Kirk DL, Nishii I
Volvox carteri as a model for studying the genetic and
cytological control of morphogenesis
DEV GROWTH DIFFER 43 (6): 621-631 DEC 2001
Abstract:
The green alga Volvox carteri has a very simple and regular adult form
that arises through a short sequence of well-defined morphogenetic steps. A
mature gonidium (asexual reproductive cell) initiates a stereotyped sequence of
rapid cleavage divisions that will produce all of the cells found later in an
adult. A predictable subset of these divisions are asymmetric and result in
production of a small set of germ cells in a precise spatial pattern.
Throughout cleavage, all intracellular components are held in predictable
spatial relationships by a cytoskeleton of unusually regular structure, while
neighboring cells are also held in fixed spatial relationships by an extensive
network of cytoplasmic bridges that form as a result of incomplete cytokinesis.
As a result of these two orienting mechanisms combined, dividing cells are
arranged around the anterior-posterior axis of the embryo with precise
rotational symmetry. These relationships are maintained by the cytoplasmic
bridge system when the embryo that was inside out at the end of cleavage turns
right-side out in the gastrulation-like process of inversion. Inversion is
driven by a cytoskeleton-mediated sequence of cell shape changes, cellular
movements and coordinated contraction. Then, by the time the cytoplasmic
bridges begin to break down shortly after inversion, a preliminary framework of
extracellular matrix (ECM) has been formed. The ECM traps the cells and holds
them in the rotational relationships that were established during cleavage, and
that must be maintained in order for the adult to be able to swim. Transposon
tagging is now being used to clone and characterize the genes regulating these
morphogenetic processes.
Sineshchekov OA, Govorunova EG
Rhodopsin
receptors of phototaxis in green flagellate algae
BIOCHEMISTRY-MOSCOW+ 66 (11): 1300-1310 NOV 2001
Abstract:
Green flagellate algae are capable of the active adjustment of their swimming
path according to the light direction (phototaxis). This direction is detected
by a special photoreceptor apparatus consisting of the photoreceptor membrane
and eyespot. Receptor photoexcitation in green flagellates triggers a cascade
of rapid electrical events in the cell membrane which plays a crucial role in
the signal transduction chain of phototaxis and the photophobic response. The
photoreceptor current is the earliest so far detectable process in this
cascade. Measurement of the photoreceptor current is at present the most
suitable approach to investigation of the photoreceptor pigment in green
flagellate algae, since a low receptor concentration in the cell makes application
of optical and biochemical methods so far impossible. A set of physiological
evidences shows that the phototaxis receptor in green flagellate algae is a
unique rhodopsin-type protein. It shares common chromophore properties with
retinal proteins from archaea. However, the involvement of photoelectric
processes in the signal transduction chain relates it to animal visual
rhodopsins. The presence of some enzymatic components of the animal visual
cascade in isolated eyespot preparations might also point to this relation. A
retinal-binding protein has been identified in such preparations, the amino
acid sequence of which shows a certain homology to sequences of animal visual
rhodopsins. However, potential function of this protein as the phototaxis receptor
has been questioned in recent time.
Kaiser D
Building a
multicellular organism
ANNU REV GENET 35: 103-123 2001
Abstract:
Multicellular organisms appear to have arisen from unicells numerous times.
Multicellular cyanobacteria arose early in the history of life on Earth.
Multicellular forms have since arisen independently in each of the kingdoms and
several times in some phyla. If the step from unicellular to multicellular life
was taken early and frequently, the selective advantage of multicellularity may
be large. By comparing the properties of a multicellular organism with those of
its putative unicellular ancestor, it may be possible to identify the selective
force(s). The independent instances of multicellularity reviewed indicate that
advantages in feeding and in dispersion are common. The capacity for signaling
between cells accompanies the evolution of multicellularity with cell
differentiation.
Fuhrmann M, Stahlberg A, Govorunova E, et al.
