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The colonial volvocine algae span the full range of organizational complexity, from four-celled species to multicellular species, and this group of algae is often used for the study of evolution. In recent years, many organelle genomes have been sequenced using the application of next generation sequencing technology; however, only a few organelle genomes have been reported in colonial volvocine algae. In this study, we determined the organelle genomes of Eudorina elegans and Eudorina cylindrica and analysed the organelle genome size, structure and gene content between these volvocine species. This provided useful information to help us understand the composition of colonial volvocine organelle genomes. Based on the chloroplast genome protein-coding genes, we conducted a phylogenomics analysis of the volvocine algae. The result revealed an unexpected phylogenetic relationship, namely, E. elegans is more closely related to Pleodorina starrii than to E. cylindrica. The substitution rate of volvocine algae was then calculated based on organelle genome protein-coding genes; our analysis suggested the possibility that the two Eudorina species may be under similar evolutionary pressure. Lastly, the synteny analysis of the mitochondrial genome showed that gene arrangements and contents are highly conserved in the family Volvocaceae, and the synteny analysis of the chloroplast genome indicated that the genus Eudorina may have experienced genomic changes.

Seven species of Volvocaceae were recorded in the lower Vistula River and its oxbow lakes, including Pleodorina californica for the first time in Poland. Three species – Eudorina cylindrica, E. illinoisensis and E. unicocca – were found in the Polish Vistula River in the 1960s and 1970s, as well as at present. They are rare species in the Polish aquatic ecosystems. Three species are common both in the oxbow lakes and in the Vistula River: Eudorina elegans, Pandorina morum and Volvox aureus. New and rare Volvocaceae species were described in terms of morphology and ecology; also photographic documentation (light microscope microphotographs) was completed.

This review covers essentially all aspects of the organisms in the green algal family Volvocaceae and suggests the genetic history of the various steps in their evolution from their unicellular ancestors.

Steering their swimming direction toward the light is crucial for the viability of Volvox colonies, the larger members of the volvocine algae. While it is known that this phototactic steering is achieved by a difference in behavior of the flagella on the illuminated and shaded sides, conflicting reports suggest that this asymmetry arises either from a change in beating direction or a change in beating frequency. Here, we report direct observations of the flagellar behavior of various Volvox species with different phyletic origin in response to light intensity changes and thereby resolve this controversy: Volvox barberi W. Shaw from the section Volvox sensu Nozaki (2003) changes the direction of the flagellar beating plane, while species encompassed in the group Eudorina (Volvox carteri F. Stein, Volvox aureus Ehrenb., and Volvox tertius Art. Mey.) decrease the flagellar beating frequency, sometimes down to flagellar arrest.

Colonial volvocacean algae engage in two types of sexual reproduction: isogamy and anisogamy/oogamy with sperm packets. This difference is an important generic diagnosis within the Volvocaceae. Although Yamagishiella differs from the anisogamous genus Eudorina in its isogamous sexual reproduction, the vegetative morphology and asexual reproduction characteristics of the two genera are indistinguishable, especially between Eudorina unicocca G. M. Smith and Yamagishiella unicocca (Rayburn et Starr) Nozaki. We re-examined morphological characteristics of E. unicocca and related species, using multiple strains of E. unicocca and Y. unicocca and molecular phylogenetic analyses. Strains from two Japanese lakes, which produced aplanospores and were solely asexual, could be assigned to either E. unicocca or Y. unicocca, based on traditional morphological diagnoses. However, a new morphological diagnosis (the difference in the distribution and number of contractile vacuoles on the cell surface) and molecular phylo...

