Meissner M, Stark K, Cresnar B, et al.
Volvox germline-specific genes that are putative targets of RegA repression encode chloroplast proteins
CURR GENET 36 (6): 363-370 DEC 1999
Abstract:
In Volvox carteri, regA acts as a master gene to suppress all germ cell
functions in somatic cells. Its product, RegA, has
features of a transcriptional repressor. Here we report cDNA
sequences representing 15 nuclear genes with properties expected of RegA targets: they are expressed strongly in germ cells and
in regA(-),
but not regA(+), somatic cells. Two of them encode
polypeptides with no recognizable features, but ten (like three previously
sequenced ones) encode chloroplast proteins of known function, and the
remaining three encode putative chloroplast proteins of unknown function. This
suggests that RegA blocks reproductive development in
somatic cells by preventing chloroplast biogenesis, thereby making it impossible
for the cells to grow enough to reproduce
Ender F, Hallmann A, Amon P, et al.
Response to
the sexual pheromone and wounding in the green alga Volvox: Induction of an extracellular glycoprotein consisting almost exclusively of
hydroxyproline
J BIOL CHEM 274 (49): 35023-35028 DEC 3 1999
Abstract:
The extracellular matrix (ECM) of Volvox
is modified during development or in response to external stimuli, like the
sex-inducing pheromone. It has recently been demonstrated that a number of
genes triggered by the sex-inducing pheromone are also inducible by wounding.
By differential screening of a cDNA library, a novel
gene was identified that is transcribed in response to the pheromone. Its gene
product was characterized as an ECM glycoprotein with a striking feature: it
exhibits a hydroxyproline content of 68% and
therefore is an extreme member of the family of hydroxyproline-rich
glycoproteins (HRGPs), HRGPs are known as constituents
of higher plant ECMs and seem to function as
structural barriers in defense responses. The Volvox
HRGP is also found to be inducible by wounding. This indicates that the wound
response scenarios of higher plants and multicellular
green algae may be evolutionary related.
Kirk DL
Evolution of multicellularity in the volvocine
algae
CURR OPIN PLANT BIOL 2 (6): 496-501 DEC 1999
Abstract:
Recent studies reveal that relationships among the volvocine
algae are more complex than was previously believed. Nevertheless, this group
still appears to provide an unrivaled opportunity to analyze an evolutionary
pathway leading from unicellularity (Chlamydomonas) to multicellularity
with division of labor (volvox). Significant
progress in this regard was made in the past year when two genes playing key
roles in volvox cellular differentiation were
cloned, and clues were uncovered regarding their mechanisms of action.
Coleman AW
Phylogenetic analysis of "Volvocacae"
for comparative genetic studies
P NATL ACAD SCI USA 96 (24): 13892-13897
Abstract:
Sequence analysis based on multiple isolates representing essentially all
genera and species of the classic family Volvocaeae
has clarified their phylogenetic relationships.
Cloned internal transcribed spacer sequences (ITS-1. and ITS-2, flanking the
5.8S gene of the nuclear ribosomal gene cistrons)
were aligned, guided by ITS transcript secondary structural features, and
subjected to parsimony and neighbor joining distance analysis. Results confirm
the notion of a single common ancestor, and Chlamydomonas
reinharditii alone among all sequenced green unicells is most similar. interbreeding
isolates were nearest neighbors on the evolutionary tree in all cases. Some taxa, at whatever level, prove to be clades
by sequence comparisons, but others provide striking exceptions. The
morphological species Pandorina morum,
known to be widespread and diverse in mating pairs, was found to encompass all
of the isolates of the four species of Volvulina. Platydorina appears to have originated early and not to
fall within the genus Eudorina, with which it can
sometimes be confused by morphology. The four species of Pleodorina
appear variously associated with Eudorina examples.
Although the species of Volvox are each clades, the genus Volvox
is not The conclusions confirm and extend prior, more
limited, studies on nuclear SSU and LSU rDNA genes
and plastid-encoded rbcL and atpB.