The abundant
retinal protein of the Chlamydomonas eye is not the photoreceptor for
phototaxis and photophobic responses
J CELL SCI 114 (21): 3857-3863 NOV 2001
Abstract:
The chlamyopsin gene (cop) encodes the most abundant eyespot protein in the
unicellular green alga Chlamydomonas reinhardtii. This opsin-related protein
(COP) binds retinal and was thought to be the photoreceptor controlling
photomovement responses via a set of photoreceptor currents. Unfortunately,
opsin-deficient mutants are not available and targeted disruption of
non-selectable nuclear genes is not yet possible in any green alga. Here we show
that intron-containing gene fragments directly linked to their intron-less
antisense counterpart provide efficient post-transcriptional gene silencing
(PTGS) in C. reinhardtii, thus allowing an efficient reduction of a specific
gene product in a green alga. In opsin-deprived transformants, Hash-induced
photoreceptor currents (PC) are left unchanged. Moreover, photophobic responses
as studied by motion analysis and phototaxis tested in a light-scattering assay
were indistinguishable from the responses of untransformed wild-type cells. We
conclude that phototaxis and photophobic responses in C. reinhardtii are
triggered by an as yet unidentified rhodopsin species.
Kirk DL
Germ-soma
differentiation in Volvox
DEV BIOL 238 (2): 213-223
Abstract:
Volvox carteri is a spherical green alga with a predominantly asexual
mode of reproduction and a complete germ-soma division of labor. Its somatic
cells are specialized for motility, incapable of dividing, and pro-rammed to
die when only a few days old, whereas its gonidia (asexual reproductive cells)
are nonmotile, specialized for growth and reproduction, and potentially
immortal. When a gonidium is less than 2 days old it divides to produce a
juvenile spheroid containing all of the somatic cells and gonidia that will be
present in an adult of the next generation. The first visible step in germ-soma
differentiation is a set of asymmetric cleavage divisions in the embryo that
set apart small somatic initials from their large gonidial-initial sister
cells. Three types of genes have been found to play key roles in germ-soma
specification. First a set of gls genes act in the embryos to shift
cell-division planes, resulting in the asymmetric divisions that set apart the
large-small sister-cell pairs. Then a set of lag genes act in the large cells
to prevent somatic differentiation, while the regA gene acts in the small cells
to prevent reproductive development. An inducible transposon was used to tag
and recover some of these and other developmentally important genes. The glsA
gene encodes a chaperone-like protein that, like another chaperone that is one
of its putative binding partners, is associated with the cell division
apparatus, although how this leads to asymmetric division remains to be
elucidated. The regA gene encodes a somatic-cell-specific nuclear protein that
appears to function by repressing genes required for chloroplast biogenesis,
thereby preventing somatic cells from growing enough to reproduce.
Somatic-cell-specific expression of regA is controlled by three intronic
enhancers. (C) 2001 Academic Press.
A
senescence-associated S-like RNase in the multicellular green alga Volvox carteri
GENE 274 (1-2): 227-235
Abstract:
Asexual individuals of the green alga Volvox carteri consist of only two
cell types. somatic and reproductive cells. The somatic cells are terminally
differentiated, post-mitotic cells which undergo gradual senescence leading to
cell death in every generation. To elucidate the self-degrading process of
macromolecules associated with senescence, we attempted to clone an RNase whose
mRNA accumulation is increased during senescence. The corresponding cDNA clone
VRN1, encoding an S-like RNase of V. carteri, is the first T-2/S-like RNase to
be cloned from green algae. Semi-quantitative RT-PCR analysis revealed that a
relative amount of VPN1 mRNA is more than three-fold higher in the senescent
somatic cells than in young somatic cells when the mRNA of ribosomal protein
S18 is used as an internal standard. VRN1 mRNA is not induced by phosphate
starvation, indicating that its accumulation during senescence is not due to a
self-induced defect in utilizing phosphates. Similar regulation has been
reported for RNS3, which encodes the S-like RNase that is induced in senescent
leaves of Arabidopsis thaliana. These observations imply that VRN1 may promote
RNA degradation during senescence of somatic cells in V. carteri, and that its
regulation has similarity with that of certain senescence-associated RNases in
higher plants. (C) 2001 Elsevier Science B.V. All rights reserved.
