Cell division in microalgae: Mitosis revealed in detail

Date:
March 7, 2022
Source:
Bielefeld University
Summary:
Cell division ensures growth or renewal and is thus vital for
all organisms. However, the process differs somewhat in animals,
bacteria, fungi, plants, and algae. Until now, little was known
about how cell division occurs in algae. Researchers have used
confocal laser scanning microscopy (CLSM) to capture the very
first high-resolution three- dimensional images of cell division
in live cells of the microalga Volvox carteri, and have identified
new cellular structures involved in the process.



FULL STORY ==========================================================================
Cell division ensures growth or renewal and is thus vital for all
organisms.

However, the process differs somewhat in animals, bacteria, fungi,
plants, and algae. Until now, little was known about how cell division
occurs in algae.

Researchers at Bielefeld University have used confocal laser
scanning microscopy (CLSM) to capture the very first high-resolution three-dimensional images of cell division in live cells of the microalga
Volvox carteri, and have identified new cellular structures involved in
the process. Professor Dr Armin Hallmann from the Faculty of Biology is
leading the study. The findings have now been published open access in
the journal The Plant Cell.


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The cell is the smallest organisational unit of life. It contains the
necessary building blocks of life in a compact form and is the place
where vital biochemical reactions take place. With the help of enzymes, substance and energy transformations take place, which are processes also
known as metabolism. The cell interior is separated and thus protected
from the environment by the cell membrane. Genetic material, the cell's information store, is often located in the cell nucleus as DNA. When a
cell divides by mitosis, it first divides its nucleus into two identical daughter nuclei with the same genetic material. Then the rest of the
cell divides and two identical daughter cells are produced. The complex, genetically determined process of mitosis in particular must take place
very precisely: the entire genetic material, divided into chromosomes,
must be segregated accurately into the two daughter cell nuclei.

Cell division of the alga Volvox carteri combines animal and plant characteristics 'Cell division is one of the most fundamental processes in living organisms. It has basically been preserved over countless millions
of years of evolution and can be found in all organisms,' says Professor
Dr Armin Hallmann, head of the Cellular and Developmental Biology of
Plants research group at Bielefeld University. Yet the mechanisms of cell division in animals, fungi, plants, and algae each have characteristic features. The multicellular green alga Volvox carteri is a particularly interesting case in point. 'It exhibits both animal and plant features
in mitosis,' says Hallmann. The researchers have now been able to clarify
this phenomenon in their study. 'Until now, researchers knew very little
about the exact process of mitosis in this green alga.' Mitosis in the microalga Volvox carteri With their analyses, the scientists have been
able to identify five characteristics that are crucial for mitosis in
the microalga Volvox carteri.



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The first two features concern the envelope of the microalgal
nucleus. 'The nuclear envelope does not disintegrate at the beginning
of mitosis, as is often the case, but remains in place until shortly
before nuclear division is completed,' says Armin Hallmann. 'Instead, it becomes porous and permeable, so that cellular components are exchanged
between the inside of the cell nucleus and the cytosol -- a fluid that surrounds the cell nucleus. Hence, for a certain period of time, the
cell nucleus loses its typical property as a confined reaction space,
although the nuclear envelope is still present.' The third feature is
related to the centrosomes of the cell. These are cell structures that
play a central role in the organisation of the mitotic spindle here. The mitotic spindle arranges the chromosomes in such a way that they can
be segregated accurately into the two newly forming cell nuclei. 'We
have been able to show that the centrosomes play a crucial role in
the mitosis of Volvox carteri even though they are located outside
the nuclear envelope. They form the basic structure for organising the
precise division of the genetic material with the help of the nuclear
division spindle within the nuclear envelope.

Until now, we only knew about an organisation of the spindle by
centrosomes from cell division in animals,' says Hallmann.

A fourth feature is the formation of a specific filamentous structure,
the phycoplast, at the end of mitosis. After the cell nucleus has divided,
the rest of the cell must also divide so that the newly formed cells can finally separate from each other. The dynamic phycoplast is the basis
for the formation of a cleavage furrow which ultimately divides the
cell, whereas plants form a different structure which ultimately leads
to the formation of a separating, solid cell wall. 'The special thing
about algae is that the phycoplast is formed directly by recycling the
nuclear division spindle, which is then no longer required,' explains
the scientist.

Finally, the researchers were able to detect an enormous dynamic of the
entire inner architecture of the cell as well as of the nuclear envelope
during cell division.

Making molecular processes visible The researchers were able to record the
cell division processes by producing fluorescent proteins (proteins that
glow when exposed to light) and tracking them in the cell using confocal
laser scanning microscopy (CLSM). For the first time, scientists have
succeeded in imaging the mitosis of microalgae in three dimensions in live cells and characterising it in detail, using Volvox carteri as an example.



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'The question we posed ourselves was: how exactly does cell division
work in green algae? Which structures are involved in mitosis and what
role do they play in the process?' says first author Dr Eva Laura von
der Heyde. She previously conducted research in Hallmann's research
group as a doctoral student and is now a postdoc. In order to be able to localise important proteins involved in cell division in the cell, their
genes are linked to the gene of a fluorescent protein using molecular
biology techniques. The proteins involved in cell division thus become fluorescent, which makes them distinguishable from all other proteins
in the cell. 'We used a special laser to excite different fluorescent
proteins to glow. Using a confocal laser scanning microscope, we were
able to detect the yellow-green glow of the microstructures formed by
the proteins in live cells,' says Eva Laura von der Heyde.

The researchers also recorded on video how the proteins move during
cell division, how they form microstructures, and how these structures
are rebuilt.

In a time-lapse video, which condenses 30 minutes of mitosis to nine
seconds and shows it simultaneously in ten optical section planes, it
becomes clear how the centrosomes organise the formation of the nuclear division spindle and how the nuclear division spindle finally transforms
into the phycoplast after the chromosomes have separated.

Insights into evolution In the long term, Armin Hallmann and Eva Laura
von der Heyde hope to be able to build on these new findings to learn
more about the evolution of cell division.

How did the different variants of cell division that are found today in animals, fungi, plants, and algae come about? 'In evolution, the first
land plants developed from primordial green algae. This is why the green
alga Volvox carteri also possesses properties that it has in common with
land plants growing today. However, it is striking that Volvox carteri
also possesses properties that can be found in animals living today. Other
of their characteristics are again only found in green algae. These
special characteristics also make this model organism so important for
our understanding of the evolution of cell division,' says Hallmann.

========================================================================== Story Source: Materials provided by Bielefeld_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Eva Laura von der Heyde, Armin Hallmann. Molecular and cellular
dynamics
of early embryonic cell divisions in Volvox carteri. The Plant Cell,
2022; DOI: 10.1093/plcell/koac004 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220307091729.htm

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