The phylogenetic tree suggests which classical
taxonomic characters are most misleading and provides a framework for molecular
studies of the cell cycle-related and other alterations that have engendered
diversity in both vegetative and sexual colony patterns in this classical
family.
Funke RP, Kovar JL, Logsdon JM, et al.
Nucleus-encoded,
plastid-targeted acetolactate synthase
genes in two closely related chlorophytes, Chlamydomonas reinhardtii and Volvox carteri: phylogenetic origins and
recent insertion of introns
MOL GEN GENET 262 (1): 12-21 AUG 1999
Abstract:
Acetolactate synthase (ALS
catalyzes the first committed step in the synthesis of branched-chain amino
acids. In green plants and fungi, ALS is encoded by a nuclear gene whose
product is targeted to plastids (in plants) or to mitochondria tin fungi). In
red algae, the gene is plastid-encoded. We have determined the complete
sequence of nucleus-encoded ALS genes from the green algae Chlamydomonas
reinhardtii and Volvox
carteri. Phylogenetic
analyses of the ALS gene family indicate that the ALS genes of green algae and
plants are closely related, sharing a recent common ancestor. Furthermore,
although these genes are clearly of eubacterial
origin, a relationship to the ALS genes of red algae and cyanobacteria
(endosymbiotic precursors of plastids) is only weakly
indicated. The algal ALS genes are distinguished from their homologs
in higher plants by the fact that they are interrupted by numerous spliceosomal introns; plant ALS
genes completely lack introns. The restricted phylogenetic distribution of these introns
suggests that they were inserted recently, after the divergence of these green
algae from plants. Two introns in the Volvox ALS gene, not found in the Chlamydomonas gene, are positioned precisely at sites which
resemble "proto-splice" sequences in the Chlamydomonas
gene.
Kobayashi K, Koyanagi R, Matsumoto M, et al.
Switching
from asexual to sexual reproduction in the planarian Dugesia
ryukyuensis: Bioassay system and basic description of
sexualizing process
ZOOL SCI 16 (2): 291-298 APR 1999
Abstract:
An assay system has been established for the sexual induction in the OH strain,
an exclusively fissiparous (asexual) strain, of Dugesia
ryukyuensis by feeding them with sexually matured
worms of Bdellocephala brunnea,
an exclusively oviparous (sexual) species. In this assay system, asexual worms
gradually differentiated sexual organs, namely the ovary, testis, genital pore
and yolk gland in this order, and eventually mated and laid cocoons filled with
fertilized eggs. Although the OH strain worms were believed not to have any
sexual organs, a pair of undeveloped ovaries with a few oogonia were detected by an intensive histological
search. Along with the progression of sexualization,
five distinct stages were histologically recognized:
In the first stage, the ovaries became larger enough to be externally apparent;
oocytes appeared first at stage 2; the primordial
testes emerged at stage 3; a genital pore opened, yolk gland primordia developed and spermatocytes
appeared at stage 4; and finally at stage 5 matured spermatozoa and yolk glands
were formed.
Hoops HJ,
A test of two
possible mechanisms for phototactic steering in Volvox carteri (Chlorophyceae)
J PHYCOL 35 (3): 539-547 JUN 1999
Abstract:
We tested two competing models that could explain how differential flagellar activity leads to phototactic
turning in spheroids of Volvox carteri f, weismannia (Powers) Iyengar. In one model, turning results from the flagella of
anterior cells in the lighted and shadowed hemispheres beating at different
frequencies. In a competing model, turning results from a change in beat
direction in these flagella. Both models successfully explain phototactic steering under constant illumination, but they
make different predictions when colonies are exposed to abrupt changes in light
intensity. If turning is due to control of flagellar
beat frequency, both progression and rotation rates will change in the same
direction and with similar magnitudes. If spheroid turning is due to a change
in flagellar beat direction, a decreased rate of
progression will accompany an increased rate of rotation and vice versa. We
used video-microscopy to observe the behavior of positively phototactic
V. carteri spheroids exposed to 10x step-up and
step-down stimuli. After a step-up stimulus, spheroids slow their progression
and rotation by equal amounts, No significant changes are reported in these
parameters after the reciprocal step-down response. These observations are
consistent with the variable flagellar frequency
model and inconsistent with the variable flagellar
direction model for phototactic turning. Switching
the direction of Light stimulus by 180 degrees results in reorientation of
positively phototactic spheroids, The
kinetics of this reorientation did not precisely match the predictions of
either model.