The volvocacean genus Pleodorina has been morphologically characterized as having small somatic cells in spheroidal colonies and anisogamous sexual reproduction with sperm packets. In this study we examined two new species that can be assigned to the genus Pleodorina based on morphology: P. starrii H. Nozaki et al. sp. nov. and P. thompsonii F. D. Ott et al. sp. nov. P. starrii was collected from Japan and had 32- or 64-celled colonies with anterior somatic cells and spheroidal individual cellular sheaths that were weakly attached to each other within the colonial envelope. P. thompsonii from Texas (USA) exhibited four or 12 somatic cells in the anterior pole of 16- or 32-celled colonies, respectively, and had a single large pyrenoid in the chloroplast of mature reproductive cells. The chloroplast multigene phylogeny placed P. starrii and P. indica (Iyenger) H. Nozaki in a clade that was robustly separated from the type species P. californica Shaw and P. japonica H. Nozaki. Pleodorina thompsonii was resolved as a basal branch within a large monophyletic group (Eudorina group) composed of Eudorina, Pleodorina and Volvox (excluding section Volvox). Thus, Pleodorina was found among three separate lineages within the Eudorina group in which Eudorina and Volvox were also resolved as nonmonophyletic. The DNA sequences from additional species/strains as well as recognition of morphological attributes that characterize the monophyletic groups within the Eudorina group are needed to construct a natural generic classification within these members of the Volvocaceae.

The volvocine algae provide an unrivalled opportunity to explore details of an evolutionary pathway leading from a unicellular ancestor to multicellular organisms with a division of labor between different cell types. Members of this monophyletic group of green flagellates range in complexity from unicellular Chlamydomonas through a series of extant organisms of intermediate size and complexity to Volvox, a genus of spherical organisms that have thousands of cells and a germ-soma division of labor. It is estimated that these organisms all shared a common ancestor about 50 +/- 20 MYA. Here we outline twelve important ways in which the developmental repertoire of an ancestral unicell similar to modern C. reinhardtii was modified to produce first a small colonial organism like Gonium that was capable of swimming directionally, then a sequence of larger organisms (such as Pandorina, Eudorina and Pleodorina) in which there was an increasing tendency to differentiate two cell types, and eventually Volvox carteri with its complete germ-soma division of labor.

Mating affinity, nuclear rDNA internal transcribed spacer (ITS) sequence, and geographic distribution of more than 100 isolates of Pandorina, Volvulina, and Yamagishiella were determined. Comparative analysis of ITS sequences reaffirmed the unity of Pandorina morum and its separation from all other species of Volvocaceae except those of Volvulina. This latter genus, represented by four species, appears to represent only morphological variants of several P. morum subclades. The P. morum clade (including also additional species such as P. smithii and P. colemaniae) encompasses an evolutionary span, as determined by comparison of ITS, greater than either of the multispecies genera Gonium or Eudorina. There are at least 30 Pandorina/Volvulina syngens, sexually isolated groups, so far as can be determined, among the current collection of strains. In addition, as in other volvocacaean genera, two clones are homothallic, capable of forming zygotes within a genetic clone. The existence of so many syngens suggests that considerable evolutionary diversification of the genes controlling gamete compatibility and intercross survival has occurred, unaccompanied by significant morphological change. Within each syngen, genetic distance increases with geographic distance between collection sites. At least half of the isolates studied must have been introduced northward since the Pleistocene. Although we probably know more about characters in this group of algae than any other algal group, in part because they are so easy to culture, we are still largely ignorant of what circumscribes their niche in nature. The study of all these organisms, distributed throughout the world presumably by the activities of shorebirds, emphasizes the evolutionary role of mating genes in the inevitable formation of new genetically isolated subclades in these eukaryotes and provides initial data on their rate of appearance versus their rate of distribution over the earth.

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 nonmonophyletic 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.

