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Research Projects
Establishing developmental polarity in Silvetia
Colocalization of sperm pronucleus (blue)
and F-actin (pink)
The egg is spherical, radially symmetric and lacks a developmental
axis. We have shown that a developmental axis is acquired at fertilization
with the sperm entry site marking the rhizoid pole of the axis
(see above). (The opposite pole is termed the thallus pole.) This
axis becomes the growth axis along which the young embryo elongates.
The growth axis in the fertilized egg (zygote) is not permanent
and it can be reoriented during early development if the zygote
receives positional information from the environment. If vectorial
information is perceived, the previous axis is abandoned and a
new axis is assembled in accordance with the new information.
The axis can be reoriented several times prior to growth, but
once growth begins the axis is irreversibly determined.
We are studying the cellular parameters that define an axis and
would like to understand the process of axis reorientation (see
Current Model). We have shown that the
axis is immediately marked by a patch of cortical F-actin and
this actin patch reorients when the axis is reoriented. Reorientation
requires a dynamic actin network and we are investigating the
role of actin binding proteins and the Arp2/3 nucleating complex
in regulating the actin arrays. Microtubules and secretion soon become targeted
to the actin patch. We are studying the targeting mechanisms and the hypothesis that microtubules transport exocytotic vesicles to the growth site.
Asymmetric cell division
Spindle double labeled for centrin (yellow)
and microtubules (green)
The first cell division during embryogenesis is an asymmetric,
invariant division oriented transverse to the growth axis. This
division produces daughter cells of different developmental fates,
the larger thallus cell is the progenitor of the stipe (stem)
and fronds (leaves) of the mature alga, and the smaller rhizoid
cell develops primarily into the holdfast that attaches the alga
to the rocks.
We are investigating the process that orients this division and
the mechanics of cytokinesis. Orientation of the division plane
transverse to the growth axis is accomplished by a rotation of
the nucleus that brings the spindle into alignment with the growth
axis (see figure). Spindle pole position then directly specifies
division plane. Microtubules play an important role in spindle
alignment and we are investigating the function of EB1 proteins
in this process. EB1 proteins bind to the plus end of microtubules
and are important in spindle alignment in yeast and metazoans.
Cytokinesis appears to be a centrifugal (inside to out) process
as it is in higher plants. We would like to know how the growing
cell plate is assembled and guided to the cortex. At present we
know that secretion is required, F-actin colocalizes with the
plate and microtubules may guide it to the division site in the
cortex.
This research has been funded by
awards to DLK from the Plant and Microbial Developmental Mechanisms Panel at the NSF.
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