Nishii I, Ogihara S
Actomyosin contraction of the posterior hemisphere is required
for inversion of the Volvox
embryo
DEVELOPMENT 126 (10): 2117-2127 MAY 1999
Abstract:
During inversion of a Volvox embryo, a series
of cell shape changes causes the multicellular sheet
to bend outward, and propagation of the bend from the anterior to the posterior
pole eventually results in an inside-out spherical sheet of cells. We use
fluorescent and electron microscopy to study the behavior of the cytoskeleton
in cells undergoing shape changes. Microtubules are aligned parallel to the
cell's long axis and become elongated in the bend. Myosin and actin filaments are arrayed perinuclearly
before inversion. In inversion, actin and myosin are
located in a subnuclear position throughout the uninverted region but this localization is gradually lost
towards the bend. Actomyosin inhibitors cause
enlargement of the embryo. The bend propagation is inhibited halfway and, as a
consequence, the posterior hemisphere remains uninverted.
The arrested posterior hemisphere will resume and complete inversion even in
the presence of an actomyosin inhibitor if the
anterior hemisphere is removed microsurgically. We
conclude that the principal role of actomyosin in
inversion is to cause a compaction of the posterior hemisphere; unless the
equatorial diameter of the embryo is reduced in this manner, it is too large to
pass through the opening defined by the already-inverted anterior hemisphere.
Huey RB, Berrigan D, Gilchrist GW, et al.
Testing the
adaptive significance of acclimation: A strong inference approach
AM ZOOL 39 (2): 323-336 APR 1999
Abstract:
Acclimation is a common phenotypic response to environmental change.
Acclimation is often thought to enhance performance and thus to be adaptive.
This view has recently been formalized as the "Beneficial Acclimation
Hypothesis" and predicts that individuals acclimated to one environment
perform better in that environment than do individuals acclimated to a
different environment. Although Beneficial Acclimation is appealing and widely
supported, recent studies with E. coli and Drosophila have challenged its
general validity. Although these challenges could be dismissed as mere
exceptions, they encourage a re-evaluation of the adaptive significance of
acclimation. Our philosophical approach differs from that of most previous
studies of acclimation, in which the prediction derived from a Beneficial
Acclimation perspective (e.g., heat tolerance is positively correlated with
acclimation temperature) is tested against the null hypothesis ("single
hypothesis approach"). Instead, we follow Huey and Berrigan
(1996) in advocating a strong inference approach (sensu
Platt, 1964), which recognizes that Beneficial Acclimation is actually one of a
set of competing hypotheses that make different predictions as to how
developmental temperature influences the thermal sensitivity of adults
("developmental acclimation"). Using this perspective, Huey and Berrigan proposed a factorial experimental design (3
developmental by 3 adult temperatures) designed to discriminate among all
competing hypotheses. We now derive a formal statistical model (ANOVA with
orthogonal polynomial contrasts) for this experimental design and use it to
evaluate simultaneously the relative impact of each competing hypothesis. We then
apply this model to several case studies (Drosophila, Volvox,
Trichogramma), and we review also a recent study with
E. coli. The influence of Beneficial Acclimation is supported (albeit often
weakly) in most cases. Nevertheless, other hypotheses (especially the Optimal
Developmental Temperature Hypothesis) often have a greater impact. Even so,
however, Beneficial Acclimation usually predicts relative performance at
extreme test temperatures. We conclude that, although rumors of its death are
premature, Beneficial Acclimation cannot be viewed as the dominant expectation,
at least with regard to developmental temperature acclimation. Moreover, our
findings reinforce the view that a strong inference approach provides a more
comprehensive portrait of complex biological responses than do
single-hypothesis approaches.