Species and varieties in the genus Eudorina Ehrenberg (Volvocaceae, Chlorophyta) were evaluated on the basis of phylogenetic analyses of the large subunit ofribulose-1,5-bis-phosphate carboxylase/oxygenase (rbcL) gene sequences from 14 strains of four Eudorina species, as well as from nine species of Pleodorina and Volvox. The sequence data suggested that 10 of the 14 Eudorina strains form three separate and robust monophyletic groups within the nonmonophyletic genus Eudorina. The first group comprises all three strains of E. unicocca G. M. Smith; the second group consists of one of the E. elegans Ehrenberg var. elegans strains, the E. cylindrica Korshikov strain, and both E. illinoisensis (Kofoid) Pascher strains; and the third group consists of two monoecious varieties of E. elegans [two strains of E. elegans var. synoica Goldstein and one strain of E. elegans var. carteri (G. M. Smith) Goldstein]. In addition, E. illinoisensis represents a poly- or paraphyletic species within the second group. The remaining four strains, all of which are assigned to E. elegans var. elegans, are nonmonophyletic. Although their position in the phylogenetic trees is more or less ambiguous, they are ancestral to other taxa in the large anisogamous/oogamous monophyletic group including Eudorina, Pleodorina, and Volvox (except for sect. Volvox). Thus, the four Eudorina groups resolved in the present molecular phylogeny do not correspond with the species concepts of Eudorina based on vegetative morphology, but they do reflect the results of the previous intercrossing experiments and modes of monoecious and dioecious sexual reproduction.

Yamagishiella, based on Pandorina unicocca Rayburn et Starr, is distinguished from Eudorina by its isogamous sexual reproduction, whereas Platydorina exhibits anisogamous sexual reproduction. In the present study, we sequenced the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) genes from five Japanese and North American strains of Y. unicocca (Rayburn et Starr) Nozaki, two Platydorina caudata Kofoid strains, and two strains of Eudorina unicocca G. M. Smith, as well as eight related colonial and unicellular species. Phylogenetic trees were constructed based on these sequence data and on previously published rbcL gene sequences from 23 volvocalean species in order to deduce phylogenetic relationships within the colonial Volvocales, with particular regard to the phylogenetic positions and status of the genera Yamagishiella and Platydorina. Two robust monophyletic groups of the anisogamous/oogamous volvocacean species were resolved in the maximum-parsimony tree as well as in the neighbor-joining distance tree. One of the two groups comprises three species of Volvox section Volvox, whereas the other is composed of other sections of Volvox as well as of all the species of Eudorina and Pleodorina. Platydorina, however, was positioned outside these two monopliyletic groups. Therefore, derivation of the Platydorina lineage may be earlier than that of such anisogamous/oogamous groups, or orgin of “anisogamy with sperm packets” in Platydorina may be independent of sperm packet evolution in Eudorina, Pleodorina, and Volvox. It was also resolved with high bootstrap values that all of the Y. unicocca strains form a monophyletic group positioned outside the large monophyletic group including Eudorina and Pleodorina. These reject the possibility of the reverse evolution of isogamy from anisogamy to give rise to Yamagishiella within the lineage of Eudorina.

The chloroplast-encoded large subunit of the ribulose-1, 5-bisphosphate carboxylase / oxygenase (rbcL) gene was sequenced from 20 species of the colonial Volvocales (the Volvacaceae, Goniaceae, and Tetrabaenaceae) in order to elucidate phylogenetic relationships within the colonial Volvocales. Eleven hundred twenty-eight base pairs in the coding regions of the (rbcL) gene were analyzed by the neighbor-joining (NJ) method using three kinds of distance estimations, as well as by the maximum parsimony (MP) method. A large group comprising all the anisogamous and oogamous volvocacean species was resolved in the MP tree as well as in the NJ trees based on overall and synonymous substitutions. In all the trees constructed, Basichlamys and Tetrabaena (Tetrabaenaceae) constituted a very robust phylogenetic group. Although not supported by high bootstrap values, the MP tree and the NJ tree based on nonsynonymous substitutions indicated that the Tetrabaenaceae is the sister group to the large group comprising the Volvocaceae and the Goniaceae. In addition, the present analysis strongly suggested that Pandorina and Astrephomene are monophyletic genera whereas Eudorina is nonmonophyletic. These results are essentially consistent with the results of the recent cladistic analyses of morphological data. However, the monophyly of the Volvocaceae previously supported by four morphological synapomorphies is found only in the NJ tree based on nonsynonymous substitutions (with very low bootstrap values). The genus Volvox was clearly resolved as a polyphyletic group with V. rousseletii Pocock separated from other species of Volvox in the rbcL gene comparisons, although this genus represents a monophyletic group in the previous morphological analyses. Furthermore, none of the rbcL gene trees supported the monophyly of the Goniaceae; Astrephomene was placed in various phylogenetic positions.