Hallmann A, Amon P, Godl K, et al.
Transcriptional
activation by the sexual pheromone and wounding: a new gene family from Volvox encoding modular proteins with
(hydroxy)proline-rich and metalloproteinase homology domains
PLANT J 26 (6): 583-593 JUN 2001
Abstract:
The green alga Volvox represents the simplest kind of multicellular
organism: it is composed of only two cell types, somatic and reproductive,
making it suitable as a model system. The sexual development of males and
females of Volvox carteri is triggered by a sex-inducing pheromone at a
concentration of < 10(-16) M. Early biochemical responses to the pheromone
involve structural modifications within the extracellular matrix (ECM). By
differential screenings of cDNA libraries made from mRNAs of pheromone-treated Volvox,
four novel genes were identified that encode four closely related Volvox
metalloproteinases that we use to define a new protein family, the VMPs. The
existence of several features common to matrix glycoproteins, such as signal
peptides, a (hydroxy)proline content of 12-25%, and Ser(Pro)(2-4) repeats,
suggest an extracellular localization of the VMPs within the ECM. Synthesis of
VMP cDNAs is triggered not only by the sex-inducing pheromone, but also by
wounding, and is restricted to the somatic cell type. Sequence comparisons
suggest that the VMPs are members of the MB clan of zinc-dependent matrix
metalloproteinases, although the putative zinc binding site of all VMPs is
QEXXHXXGXXH rather than HEXXHXXGXXH. The presence of glutamine instead of
histidine in the zinc binding motif suggests a novel family, or even clan, of
peptidases. Like the matrixin family of human collagenases, Volvox VMPs
exhibit a modular structure: they possess a metalloproteinase homology domain
and a (hydroxy)proline-rich domain, and one of them, VMP4, also has two
additional domains. Metalloproteinases seem to be crucial for biochemical
modifications of the ECM during development or after wounding in the lower
eukaryote Volvox with only two cell types, just as in higher organisms.
Kaczanowski S, Jerzmanowski A
Evolutionary
correlation between linker histones and microtubular structures
J MOL EVOL 53 (1): 19-30 JUL 2001
Abstract:
Histones of the H1 group (linker histones) are abundant components of chromatin
in eukaryotes, occurring on average at one molecule per nucleosome. The recent
reports on the lack of a clear phenotypic effect of knock-out mutations as well
as overexpression of histone H1 genes in different organisms have seriously
undermined the long-held view that linker histones are essential for the basic
functions of eukaryotic cells. In an attempt to resolve the paradox of an
abundant conserved protein without a clear function, we re-examined the
molecular and phylogenetic data on linker histones to see if they could reveal
any correlation between the features of H1 and the functional or morphological
characteristics of cells or organisms. Because of an earlier demonstration that
in sea urchin the chromatin-type histone HI is also found in the flagellar
microtubules (Multigner et al. 1992), we focused on the correlation between the
features of H1 and those of microtubular structures. A phylogenetic tree based
on multiple alignment of over 100 available H1 sequences suggests that the
first divergence of the globular domain of H1 (GH1) resulted in branching into
separate types characteristic for plants/Dictyostelium and for
animals/ascomycetes, respectively. The GH1s of these two types differ by a
short region (usually 5 amino acids) placed at a specific location within the
C-terminal wing subdomain of GH1. Evolutionary analysis of the diversification
of H1 mRNA into cell-cycle-dependent (polyA(-)) and independent (polyA(+))
forms showed a mosaic occurrence of these two forms in plants and animals,
despite the fact that the H1 proteins of plants and animals belong to two
well-distinguished groups. However, among organisms from both animal and plant
kingdom, only those with H1 mRNA of a polyA- type have flagellated gametes.
This correlation as well as the demonstration that in Volvox carteri the
accumulation of polyA- mRNA of H1 occurs concurrently with the production of
new flagella (Lindauer et al. 1993), suggests a direct link between polyA-
phenotype of histone H1 mRNA and flagello-genesis.