Hallmann A
Enzymes in
the extracellular matrix of Volvox: an inducible,
calcium-dependent phosphatase with a modular
composition
J BIOL CHEM 274 (3): 1691-1697
Abstract:
The volvocine algae provide the unique opportunity
for exploring development of an extracellular matrix.
Volvox is the most advanced member of this
family and represents the simplest multicellular
organism, with differentiated cells, a complete division of labor, and a
complex extracellular matrix, which serves structural
and enzymatic functions. In Volvox carteri a glycosylated extracellular phosphatase was
identified, which is partially released from the extracellular
matrix into the growth medium. The phosphatase is
synthesized in response to inorganic phosphate starvation and is strictly to
inorganic phosphate starvation and is strictly calcium-dependent. The metalloenzyme has been purified to homogeneity and
characterized. Its gene and cDNA have been cloned.
Comparisons of genomic and cDNA sequences revealed an
extremely intron-rich gene (32 introns),
With an apparent molecular mass of 160 kDa the Volvox extracellular phosphatase is the largest phosphatase
cloned, with no sequence similarity to any other phosphatase.
This enzyme exhibits a modular composition. There are two large domains and a
small one. The large domains are highly homologous to each other and therefore
most likely originated from gene duplication and fusion. At least one EF-hand
motif for calcium binding was identified in this extracellular
protein Volvox extracellular
phosphatase is the first calcium-dependent extracellular phosphatase to be
cloned.
Rodriguez H, Haring MA, Beck CF
Molecular
characterization of two light-induced, gamete-specific genes from Chlamydomonas reinhardtii that
encode hydroxyproline-rich proteins
MOL GEN GENET 261 (2): 267-274 MAR 1999
Abstract:
Gametic differentiation in Chlamydomonas
reinhardtii is a two-step process, which is
controlled by the sequential action of the two extrinsic signals, nitrogen
starvation and blue light. The gamete-specific genes GAS28 and GAS29 are
expressed in the late phase of gametogenesis. Their
light-induced expression is restricted to cells that have completed the first,
nitrogen starvation-activated, phase of differentiation. A comparison of the
two genes revealed striking similarities as well as differences. Their most
prominent shared feature is an extended sequence homology of over 90% in their
5'-untranslated regions, suggesting a role in translational regulation. GAS28
and GAS29 both encode hydroxyproline-rich proteins (HRGPs) of very similar sizes that exhibit typical features
of volvocalean cell wall constituents. GAS28 shows a
high degree of homology with the Volvox pherophorin gene family, suggesting a relationship between
these genes.
Bischoff F, Molendijk A, Rajendrakumar CSV, et al.
GTP binding
proteins in plants
CELL MOL LIFE SCI 55 (2): 233-256 FEB 1999
Abstract:
GTP-binding proteins are found in all organisms. They are important switches
that cycle between an active and an inactive state, ensuring vectorial flow of information on the expense of guanosine triphosphate (GTP). In
this review, we discuss current progress in the molecular characterization and
functional analysis of plant genes encoding heterotrimeric
and small GTPases. An up-to-date list in eluding all
cloned plant GTPase genes is given and a systematic
classification is proposed.
Kirk MM, Stark K, Miller SM, et al.
regA, a Volvox gene that plays a
central role in germ-soma differentiation, encodes a novel regulatory protein
DEVELOPMENT 126 (4): 639-647 Feb 1999
Abstract:
Volvox has two cell types: mortal somatic
cells and immortal germ cells. Here we describe the transposon-tagging,
cloning and characterization of regA, which plays a
central role as a master regulatory gene in Volvox
germsoma differentiation by suppressing reproductive
activities in somatic cells. The 12.5 kb regA
transcription unit generates a 6,725 nucleotide mRNA that appears at the
beginning of somatic cell differentiation, and that encodes a 111 kDa RegA protein that localizes
to the nucleus, and has an unusual abundance of alanine,
glutamine and proline, This is a compositional
feature shared by functional domains of many 'active' repressors, These
findings are consistent with the hypothesis that RegA
acts in somatic cells to repress transcription of genes required for growth and
reproduction, including 13 genes whose products are required for chloroplast
biogenesis.
