A new male-specific gene “<Emphasis Type="Italic">OTOKOGI</Emphasis>” in <Emphasis Type="Italic">Pleodorina starrii</Emphasis> (Volvocaceae,Chlorophyta) unveils the origin of male and female

Eukaryotic sex was initially isogametic and it is assumed that anisogamy/oogamy evolved independently in many lineages including animals, land plants and volvocine green algae. The exact evolutionary mechanisms that were responsible for the evolution of oogamy from isogamy were poorly understood until Nozaki et al. (2006) introduced the use of molecular-genetic data in elucidating the evolutionary origin of oogamy from isogamy in the colonial volvocacean Pleodorina starrii. In the close relative Chlamydomonas reinhardtii, sexual reproduction is isogametic with mating-types plus and minus. Mating type minus represents a “dominant sex” because the MID (“minus-dominance”) gene of C. reinhardtii is both necessary and sufficient to cause the cells to differentiate as isogametes of the minus mating type. No sex-specific genes had been identified in the volvocine green algae until Nozaki et al. (2006a) successfully cloned the MID gene of P. starrii. This “OTOKOGI” (PlestMID) gene is present only in the male genome, and encodes a protein localized abundantly in the nuclei of mature sperm. Thus, P. starrii maleness evolved from the dominant sex (mating type minus) of its isogamous ancestor. This breakthrough provides an opportunity to address various extremely interesting questions regarding the evolution of oogamy and the male-female dichotomy. Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007, Slovakia.     

Morphology and phylogeny of a new wall‐less freshwater volvocalean flagellate,Hapalochloris nozakii gen. et sp. nov. (Volvocales,Chlorophyceae)

New strains of a wall‐less unicellular volvocalean flagellate were isolated from a freshwater environment in Japan. Observations of the alga, described here as Hapalochloris nozakii Nakada, gen. et sp. nov., were made using light, fluorescence, and electron microscopy. Each vegetative cell had two flagella, four contractile vacuoles, and a spirally furrowed cup‐shaped chloroplast with an axial pyrenoid, and mitochondria located in the furrows. Based on the morphology, H. nozakii was distinguished from other known wall‐less volvocalean flagellates. Under electron microscopy, fibrous material, instead of a cell wall and dense cortical microtubules, was observed outside and inside the cell membrane, respectively. Based on the phylogenetic analyses of 18S rRNA gene sequences, H. nozakii was found to be closely related to Asterococcus, Oogamochlamys, Rhysamphichloris, and “Dunaliella” lateralis and was separated from other known wall‐less flagellate volvocaleans, indicating independent secondary loss of the cell wall in H. nozakii. In the combined 18S rRNA and chloroplast gene tree, H. nozakii was sister to Lobochlamys.     

EVIDENCE FOR TUBULAR MATING STRUCTURES INDUCED IN EACH MATING TYPE OF HETEROTHALLIC GONIUM PECTORALE (VOLVOCALES,CHLOROPHYTA)1

Gametes were induced separately in cultures of each mating type of the heterothallic, isogamous colonial volvocalean Gonium pectorale O. F. Müll. to examine the tubular mating structure (TMS) of both mating types plus and minus (plus and minus), referred to as “bilateral mating papillae.” Addition of dibutyryl cyclic adenosine monophosphate (DcAMP or db‐cAMP) and 3‐isobutyl‐1‐methylxanthine (IBMX) to approximately 3‐week‐old cultures of each mating type induced immediate release of naked gametes from the cell walls. Both plus and minus gametes formed a TMS in the anterior region of the protoplasts. Accumulation of actin was visualized by antibody staining in the TMS of both mating types as occurs in the TMS (fertilization tubule) of the plus gametes of the unicellular volvocalean Chlamydomonas reinhardtii P. A. Dang. Induction of naked gametes with a TMS in each mating type will be useful for future cell biological and evolutionary studies of the isogametes of colonial volvocalean algae.     

PHYLOGENETIC ANALYSIS OF MORPHOLOGICAL SPECIES OF CARTERIA (VOLVOCALES,CHLOROPHYTA) BASED ON rbcL GENE SEQUENCES1

Four related species in the unicellular volvocalean genus Carteria [C. crucifera Pascher, C. eugametos Mitra, C. inversa (Korshikov) Bourrelly and C. cerasiformis Nozaki et al.] were delineated on the basis of recent comparative light and electron microscopy of a large number of culture strains. However, the species thus delineated may not represent natural or monophyletic entities. In the present study, 1128 base pairs of the chloroplast protein-coding gene (large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase gene) from 12 Carteria strains representing the four species as well as from related volvocalean species were analyzed to elucidate the phylogenetic status of the taxonomic or morphologic species of Carteria. The sequence data showed that the 12 Carteria strains exhibit four robust monophyletic groups which are strictly consistent with the four taxonomic species. These results are discussed in relation to contrasting results found in other microalgal genera. It is concluded that phylogenetic analysis, based on DMA sequence data and comparative morphologic characterization of species and using a large number of culture strains, is essential to a natural system of microalgal species taxonomy.     

The morphology and reproduction of Sphondylothamnion multifidum (Hudson) Naegeli (Ceramiaceae)

Sphondylothamnion multifidum (Huds.) Naeg., while a typical member of the Ceramiaceae in its structure and reproduction, differs in post-fertilisation details from other known genera. The sterile cells associated with the procarp system, i.e. the apical cell, the sterile cell on the supporting cell, and the sterile pericentral cell, all divide actively after fertilisation to produce branched involucral systems which surround the gonimoblast.     

