Rc. Brown et al., MORPHOGENETIC PLASTID MIGRATION AND MICROTUBULE ARRAYS IN MITOSIS ANDCYTOKINESIS IN THE GREEN-ALGA COLEOCHAETE ORBICULARIS, American journal of botany, 81(2), 1994, pp. 127-133
This study provides data on cell division in Coleochaete orbicularis,
an important taxon in evolutionary theories deriving land plants from
green algae. Vegetative growth in discoid species of Coleochaete resul
ts from marginal cell division in two planes-radial and circumferentia
l. Like many algae and certain of the simple land plants, Coleochaete
is monoplastidic. Prior to mitosis, the single plastid migrates to a p
osition where it will divide and be distributed into the daughter cell
s. Unlike monoplastidic cell division in hornworts, mosses, and lycops
ids; microtubule nucleation is not intimately associated with the plas
tids. Instead, microtubule organization is associated with centriolar
centrosomes throughout the cell cycle, as is common in algae. The cyto
kinetic apparatus lacks preprophase bands of microtubules, but include
s typical phragmoplasts consisting of brushlike arrays of microtubules
on either side of a dark zone. However, the origin and role of phragm
oplasts is unusual. Phragmoplasts appear to develop among microtubules
that emanate from the polar centrosomes rather than from nuclear enve
lopes and/or plastids. The function of phragmoplasts in Coleochaete is
unclear, as the process of cytokinesis is not strictly centrifugal. S
ome infurrowing occurs in radial division, and cytokinesis appears to
be entirely centripetal by infurrowing in circumferential division. Th
e cortical arrays of microtubules differ from those typical of land pl
ants in that they develop as a network in association with centrosomes
after mitosis.