Copyright © 2010 by Prof. Dr. Armin Hallmann, University of Bielefeld, Germany
Green algae of the order Volvocales provide an unrivalled opportunity for exploring
the transition from unicellularity to multicellularity. They range from unicells,
like Chlamydomonas, through homocytic colonial forms with increasing cooperation
of individual cells, like Gonium or Pandorina, to heterocytic multicellular
forms with different cell types and a complete division of labour, like Volvox.
A fundamental requirement for the evolution of multicellularity is the development
of a complex, multifunctional extracellular matrix (ECM). The ECM has many functions,
which can change under developmental control or as a result of environmental
factors. Here molecular data from 15 novel proteins are presented. These proteins
have been identified in Chlamydomonas reinhardtii, Gonium pectorale, Pandorina
morum and Volvox carteri, and all belong to a single protein family, the pherophorins.
Pherophorin-V1 is shown to be a glycoprotein localized to the 'cellular zone'
of the V. carteri ECM. Pherophorin-V1 and -V2 mRNAs are strongly induced not
only by the sex inducer, which triggers sexual development at extremely low
concentrations, but also by mechanical wounding. Like the extensins of higher
plants, which are also developmentally controlled or sometimes inducible by
wounding, the pherophorins contain a (hydroxy-)proline-rich (HR) rod-like domain
and are abundant within the extracellular compartment. In contrast to most extensins,
pherophorins have additional globular A and B domains on both ends of the HR
domains. Therefore pherophorins most closely resemble a particular class of
higher plant extensin, the solanaceous lectins (e.g. potato lectin), suggesting
multivalent carbohydrate-binding functions are present within the A and B domains
and are responsible for cross-linking. Our results suggest that pherophorins
are used as the building blocks for the extracellular scaffold throughout the
Volvocales, with the characteristic mesh sizes in different ECM structures being
a result of the highly diverse extensions of the HR domains. Pherophorins have
therefore been a versatile element during the evolution of ECM architecture
in these green algae.