Stark K, Kirk DL, Schmitt R
Two enhancers
and one silencer located in the introns of regA control somatic cell
differentiation in Volvox
carteri
GENE DEV 15 (11): 1449-1460
Abstract:
The regA gene plays a central role in germ-soma differentiation of Volvox
carteri by suppressing all reproductive functions in somatic cells. Here we
show that the minimal promoter of regA consists of only 42 bp immediately
upstream of the transcription start site, and that it contains no discernible
regulatory elements. However, introns 3 and 5 are both required for regA
expression in somatic cells, and intron 7 is essential for silencing regA in
gonidia (asexual reproductive cells). A regA gene lacking intron 7 rescues the
normal phenotype of mutant somatic cells, but also results in gonidia that
reproduce only weakly and soon die out. The same phenotype is observed when a
regA gene containing intron 7 is placed under control of a constitutive
promoter, suggesting that the silencing activity of intron 7 is promoter
specific. Intron 7 is unusual in that it contains a potential
Babinger P, Kobl I, Mages W, et al.
A link between
DNA methylation and epigenetic silencing in transgenic Volvox carteri
NUCLEIC ACIDS RES 29 (6): 1261-1271
Abstract:
Epigenetic silencing of foreign genes introduced into plants poses an unsolved
problem for transgenic technology. Here we have used the simple multicellular
green alga Volvox carteri as a model to analyse the relation of DNA
methylation to transgenic silencing. Volvox DNA contains on average 1.1%
Ei-methylcytosine and 0.3% N6-methyladenine, as revealed by electrospray mass spectrometry
and phosphoimaging of chromatographically separated P-32-labelled nucleotides.
In two nuclear transformants of V.carteri, produced in 1993 by biolistic
bombardment with a foreign arylsulphatase gene (C-ars), the transgene is still
expressed in one (Hill 181), but not in the other (Hill 183), after an
estimated 500-1000 generations. Each transformant clone contains multiple
intact copies of C-ars, most of them integrated into the genome as tandem
repeats. When the bisulphite genomic sequencing protocol was applied to examine
two select regions of transgenic C-ars, we found that the inactivated copies
(Hill 183) exhibited a high-level methylation (40%) of CpG dinucleotides,
whereas the active copies (Hill 181) displayed low-level (7%) CpG methylation.
These are average values from 40 PCR clones sequenced from each DNA strand in
the two portions of C-ars. The observed correlation of CpG methylation and
transgene inactivation in a green alga will be discussed in the light of
transcriptional silencing.
Ferris PJ, Woessner JP, Waffenschmidt S, et al.
Glycosylated
polyproline II rods with kinks as a structural motif in plant
hydroxyproline-rich glycoproteins
BIOCHEMISTRY-US 40 (9): 2978-2987
Abstract:
Hydroxyproline-rich glycoproteins (HRGPs) are the major proteinaceous
components of higher plant walls and the predominant components of the cell
wall of the green alga Chlamydomonas reinhardtii. The GPI protein, an HRGP of
the C. reinhardtii wall, is shown to adopt a polyproline II helical
configuration and to carry a complex array of arabinogalactoside residues, many
branched, which are necessary to stabilize the helical conformation. The deduced
GP1 amino acid sequence displays two Ser-Pro-rich domains, one with a repeating
(SP), motif and the other with a repeating (PPSPX)(x) motif. A second cloned
gene a2 also carries the PPSPX repeat, defining a novel gene family in this
lineage. The SP-repeat domains of GP1 form a 100-nm shaft with a flexible kink
28 nm from the head. The gp1 gene encodes a PPPPPRPPFPANTPM sequence at the
calculated kink position, generating the proposal that this insert interrupts
the PPII helix, with the resultant kink exposing amino acids necessary for GP1
to bind to partner molecules. It is proposed that similar kinks in the higher
plant HRGPs called extensins may play a comparable role in wall assembly.
Kirk DL
Seeking the
ultimate and proximate causes of Volvox
multicellularity
AM ZOOL 41 (6): 1493-1493 DEC 2001
Nishii I, Kirk DL
The invA gene
of Volvox encodes a novel
kinesin that is required for inversion of the embryo.
DEV BIOL 235 (1): 115