Microtubule organization and plastid distribution during meiosis of Haplomitrium mnioides (Haplomitriopsida)

The organization of microtubules and plastid distribution of the liverwort, Haplomitrium mnioides (Haplomitriopsida), was studied during the meiotic phase lasting for six months. In the late fall, the cytoplasm of early sporocytes forms four lobes of future spore domains before meiotic prophase. Microtubules align at the cytoplasmic cleavage furrow regions as girdling bands in the four-lobed sporocytes. Finally, the cleavage furrows are proximal to the nucleus positioned in the center of the sporocyte, and the girdling bands of the microtubule (GBM) disappear. Subsequently, the nucleus moves into one of the cytoplasmic lobes, and sporocytes pass the winter season at this stage. In early spring, the nucleus returns to the central position of the lobed cytoplasm, concurrent with plastid repositioning around the nucleus. Plastids are then distributed equally to each of the four lobes as a plastid cluster. Astral microtubules emanate from the plastid cluster in each spore domain and encage prophase nuclei as a quadripolar microtubule system (QMS). The QMS changes into a twisted spindle of metaphase I with broad poles, while spindles of metaphase II also emanate from the four plastid clusters. Cytokinesis is completed through the centrifugal cell plate formation in telophase II. The division axes of two successive nuclear divisions appear to be determined by plastid-based QMS, and the future site of cytokinesis is marked by cytoplasmic furrows associated with GBM. The phylogenetic distribution of GBM and QMS suggests that the meiotic system involving these structures is an ancestral trait of liverworts. Long-term dormancy in diploid sporocytes rather than haploid spores may represent transitional traits from charophycean green algae to land plants.