Sporogenesis in eusporangiate ferns: II. Polyplastidic meiosis in Ophioglossum (Ophioglossales)

Structural studies of sporogenesis using combined methods of immunofluoresc ence labelling of microtubules and transmission electron microscopy provide details of meiosis and cytokinesis in Ophioglossum petiolatum. Unlike Angi opteris (Marattiales), which is monoplastidic, Ophidglossum undergoes polyp lastidic melosis like members of the fern-seed plant clade. The meiotic spi ndle is distinctly multipolar in origin and is consolidated into a bipolar spindle that is variously twisted and curved to accommodate the large numbe r of chromosomes. Although a phragmoplast forms after first meiosis, no wal l is deposited. Instead, an organelle band consisting of intermingled plast ids and mitochondria is formed in the equatorial region between the dyad do mains. Following second meiosis, a complex of phragmoplasts forms among sis ter and non-sister nuclei. Cell plates are deposited first between sister n uclei and then in the region of the organelle band resulting in a tetrad of spores each with a equal allotment of organelles.