication of premeiotic establishment of quadripolarity, as is observed in flowering plants. In these cases, the shape of sporocytes remains spherical or elliptical and numerous plastids are distributed randomly in the cytoplasm during meiosis. Through a survey of sporocyte morphology in marchantialean liverworts, we newly report the occurrence of apolar sporocytes in Sauteria japonica and Athalamia nana (Cleveaceae; Marchantiales). Molecular phylogenetic analyses revealed that the quadri-lobing of cytoplasm and quadri-partitioning of plastids were lost independently several times during the evolution of marchantialean liverworts. In addition, our phylogenetic analyses indicate that the simplified sporophytes of several marchantialean liverworts are not a primitive condition but rather represent the result of reductive evolution. The loss of the quadripolarity of sporocytes appears to correlate with the evolutionary trend of the sporophyte towards reductions. Through the evolution of the simplified sporophytes, suppression of mitotic divisions of sporogenous cells might had caused not only the modification of sporophyte ontogeny but also the drastic cytological change of sporocyte.