Differentiation of germinal and somatic cells in Volvox carteri

Volvox carteri is a spherical alga with a complete division of labor between around 2000 biflagellate somatic cells and 16 asexual reproductive cells (gonidia). It provides an attractive system for studying how a molecular genetic program for cell-autonomous differentiation is encoded within the genome. Three types of genes have been identified as key players in germ-soma differentiation: a set of gls genes that act in the embryo to shift cell-division planes, resulting in asymmetric divisions that set apart the large-small sister-cell pairs; a set of lag genes that act in the large gonidial initials to prevent somatic differentiation; and the regA gene, which acts in the small somatic initials to prevent reproductive development. Somatic-cell-specific expression of regA is controlled by intronic enhancer and silencer elements.     

Differentiation of Reproductive Cells in Volvox carteri*

SYNOPSIS. The life cycle of Volvox carteri was studied in axenic culture using the NB-3 and the NB-7 strains isolated from Nebraska. Vegetative colonies of both strains contain 8–12 asexual reproductive cells (gonidia) which divide to form daughter colonies. During daughter colony formation, the reproductive cells of the daughters are delimited at an early stage of cleavage. Gonidia are delimited at the division from 16 to 32 cells, but eggs and male initial cells are not differentiated until the division of the 32-celled stage. In all instances the reproductive cells are the products of unequal cleavages. Male and female colonies are formed in separate clones. Female colonies contain approximately 20 eggs. Male colonies have approximately 50 male initial cells, each of which forms a sperm bundle containing 64 or 128 sperm. Sperm bundles penetrate female colonies and fertilize the eggs. Zygote formation, zygote germination, and the development of gone colonies is described. Sexual type was inherited in a 1:1 ratio. Male colonies appear spontaneously in the male strain, but female colonies were formed in the female strain only in the presence of a substance produced by colonies from male cultures. This female inducing substance is produced in male cultures primarily, if not exclusively, by male colonies rather than by vegetative colonies. The female inducing substance is heat labile and non-dialyzable. Activity is destroyed by Pronase, but not by trypsin, chymotrypsin or ribonuclease. Gonidia appear to be most susceptible to female induction during the early stages of their expansion prior to cleavage.     

Ultrastructure of Volvox carteri

Summary Somatic cells of mature asexual colonies of Volvox carteri do not possess a true cell wall, but are otherwise similar in ultrastructure to Chlamydomonas. Somatic cells are embedded in multilayered fibrillar material of the colonial matrix. The reproductive cells (gonidia) of Volvox carteri lie internal to the somatic cell layer of the colony matrix in an apparently structureless portion of the colony matrix. Mature gonidia are large vacuolate cells with a central nucleus and parietal chloroplasts and mitochondria. They are non-flagellated at maturity, but each contains a pair of kinetosomes.     

Two photophobic responses in Volvox carteri

Phototaxis of Volvox carteri is the result of two photophobicresponses: stop and accelerationof flagellar activity in theanterior region of the colony. These responses of a colony fixedunder a microscope were analyzed quantitatively by cine-micrography. The phototactic sign of Volvox is temperature-dependent: itis positive at room temperature and negative at low temperature.When the temperature was lowered, the stop response to the on-stimuluswas reduced and changed to the acceleration response, whilethe acceleration response to the off-stimulus changed to thestop response. Decrease in light inteasity resulted in reduction in both stopresponses, i.e., the response to the on-stimulus at 24?C andthat to the off-stimulus at 16?C, but scarcely affected theacceleration responses. The action spectra of the photophobic responses at 24? and 14?Care similar, with a peak at 520 nm. (Received January 24, 1979; )     

Lipid Composition and Metabolism of Volvox carteri

The membrane structural lipids of somatic cells and gonidia isolated from Volvox carteri f. nagariensis spheroids have been characterized. The principal polar lipid components of both cell types are sulfoquinovosyl diglyceride, mono- and digalactosyl diglyceride, phosphatidylglycerol, phosphatidylethanolamine, and 1(3), 2-diacylglyceryl-(3)-O-4′-(N,N,N,-trimethyl)homoserine. Light-synchronized cultures of spheroids were shown to incorporate [¹⁴C]bicarbonate, [³⁵S]sulfate, [¹⁴C]palmitic acid, and [¹⁴C]lauric acid into complex lipids. [¹⁴C]Palmitic acid was incorporated mainly into diacylglyceryltrimethylhomoserine and was not significantly modified by elongation or desaturation. In contrast, [¹⁴C]lauric acid was incorporated into a wider variety of complex lipids and was also converted into longer chain saturated and unsaturated fatty acids. Volvox is a promising system for studying the role of membranes in algal cellular differentiation.     

Mutants affecting vegetative cell orientation in Volvox carteri

The vegetative cells of Volvox are highly oriented with respect to each other and to the spheroid on which they occur. Mutants have been isolated which affect this orientation either directly or indirectly. Mutants affecting flagella (flg), including a temperature-sensitive mutant, demonstrate that flagella must be present during or following embryo inversion for proper cell orientation. Other mutants (eye) which have apparently functional flagella lead to the conclusion that the presence of functional flagella is not sufficient for proper cell orientation. These mutants also demonstrate the necessity of proper cell orientation for the phototactic motion of the spheroid.     

microRNAs in a multicellular green alga Volvox carteri

microRNAs(miRNAs)have emerged as key components in the eukaryotic gene regulatory network.We and others have previously identified many miRNAs in a unicellular green alga,Chlamydomonas reinhardtii.To investigate whether miRNA-mediated gene regulation is a general mechanism in green algae and how miRNAs have been evolved in the green algal lineage,we examined small RNAs in Volvox carteri,a multicellular species in the same family with Chlamydomonas reinhardtii.We identified 174 miRNAs in Volvox,with many of them being highly enriched in gonidia or somatic cells.The targets of the miRNAs were predicted and many of them were subjected to miRNA-mediated cleavage in vivo,suggesting that miRNAs play regulatory roles in the biology of green algae.Our catalog of miRNAs and their targets provides a resource for further studies on the evolution,biological functions,and genomic properties of miRNAs in green algae.     