Miller SM, Kirk DL
glsA, a Volvox gene required for
asymmetric division and germ cell specification, encodes a chaperone-like
protein
DEVELOPMENT 126 (4): 649-658 FEB 1999
Abstract:
The gls genes of Volvox
are required for the asymmetric divisions that set apart cells of the germ and
somatic lineages during embryogenesis. Here we used transposon
tagging to clone glsA, and then showed that it is
expressed maximally in asymmetrically dividing embryos, and that it encodes a
748-amino acid protein with two potential protein-binding domains.
Site-directed mutagenesis of one of these, the J domain (by which Hsp40-class
chaperones bind to and activate specific Hsp70 partners) abolishes the capacity
of glsA to rescue mutants. Based on this and other
considerations, including the fact that the GlsA
protein is associated with the mitotic spindle, we discuss how it might
function, in conjunction with an Hsp70-type partner, to shift the division
plane in asymmetrically dividing cells.
Nozaki H, Ohta N, Takano H, et al.
Reexamination
of phylogenetic relationships within the colonial Volvocales (Chlorophyta): An
analysis of atpB and rbcL
gene sequences
J PHYCOL 35 (1): 104-112 FEB 1999
Abstract:
The chloroplast-encoded atpB gene was sequenced from
33 strains representing 28 species of the colonial Volvocales
(the Volvocaceae and its relatives) to reexamine phylogenetic relationships as previously deduced by
morphological data and rbcL gene sequence data.1128
base pairs in the coding regions of the atpB gene
were analyzed by MP, NJ, and ML analyses, Although supported with relatively
low bootstrap values (75% and 65% in the NJ and ML analyses, respectively),
three anisogamous/oogamous volvocacean
genera-Eudorina, Pleodorina,
and Volvox, excluding the section Volvox(= Euvolvox,
illegitimate name), constituted a large monophyletic group (Eudorina
group), Outside the Eudorina group, a robust Lineage
composed of three species of Volvox sect, Volvox was resolved as in the rbcL
gene trees, rejecting the hypothesis of the previous cladistic
analysis based on morphological data that the genus Volvox
is monophyletic, In addition, the NJ and ML trees suggested that Eudorina is a non-monophyletic genus as inferred from the
morphological data and rbcL gene sequences. Although phylogenetic status of the genus Gonium is ambiguous in the
rbcL gene trees and the paraphyly
of this genus is resolved in the cladistic analysis
based on morphological data, the atpB gene sequence
data suggest monophyly of Gonium with relatively low
bootstrap values (56-61%) in the NJ and ML trees. On the basis of the combined
sequence data (2256 base pairs) from atpB and rbcL genes, Gonium was resolved as a robust monophyletic
genus in the NJ and ML trees (with 68-86% bootstrap values), and Eudorina elegans Ehrenberg
represented a paraphyletic species positioned most
basally within the Eudorina group, However, phylogenetic status and relationships of the families of
the colonial Volvocales were still almost ambiguous
even in the combined analysis.
Braun FJ, Hegemann P
Two
light-activated conductances in the eye of the green
alga Volvox
carteri
BIOPHYS J 76 (3): 1668-1678 MAR 1999
Abstract:
Photoreceptor currents of the multicellular green
alga Volvox carteri
were analyzed using a dissolver mutant. The photocurrents are restricted to the
eyespot region of somatic cells. Photocurrents are detectable from intact cells
and excised eyes. The rhodopsin action spectrum
suggests that the currents are induced by Volvox
rhodopsin. Flash-induced photocurrents are a
composition of a fast Ca2+-carried current (P-F) and a slower current (P-S),
which is carried by H+. P-F is a high-intensity response that appears with a
delay of less than 50 mu s after flash. The
stimulus-response curve of its initial rise is fit by a single exponential and
parallels the rhodopsin bleaching. These two
observations suggest that the responsible channel is closely connected to the rhodopsin, both forming a tight complex. At low flash
energies P-S is dominating. The current delay increases up to 10 ms, and the
P-S amplitude saturates when only a few percent of the rhodopsin
is bleached. The data are in favor of a second signaling system, which includes
a signal transducer mediating between rhodopsin and
the channel. We present a model of how different modes of signal transduction
are accomplished in this alga under different light conditions.