A cladistic analysis was used to deduce the phylogenetic relationships within the colonial Volvocales. Forty-one pairs of characters related to gross morphology and ultrastructure of vegetative colonies as well as asexual and sexual reproduction were analyzed based on parsimony, using the PAUP 3.0 computer program, for 25 species belonging to nine volvocacean and goniacean genera of the colonial Volvocales. Chlamydomonas reinhardtii Dangeard was the outgroup. The strict consensus tree indicated the presence of two monophyletic groups, one composed of all the volvocacean species analyzed in this study and the other containing the goniacean species except for the four-celled species Gonium sociale (Dujardin) Warming. In addition, these two groups constitute a large monophyletic group, to which G. sociale is a sister group. A new combination Tetrabaena socialis (Dujardin) Nozaki et Itoh and a new family Tetrabaenaceae Nozaki et Itoh are thus proposed for G. sociale. In addition, the analysis suggests that the volvocacean genera Eudorina and Pleodorina are paraphyletic groups, respectively, and that the monotypic genus Yamagishiella has no autapomorphic characters and represents primitive features of the anisogamous and oogamous genera of the Volvocaceae. Phylogenetic relationships within the Volvocaceae and the Goniaceae, as well as the various modes of sexual reproduction exhibited by these organisms, are discussed on the basis of the analysis.

Vegetative colonies of Pandorina unicocca Rayburn et Starr, four taxa of Eudorina [E. elegans Ehrenberg (type species), E. illinoisensis (Kofoid) Pascher, E. unicocca G.M. Smith var. unicocca andE. unicocca var. peripheralis Goldstein] and two species of Pleodorina [Po californicaShaw (type species) and P. indica (Iyengar) Nozaki] were examined with electron microscopy in order to characterize the structure of the extracellular matrix. Each cell of the colonies of all the taxa examined was tightly enclosed by a dense fibrillar zone of the extracellular matrix (cellular envelope) with sparse fibrillar material filling the space outside the cellular envelopes within the tripartite colonial boundary of the matrix. This arrangement is essentially different from that of Pandorina morum (O.F. Miiller) Bory (type species) and P. colemaniae Nozaki. As Eudorina and Pleodorina both have anisogamous sexual reproduction with sperm packets (bundles of male gametes), a new genus, Yamagishiella Nozaki, is propo...

A combined data set of DNA sequences (6021 bp) from five protein-coding genes of the chloroplast genome (rbcL, atpB, psaA, psaB, and psbC genes) were analyzed for 42 strains representing 30 species of the colonial Volvocales (Volvox and its relatives) and 5 related species of green algae to deduce robust phylogenetic relationships within the colonial green flagellates. The 4-celled family Tetrabaenaceae was robustly resolved as the most basal group within the colonial Volvocales. The sequence data also suggested that all five volvocacean genera with 32 or more cells in a vegetative colony (all four of the anisogamous/oogamous genera, Eudorina, Platydorina, Pleodorina, and Volvox, plus the isogamous genus Yamagishiella) constituted a large monophyletic group, in which 2 Pleodorina species were positioned distally to 3 species of Volvox. Therefore, most of the evolution of the colonial Volvocales appears to constitute a gradual progression in colonial complexity and in types of sexual reproduction, as in the traditional volvocine lineage hypothesis, although reverse evolution must be considered for the origin of certain species of Pleodorina. Data presented here also provide robust support for a monophyletic family Goniaceae consisting of two genera: Gonium and Astrephomene.

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.