Characterization of a sperm lysin of Volvox carteri

Summary A cell-wall degrading enzyme has been isolated from mature sperm packets of the green flagellate Volvox carteri (Poona strain). This sperm lysin (S-lysin) is a Ca²⁺-dependent protease of 34 kDa with an essential serine group in its active centre. Neither SH group-blocking reagents nor transition metal chelators inhibit its action. S-lysin degrades the hydroxyproline-rich glycoprotein structures of the cell walls of sheath cells and gonidia (eggs) of vegetative and sexual spheroids in a characteristic manner. In asexual spheroids the somatic envelope is totally disintegrated, whereas in sexual spheroids pores are formed by local lysis at sites of adjacent eggs. Although S-lysin is very similar to the G-lysin of the closely related Chlamydomonads, it is species specific and does not attack the mother or daughter cell walls of Chlamydomonas reinhardtii. S-lysin resembles the aerosin of animal sperm cells in some aspects of its action.Dedicated to Professor Richard C. Starr on the occasion of his 65th birthday. He called the piper and gave the tune     

Two Different Rickettsial Bacteria Invading Volvox carteri

BackgroundBacteria of the family Rickettsiaceae are principally associated with arthropods. Recently, endosymbionts of the Rickettsiaceae have been found in non-phagotrophic cells of the volvocalean green algae Carteria cerasiformis, Pleodorina japonica, and Volvox carteri. Such endosymbionts were present in only C. cerasiformis strain NIES-425 and V. carteri strain UTEX 2180, of various strains of Carteria and V. carteri examined, suggesting that rickettsial endosymbionts may have been transmitted to only a few algal strains very recently. However, in preliminary work, we detected a sequence similar to that of a rickettsial gene in the nuclear genome of V. carteri strain EVE.Conclusion/SignificanceAt least two different rickettsial organisms may have invaded the V. carteri lineage, one of which may be the direct ancestor of the endosymbiont of V. carteri strain UTEX 2180, whereas the other may be closely related to the endosymbiont of P. japonica. Endosymbiotic gene transfer from the latter rickettsial organism may have occurred in an ancestor of V. carteri. Thus, the rickettsiae may be widely associated with V. carteri, and likely have often been lost during host evolution.     

Cyclic AMP as an intraspheroidal differentiation signal in Volvox carteri

The action of the macromolecular inducer glycoprotein on sexual reproduction in the green alga Volvox carteri can be modified by altering the external (intraspheroidal) cAMP concentration. Direct proof for the presence of cAMP in the spheroids is given. Protein binding assay and HPLC-fluorimetric analysis independently demonstrate the existence of cAMP in the matrix, cells, and culture medium. Its concentration is higher in sexual cultures, pointing to a transmitting function in sex induction. The presence in the matrix of other members of a protein phosphorylation system suggests an induction-specific signal cascade in this plant.     

Sexual induction in Volvox carteri f. nagariensis by aldehydes

Summary Sexual induction in the Gone 12 mutant of Volvox carteri can be achieved by shortterm treatment with glutardialdehyde or formaldehyde, followed by capture of the aldehyde by means of amino acids at slightly acidic pH. The same effect is obtained by exposure to anthranilic acid formalide, the condensation product of 2-amino benzoic acid with formaldehyde, at low concentration for several minutes. This is in contrast to the prolonged exposure required by the specific glycoprotein inducer. In both situations the asexual reproductive cells are affected in such a way that they change their pattern of cleavage to form sexual embryos rather than asexual ones. Thus, besides the natural messenger molecule, a physical (UV light, heat) or molecular shock may trigger the chain reaction leading to expression of sexual induction.     

Effect of external ionic environment on phototaxis of Volvox carteri

The sign of phototaxis in Volvox carteri is temperature-dependent;positive at room temperature and negative at low temperature.Modification of the tactic sign by external ions, pH and chemicalswas studied. The addition of 30 mM potassium ion to the mediumchanged the tactic sign from positive to negative, and a mediumwith a high pH elicited positive phototaxis. An increase inthe potassium or hydrogen ion concentration raised the reversaltemperature of the phototactic sign, and the addition of magnesiumor calcium ion also raised the reversal temperature of the signslightly. Valinomycin, a highly specific ionophore of potassium,raised the reversal temperature. CCCP, DCMU and DCCD, whichdepolarize biological membranes, also raised the reversal temperature.These results show that the sign of phototaxis is determinedby membrane polarization; on depolarization of the membranethe sign of phototaxis changes from positive to negative. Ethanol lowered the reversal temperature, and sodium azide inhibitedboth positive and negative phototaxis. The effects of ethanoland azide indicate that depolarization of the membrane is notthe only factor that induces change in the phototactic sign. (Received July 23, 1979; )