A two-step cell polarization in algal zygotes

During earth's history, complex multicellular life has evolved from unicellular organisms along at least five independent paths, which are: animals, plants, fungi, red algae and brown algae. While symmetric cell divisions merely amplify the number of equivalent cells in an organism, asymmetric divisions are essential to create diverse cell types.

First cell undergoes polarization during which molecular components such as mRNA, proteins or even organelles become unequally distributed in the cell. The result is that, after the asymmetric cell division, the two daughters cells are not equivalent anymore and will receive different cell fates.

Polarization and asymmetric cell division are intensively studied in animals for which elegant and easily accessible model systems are available such as the round worm C. elegans and fruitfly Drosophila. However, plants have developed multicellularity in the presence of a rigid cell wall restricting cell movement and hindering intercellular contact. This is not the case for animals that evolved different mechanisms to generate asymmetries during division. The deviating evolutionary trajectory that plants and animals have followed is probably the reason why homologous asymmetric division components between the two groups are lacking.

Since many years, the Phycology Group of Prof. Dr. Olivier De Clerck is studying the diverse biological aspects of the brown alga Dictyota. With respect to the study of cell polarization and asymmetric cell division, brown algae represent an exciting study object. They evolved multicellularity together with cell walls just as in plants. Fertilization and early polarization take place in the naked egg which is more easily accessible for the researcher then those of higher plants which are internalized in the flowers and cones.

This was the reason why the Phycology Group teamed up with the Root Development Group in the VIB of Prof. Dr. Tom Beeckman. This collaboration resulted in the discovery by Dr Kenny Bogaert of an unknown process of cell polarization in the brown alga Dictyota that is characterized by two steps, first, upon fertilisation, the egg cell immediately polarizes along a preprogrammed axis (Bogaert et al., 2017, Nature Plants, in press). At this point the sense of the polarization is not yet determined, or in other words, it is not decided yet which side is going to develop into the leaf-like upper part or the lower root-like part of the alga. This happens in a next step characterized by de novo zygotic transcriptional activity and is dependent on external stimuli such as light.

In the classic view the axis and the sense of polarization during embryogenesis are installed in one single step. The availability of a model system in which these process are phased and thus distinguishable opens great opportunities to disentangle processes involved in cell polarization and ACD in a cell wall-based multicellular context.