Phylogenetic studies of approximately 2,000 bases of sequence from the large and small nuclear-encoded ribosomal RNAs are used to investigate the origins of the genus Volvox. The colonial and multicellular genera currently placed in the family Volvocaceae form a monophyletic group that is significantly closer phylogenetically to Chlamydomonas reinhardtii than it is to the other unicellular green flagellates that were tested, including Chlamydomonas eugametos, Chlorella pyrenoidosa, and Haematococcus lacustris. Statistical analysis of 251 phylogenetically informative nucleotide positions rejects the "volvocine lineage" hypothesis, which postulates a monophyletic evolutionary progression from unicellular organisms (such as Chlamydomonas), through colonial organisms (e.g., Gonium, Pandorina, Eudorina, and Pleodorina) demonstrating increasing size, cell number, and tendency toward cellular differentiation, to multicellular organisms having fully differentiated somatic and reproductive cells (in the genus Volvox). The genus Volvox appears not to be monophyletic. Volvox capensis falls outside a lineage containing other representatives of Volvox (V. aureus, V. carteri, and V. obversus), and both of these Volvox lineages are more closely related to certain colonial genera than they are to each other. This implies either a diphyletic origin of Volvox from different colonial volvocacean ancestors, a phylogenetic derivation of some of the colonial genera from a multicellular (i.e., Volvox) ancestor, or both. Considered together with previously published observations, these results suggest that the different levels of organizational and developmental complexity found in the Volvocaceae represent alternative stable states, among which evolutionary transitions have occurred several times during the phylogenetic history of this group.

Publisher Summary This chapter presents a summary of the morphological and biochemical evidence that supports the volvocine-lineage hypothesis. It describes attempts to evaluate the validity of this hypothesis objectively with molecular phylogenetic methods. The chapter also reviews advances that have occurred in three areas of Volvox research:(1) the genetic and molecular control of germ or soma differentiation, (2) the mechanism of action of the sexual pheromone, and (3) the comparative anatomy and evolution of selected structural genes of Volvox and Chlamydomonas. The volvocine algae (Chlamydomonas and the members of the family Volvocaceae) all have a fundamentally similar and distinctive pattern of subcellular organization. However, the volvocine algae can be conceptually lined up in a series (Chlamydomonas, Gonium, Pandorina, Eudorina, Pleodorina, and Volvox), in which there is a progressive increase in each of the parameters—cell number, organismic size, tendency for some cells to differentiate terminally as somatic cells, and ratio of somatic-cell number to reproductive-cell number. The volvocine lineage hypothesis postulates that this conceptual series may actually reflect the evolutionary history of the group. The aspect of Volvox carteri development that is most interesting from an evolutionary genetic standpoint is the aspect that most clearly differentiates it from other genera of volvocine algae, and from most other species of Volvox, i.e., the early and complete segregation of separate germ and somatic-cell lineages.

The morphology, reproduction and taxonomy of Pleodorina japonica Nozaki sp. nov. collected in Japan were studied using clonal cultures. This taxon is distinguished from the other species of Pleodorina and Eudorina by its unique vegetative colonies, in which only 20–27% of the cells are somatic and anterior, and in which the gelatinous matrix does not form individual sheaths. Zygote germination was observed for the first time in Pleodorina. A single biflagellate gone-cell moved out from the zygote wall. In addition, abundant bacteria-like endosymbionts in the cytoplasm of somatic and reproductive cells as well as in the male and female gametes and zygotes were revealed by means of 4'-6-diamidino-2-phenylindole (DAPI) staining and electron microscopy. The endosymbiont was rigidly rod-shaped, measured 1.5–1.9 μm long and emitted strong blue-white fluorescence under the epifluorescence microscope after DAPI staining, and it was surrounded by electron transparent areas. The generic delimitation betwee...

