Open Mitosis: Nuclear Envelope Dynamics 213
contain a phycoplast where the spindle apparatus collapses and microtubules
align parallel to the plane of cell division in telophase. In contrast, theZygne-
matales,Coleochaetales,andCharalescontain a phragmoplast where the spin-
dle persists into telophase and microtubules align perpendicular to the plane
of cell division (McCourt et al. 2004). These characteristics coincide with the
division of the green algae intoCharophytaand otherChlorophyta.Outside
theCharophyta,onlyafewspeciesofalgaeintheUlvophyceanclade utilize
phragmoplasts as well (Lopez-Bautista et al. 2003).
TheCharophyta, and within this group theCharales(stoneworts), are con-
sidered the closest relatives to the land plants based on their mitotic behavior
as well as phylogeny (Battacharya and Medlin 1998; Karol et al. 2001; Lewis
and McCourt 2004). Morphological and sequence-based data suggest that the
ancestor of the charophytes belonged to the green algae groupPrasinophyceae
(Battacharya and Medlin 1998). Cell division within thePrasinophyceaeis
diverse and includes open and closed mitosis with a persistent spindle and
cleavage furrow (PyramimonasandNephroselmis, respectively) and closed
mitosis with a non-persistent spindle and cell plate formation via phycoplast
(Tetraselmis).
Putting phylogeny and differences and similarities in mitosis together,
the following order of events might have led to the development of the
phragmoplast-type open mitosis of higher plants:
- Spindle persists into telophase (somePrasinophyceae,allCharophyta)
- Nuclear envelope breaks down during mitosis (somePrasinophyceae,all
Charophyta) - Telophase spindle becomes more prominent (allCharophyta)
- Phragmoplast with cell plate forms (someCharophyta)
- Centrioles are lost in higher plants
Schematic examples of proposed stages along the development of higher plant
type open mitosis are shown in Fig. 3.
3
Role of Nuclear Envelope Components in Mitosis and Meiosis
The nuclear envelope plays a role in several processes during cell division,
such as:
- Chromosome condensation
- Telomere attachment and homologous chromosome synapsis in meiois
- Spindle assembly and orientation
- Kinetochore assembly and function
- Mitotic checkpoint control
- Cell plate formation in higher plants