Phylogenetic Position of the Phacotaceae Within the Chlamydophyceae as Revealed by Analysis of 18S rDNA and rbcL Sequences

Four genera of the Phacotaceae (Phacotus, Pteromonas, Wislouchiella, Dysmorphococcus), a family of loricated green algal flagellates within the Volvocales, were investigated by means of transmission electron microscopy and analysis of the nuclear encoded small-subunit ribosomal RNA (18S rRNA) genes and the plastid-encoded rbcL genes. Additionally, the 18S rDNA of Haematococcus pluvialis and the rbcL sequences of Chlorogonium elongatum, C. euchlorum, Dunaliella parva, Chloromonas serbinowii, Chlamydomonas radiata, and C. tetragama were determined. Analysis of ultrastructural data justified the separation of the Phacotaceae into two groups. Phacotus, Pteromonas, and Wislouchiella generally shared the following characters: egg-shaped protoplasts, a single pyrenoid with planar thylakoid double-lamellae, three-layered lorica, flagellar channels as part of the central lorica layer, mitochondria located in the central cytoplasm, lorica development that occurs in mucilaginous zoosporangia that are to be lysed, and no acid-resistant cell walls. Dysmorphococcus was clearly different in each of the characters mentioned. Direct comparison of sequences of Phacotus lenticularis, Pteromonas sp., Pteromonas protracta, and Wislouchiella planctonica revealed DNA sequence homologies of ≥98.0% within the 18S gene and 93.9% within the rbcL gene. D. globosus was quite different from these species, with a maximum of 92.9% homology in the 18S rRNA and ≤86.6% in the rbcL gene. It showed major similarities to the 18S rDNA of Dunaliella salina, with 95.3%, and to the rbcL sequence of Chlamydomonas tetragama, with 90.3% sequence homology. Additionally, the Phacotaceae sensu stricto exclusively shared 10 (rbcL: 4) characters which were present neither in other Chlamydomonadales nor in Dysmorphococcus globosus. Different phylogenetic analysis methods confirmed the hypothesis that the Phacotaceae are polyphyletic. The Phacotaceae sensu stricto form a stable cluster with affinities to the Dunaliellaes and possibly Haematococcus pluvialis. Dysmorphococcus globosus represented an independent lineage that is possibly related to Chlamydomonas moewusii and C. tetragama. Received: 9 June 1997 / Accepted: 17 October 1997     

Sexual reproduction inEudorina elegans (chlorophyta,volvocales)

Sexual reproduction inEudorina elegans Ehr. was studied in detail in laboratory cultures, with particular regard to conjugation between gametes and gone colony formation. Male and female gametes fused after being induced by changing the medium. The anterior end, including the flagellar base, of the male gamete entered the anterior region of the female gamete. Fusion of the two protoplasts proceeded laterally and posteriorly. The male gamete bore a slender cytoplasmic protrusion at the base of the flagella. This structure has not been previously described in the male gamete ofEudorina, and may participate in plasmogamy. A biflagellate gone cell swam from the germinating zygote and secreted a gelatinous envelope. It then divided to form a gone colony within the gelatious envelope, which moved during colony formation by means of the two flagella which were retained intact from the original gone cell.     

Chlamydomonas neoplanoconvexa nom. nov. and its unique phylogenetic position within Volvocales (Chlorophyceae)

Chlamydomonas Ehrenb. is a unicellular volvocalean genus consisting of 400–600 species, most of which are known solely based on microscopy. In this study, a newly isolated strain of Chlamydomonas neoplanoconvexa (M. O. P. Iyengar) Nakada nom. nov. (≡Chlamydomonas planoconvexa M. O. P. Iyengar non J. W. G. Lund) was examined using light microscopy and combined 18S rRNA, rbcL and psaB gene phylogeny. The C. neoplanoconvexa strain was quite similar to Chlamydomonas perpusilla (Korshikov) Gerloff in terms of its small fusiform vegetative cells (～10 µm in length), parietal chloroplast containing a single pyrenoid, and nucleus posterior to the pyrenoid. However, C. neoplanoconvexa was distinguished from C. perpusilla based on the morphology of the papillae and pyrenoids. The two species belong to the clade Caudivolvoxa in the order Volvocales, but are distantly related to each other. Chlamydomonas perpusilla was shown to be sister to Chlorogonium Ehrenb. (clade Chlorogonia, within Caudivolvoxa), while C. neoplanoconvexa represented a second basal lineage within Caudivolvoxa, next to the clade Characiosiphonia. Although the morphology of C. neoplanoconvexa was not particularly outstanding, its unique phylogenetic position will encourage further investigation of this species and its uncultivated relatives.     

Studies on the fine structure of Glaucocystis nostochinearum Itzigs. II. Membrane morphology and taxonomy

The fine structure of the alga Glaucocystis nostochinearum Itzigs. has been studied by thinsectioning and freeze-etching. Many of the features already established by sectioning have been verified but different conclusions are drawn, particularly with regard to the photosynthetic organs. The action of glycerol solutions on the morphology of the plasmalemma as revealed by freeze-etching is shown to be of general importance for the freeze-etching of plant cells and of particular importance in elucidating the true form of the plasmalemma in Glaucocystis. The literature regarding the taxonomic position of Glaucocystis has been reviewed, and, along with a completely new demonstration of plasmalemma structure, an attempt has been made to reclassify this organism.     

团藻目分子系统学研究进展

包含许多重要类群的团藻目在绿色植物系统演化中占有十分重要的地位。普遍认为现在的绿色植物起源于团藻目的绿色鞭毛类 ,有人认为鞭毛细胞表面覆盖有鳞片的塔胞藻类是绿色植物的祖先。团藻目是分子系统学研究比较多和比较深入的一个类群 ,通过研究证实其有单一共同祖先 ,而莱茵衣藻在所有已经经过DNA测序分析系统发育的绿色单细胞类群中是最可能的种类。本文结合自己的工作 ,对国内外这一研究领域概况进行综述。