The colonial matrices of the volvocacean algae were examined for the presence of sulfated and carboxylated polysaccharides. These results were compared to a similar examination of the single-celled Chlamydomonas reinhardtii Dang. The colonial algae examined were Pandorina morum Bory, Eudorina elegans Ehr., Platydorina caudata Kofoid, Pleodorina californica Shaw, Pleodorina illinoisensis Kofoid and Volvox carteri var. nagariensis Iyengar. Alcian blue staining of whole colonies at pH 0.5 and 2.5 showed evidence for the presence of both sulfated and carboxylated polysaccharides in the extracellular matrix. Quantitative measurement of alcian blue bound to solubilized matrices supported the in vivo results. There was a trend toward an increase in sulfated polysaccharides in the more evolutionary advanced forms with the exception of Pleodorina. This trend was readily seen in the sulfate: carboxyl ratios: Pandorina morum—0.4, Eudorina elegans—1.0, Platydorina caudata—2.1 and Volvox carteri—2.2. The acidic nature of the Pleodorina matrix with a sulfate: carboxyl ratio of 0.2 appeared to be more like that of Pandorina rather than that of the more advanced Volvox.

PREVIOUS DESCRIPTIONS of mitosis in those colonial forms ascribed by Smith (1950) to the Volvocaceae present a very confusing picture. Considerable variation has been reported from genus to genus, especially with respect to the centrioles, persistence of the nuclear membrane, and nucleolus. These variations may be due to the fact that, with the exception of Pleodorina illinoisensis Kofoid, which has been investigated twice (Merton, 1908; Hovasse, 1937), the investigators were dealing with different species; or they may be due to the cytological methods used, or to differences in interpretation of the structures observed. Since, of necessity, one is usually dealing with fixed material in the study of mitosis, different interpretations of the chronology of events would lead to entirely different pictures of the process. Dangeard (1900), Merton (1908), and Metzner (1945) were unable to see centrioles or centrosomes in Pandorina morum Bory, Pleodorina illinoisensis Kofoid, and Volvox Carteri Stein, respectively, On the other hand, in Eudorina elegans Ehr. Hartmann (1921) described a small granule adjacent to the nucleolus (Binnenk6rper) which divided in prophase. The division products moved apart to form the spindle and could be seen as centrioles at the poles in metaphase. Zimmermann (1921) found a small body (Randk6rper) attached to the nucleolus (Binnenkorper) in the metabolic nucleus of Volvox aureus Ehr. He considered it to be an ergastic structure since it disappeared at prophase, and thought the centriole of Hartmann was just such a body. Zimmermann stated that centrioles were often seen at the poles of the intranuclear spindle. Doraiswami (1940) pictured a dark body near the nucleolus at prophase in Eudorina indica Iyengar. Although he did not see it divide, he found centriole-like structures at the poles at metaphase and thought it possible that they arose from division of the body observed at prophase. Chatton (1910) made similar observations on Pleodorina californica Shaw, and in addition, showed the centrioles connected by a "centrodesmose axiale." In Eudorina (Pleodorina) illinoisensis (Kofoid) Pascher, Hovasse (1937) termed the small body seeen in prophase a "satellite du caryosome." Since sometimes several bodies were present, he considered them merely the chromatic granules of the prophase chromosomes. The centrosomes which he noted throughout the mitotic cycle arose from the basal granules (grains basaux) of the flagella. In Vol

SYNOPSIS. A comparative morphological study was made of 73 clones of Eudorina isoated from 44 natural populations and grown under controlled environmental conditions. Utilizing the information obtained in this study in association with the existing taxonomic and experimental literature concerning Eudorina a monographic treatment of the genus was prepared. Nine species and 4 varieties are now recognized. Sexual compatibility was investigated with 22 heterothallic pairs, 3 male strains and one female strain representing 4 species and one variety of Eudorina. Five partially or completely sexually-isolated groups were found: 3 groups containing strains of a single species, a fourth group composed of strains of another species, and a fifth group made up of strains of 3 different species. Strains with different zygote arrangements were found generally not to intercross. Intraspecific crosses indicated that a single pair of alleles controls the inheritance of mating types and that gene flow occurs between strains representing different natural populations. Interspecific crosses resulted in the formation of polyploid or aneuploid F1 offspring which shewed an abnormal segregation of the mating types. Abnormal mating type segregation was also obtained in F2 crosses involving certain of the hybrid polyploid offspring.

