This repositioning of MTOCs that generate opposing microtubules in the interzone also occurs after mitosis in higher plants (Brown & Lemmon, 2007).
All three types of MTOCs (POs, P-MTOCs, and NE-MTOCs) have been discovered in meiosis of bryophytes (Brown & Lemmon, 2007; Brown et al.
The four plastids serve as MTOCs and cones of microtubules emanating from them interact to form a quadripolar microtubule system (QMS) (Fig.
All forms of bryophyte MTOCs are known to organize the quadripolar spindles in sporogenesis.
The quadripolar spindle forms from four cones of microtubules emanating from MTOCs located in the four spore domains (Fig.
When the resulting four plastids reach the poles, they serve as MTOCs for the QMS which consists of distinct spindle-like BMAs interconnecting the four plastids in tetrahedral arrangement (Fig.
Tips of the two plastids are at the vertices of a tetrahedron and serve as the MTOCs that nucleate microtubules of the QMS, but the plastids do not actually complete division until metaphase I (Busby & Gunning, 1988a, b).
In most cases, there is no evidence of a discrete structure associated with the poles, but in the bryopsid moss Rhynchostegium serrulatum, spindle microtubules appear to be focused at electron dense clusters (Brown and Lemmon, 1982a) that are thought to constitute the MTOCs.
Although the plastid MTOCs control quadripolarity and generate the spindles, the behavior of [gamma]-tubulin includes a series of precisely programmed migrations away from the plastids during meiosis I and back to the plastids in meiosis II, followed by a condensation and dramatic migration to the site of aperture development in the four spores following cytokinesis.
The plastids, not yet in tetrahedral position, are MTOCs as evidenced by the presence of [gamma]-tubulin (Fig.
The plastids divide and the four resulting plastids serve as MTOCs, from which cones of microtubules forma QMS surrounding the nucleus (Fig.