Hallmann A, Rappel A
Genetic
engineering of the multicellular green alga Volvox: a
modified and multiplied bacterial antibiotic resistance gene as a dominant
selectable marker
PLANT J 17 (1): 99-109 JAN 1999
Abstract:
The green alga Volvox represents the simplest multicellular organism: Volvox
is composed of only two cell types, somatic and reproductive. Volvox, therefore, is an attractive model system for
studying various aspects of multicellularity. With
the biolistic nuclear transformation of Volvox carteri, the
powerful molecular genetic manipulation of this organism has been established,
but applications have been restricted to an auxotrophic
mutant serving as the DNA recipient. Therefore, a dominant selectable marker
working in all strains and mutants of this organism is required. Among several
gene constructs tested, the most advantageous results were obtained with a chimeric gene composed of the coding sequence of the
bacterial ble gene, conferring resistance to the
antibiotic zeocin, modified with insertions of two
endogenous introns from the Volvox
arylsulfatase gene and fused to 5' and 3' untranslated regions from the Volvox
beta 2-tubulin gene. In the most suitable plasmid used, the gene dosage was
increased 16-fold by a technique that allows exponential multiplication of a
DNA fragment. Go-transformation of this plasmid and a non-selectable plasmid
allowed the identification of zeocin resistant transformants with nuclear integration of both selectable
and non-selectable plasmids. Stable expression of the ble
gene and of genes from several non-selectable plasmids is demonstrated. The
modified ble gene provides the first dominant marker
for transformation of both wild-type and mutant strains of Volvox.
Fabry S, Kohler A, Coleman AW
Intraspecies analysis: Comparison of ITS sequence data and gene intron sequence data with breeding data for a worldwide
collection of Gonium pectorale
J MOL EVOL 48 (1): 94-101 JAN 1999
Abstract:
The morphologically uniform species Gonium pectorale
is a colonial green flagellate of worldwide distribution. The affinities of 25
isolates from 18 sites on five continents were assessed by both DNA sequence
comparisons and sexual compatibility. Complete sequences were obtained (i) for the internal transcribed spacer ITS-1 and ITS-2
regions of ribosomal DNA and (ii) for each of three single-copy spliceosomal introns, two in a
small G protein and one in the actin gene. ITS
sequences appeared to homogenize sufficiently rapidly to behave as a single
copy gene. Intron sequence differences between
isolates in this species reached nucleotide substitution saturation, while ITS
sequences did not. Parsimony and evolutionary distance analysis of the two
types of DNA data gave essentially the same tree conformation. By all these
criteria, the group of G. pectorale isolates fell
into two main clades, A and B. Clade
A, with isolates from four continents, was comprised of four subclades of quite closely related isolates, plus one
strain of ambiguous affinity. Clade B was comprised
of two subclades represented by South African and
South American isolates, respectively; thus, only subclades
of clade B showed geographical localization. With
respect to mating, all isolates except one homothallic strain and one
apparently sterile strain fell into either one or the other of two mating
types. Pairings in all possible combinations revealed that isolates from the
same site formed abundant zygotes, which germinated to produce new, sexually
active organisms. Zygotes were also formed in many pairings of other
combinations, including crosses of clade A with clade B organisms, but none of the latter produced viable germlings. The ability to mate and produce viable progeny
that were themselves capable of sexual reproduction was restricted to members
of subclades established on the basis of DNA sequence
similarities. Thus, the grades of difference in both nuclear intron sequences and rDNA ITS
sequences paralleled those observed in the sexual analysis.
Miller SM, Kirk DL
glsA, a Volvox gene required for
asymmetric division and germ cell specification, encode a chaperone-like
protein that co-localizes with the mitotic spindle
MOL BIOL CELL 10: 2265 Suppl. S NOV 1999