Polar Organizers in Meiosis of Sphaerocarpos texanus

Whereas POs are the only MTOCs known in liverwort mitosis, their occurrence in meiosis is sporadic. POs are usually associated with meiotic quadripolarity in the Jungermanniopsida where sporocytes are deeply quadrilobed (Fariner, 1895; Brown et al., 1986). In these liverworts and the early divergent marchantiod liverwort Blasia, quadripolarity is initiated by pre-meiotic bands of cortical microtubules that mark the sites of cytoplasmic infurrowing and the eventual cytokinetic planes (Brown & Lemmon, 2006). In prophase, distinct POs develop adjacent to the nuclear envelope in the deeply lobed sporocytes and the nucleus is drawn into a tetrahedron.

Interestingly, one of the most extreme examples of nuclear shaping in marchantioids occurs in association with POs in S. texanus, a species with large, unlobed polyplastidic sporocytes. Sphaerocarpos along with Riella and Geothallus comprise the small order Sphaerocarpales, a distinct and early divergent order. The sporocytes of the Sphaerocarpales are difficult to study by immunofluorescence as they contain abundant lipids and have a resistant wall. Neidhardt (1978) encountered similar difficulties in attempting TEM studies of sporogenesis in the aquatic R. affinis.

In S. texanus, the archesporium differentiates into sporocytes and accompanying nurse cells (Fig. 20a, b). There are no elaters. The small nurse cells are peculiar; they are thought to be nutritive but there is little evidence for this role (Kelley & Doyle, 1975). The nurse cells undergo several rounds of mitosis without cytokinesis (Fig. 20c, d). The sporocytes differentiate into large spherical cells (Fig. 21a-c) with no cytoplasmic evidence of quadripolarity. An extensive RMS (Fig. 21a) emanates from the relatively small nucleus (Fig. 21c). In early prophase I, microtubules (Fig. 21d) emanate from four POs with distinct concentrations of [gamma]-tubulin (Fig. 21e) and the nucleus is drawn into a tetrahedron (Fig. 21f). In spite of this QMS in early prophase, the prometaphase spindle is multipolar in origin (Fig. 21g-i). The [gamma]-tubulin becomes associated with the microtubules that enmesh the nucleus. In this manner transition to the bipolar metaphase I spindle is accomplished. By metaphase I (Fig. 21j-l) the spindle is rod-shaped (Fig. 21j) with [gamma]-tubulin concentrated at the polar regions (Fig. 21k).

As in other marchantioids, microtubules radiate from the telophase nuclei and an interzonal phragmoplast develops (not shown) but no wall is deposited. Meiosis II occurs in the undivided cytoplasm. Microtubules (Fig. 22a) emanating from distinct POs (Fig. 22b) that reform at opposite tips of the fusiform nuclei (Fig. 22c) form extensive astral arrays and outline nuclear envelopes to initiate the prophase II spindles. The second division spindles (Fig. 22d-f) are rod shaped with poles that are more focused than those of first division, [gamma]-Tubulin is distributed along the spindle fibers (Fig. 22e). Following telophase II a phragmoplast complex expands centrifugally and intersporal septae are deposited (Fig. 22g-i). In S. texanus, the spores remain united in permanent tetrads. Extensive RMSs emanate from the spore nuclei and a system of opposing microtubules develops at the spore boundaries (Fig. 22j). It is possible that this system of microtubules...