208 A. Rose
chromosome regions, the centromers, to separate the condensed chromo-
somes- To achieve the separation of one membrane-bound nucleus into two iden-
tical daughter nuclei, a variety of mechanisms has evolved ranging from
karyokinesis in closed mitosis to complete breakdown and reassembly of
the nuclear envelope in open mitosis
The evolution of both processes appears to be tightly interlocked, with the
nuclear envelope playing crucial roles in mitotic checkpoint control and spin-
dle assembly. This chapter takes a closer look at the evolution of open mitosis
in the “green” lineage and the dynamics and functional roles of the nuclear
envelope in plant cells compared to the other branches of the eukaryotic do-
main.
2
Evolution of the Nuclear Envelope
The exact events leading to the development of the nucleus as an organelle are
subject to controversial discussion. Two basic models for the formation of the
nuclear envelope have been proposed:
- Karyogenic hypothesis: formation of the nuclear envelope through invagi-
nation of the plasma membrane in a proto-eukaryote - Endokaryotic hypothesis: formation of the nucleus (and centrosomes) by
endosymbiosis, with the nuclear envelope a remnant of a phagocytic event
The possibility of analyzing fully sequenced genomes to determine likely
prokaryotic origins of eukaryotic signature proteins has renewed the debate.
While sequence data seems to support the theory of endosymbiosis of an ar-
chaebacterium by a eubacterial host (Horiike et al. 2002) or endosymbiosis
of a number of archaea and bacteria by a postulated “chronocyte” (Hart-
man and Fedorov 2002), the endokaryotic hypothesis remains controversial
(Martin 2005). Another hypothesis argues that not endosymbiosis, but the
formation of a chimera between an archaebacterium and motile eubacteria
led to the development of the karymastigont, a nuclear structure coupled with
a microtubular flagellum that is found in amitochondriate protists (Margulis
et al. 2000, 2006).
A common premise of these hypotheses is the coupling of the emergence
of a nuclear envelope to the presence of an endoskeleton and its co-evolution
with mitosis (Cavalier-Smith 2002; Dolan et al. 2002). In the compartmental-
ization model, the appearance of the nucleus would have been predated by
the evolution of an endoskeleton allowing for motility and vesicle transport
and laying the basis for the development of the mitotic apparatus (Martin
1999). Membrane enclosure of the nucleus might have been triggered by
the need to protect the genetic material from digestive enzymes used dur-