Background Volvocine algae, which range from the unicellular Chlamydomonas to the multicellular Volvox with a germ–soma division of labor, are a model for the evolution of multicellularity. Within this group, the spheroidal colony might have evolved in two independent lineages: Volvocaceae and the goniacean Astrephomene. Astrephomene produces spheroidal colonies with posterior somatic cells. The feature that distinguishes Astrephomene from the volvocacean algae is lack of inversion during embryogenesis; the volvocacean embryo undergoes inversion after successive divisions to orient flagella toward the outside. The mechanisms of inversion at the molecular and cellular levels in volvocacean algae have been assessed in detail, particularly in Volvox carteri. However, embryogenesis in Astrephomene has not been subjected to such investigations. Results This study relied on light microscopy time-lapse imaging using an actively growing culture of a newly established strain to conduct a developmental analysis of Astrephomene as well as to perform a comparison with the similar spheroidal volvocacean Eudorina. During the successive divisions involved in Astrephomene embryogenesis, gradual rotation of daughter protoplasts resulted in movement of their apical portions toward the embryonic posterior, forming a convex-to-spheroidal cell sheet with the apical ends of protoplasts on the outside. Differentiation of the posterior somatic cells from the embryo periphery was traced based on cell lineages during embryogenesis. In contrast, in Eudorina, the rotation of daughter protoplasts did not occur during successive cell divisions; however, inversion occurred after such divisions, and a spheroidal embryo was formed. Indirect immunofluorescence microscopy of basal bodies and nuclei verified this difference between Astrephomene and Eudorina in the movement of embryonic protoplasts. Conclusions These results suggest different tactics for spheroidal colony formation between the two lineages: rotation of daughter protoplasts during successive cell divisions in Astrephomene, and inversion after cell divisions in Eudorina. This study will facilitate further research into the molecular and genetic mechanisms of the parallel evolution of the spheroidal colony in volvocine algae. Electronic supplementary material The online version of this article (doi:10.1186/s12862-016-0794-x) contains supplementary material, which is available to authorized users.

SUMMARY A culture strain, CCFA 646, originating from the People's Republic of China and designated as ‘Eudorina sp.’ in the Culture Collection of Freshwater Algae at the Institute of Hydrobiology, Chinese Academy of Sciences, was re-examined taxonomically based on light and electron microscopy and molecular phylogenetic analyses. Vegetative colonies of CCFA 646 were sub-spherical or ovoid and contained 16 or 32 cells of identical size. The cells had a cup-shaped chloroplast with a single basal pyrenoid. During asexual reproduction, each daughter colony developed within a transparent vesicle inside the parental colony. Under the electron microscope, the vegetative colony exhibited a tripartite colonial boundary of the extracellular matrix. Inside the boundary, each cell was tightly enclosed by a dense fi-brillar layer of the matrix (cellular envelope). These morphological characteristics suggested that CCFA 646 is assignable to Eudorina unicocca G. M. Smith or Ya-magishiella unicocca (Rayburn et Starr) Nozaki. In addition, 1128 base pairs of the large subunit of ribulose-1, 5-bisphosphate carboxylase/oxygenase (rbcL) gene were sequenced from CCFA 646 and NIES-578 of Y. unicocca. Phylogenetic trees were constructed based on these sequence data as well as on the previously published rbcL gene sequences from five strains of Y. unicocca, six strains of four Eudorina species and 28 related species. The sequence data suggested with high bootstrap values that CCFA 646 and all six strains of Yama-gishiella form a monophyletic group positioned outside the large monophyletic group comprising Eudorina, Pleo-dorina and Volvox (except for sect. Volvox). Therefore, CCFA 646 can be assigned to Y. unicocca, which has not previously been reported from P. R. China.