Earlier studies were unable to resolve the position of the species, and it was often placed as a sister to all other green algae, as one of the basal members of the Streptophyta, or as close to Chaetosphaeridium. More recent studies agree that Mesostigma, Chlorokybus, Spirotaenia form a clade, being the earliest diverging green algae.
Marin & Melkonian, 1999
Mesostigmatales Cavalier-Smith 1998
Mesostigmataceae Moestrup & Throndsen 1988
Mesostigma Lauterborn 1894 emend. McCourt 2005
|Mesostigma viride Lauterborn|
Charophyta is a group of freshwater green algae, sometimes treated as a division, but also as a superdivision, or an unranked clade. The terrestrial plants, the Embryophyta most likely emerged within Charophyta, possibly from terrestrial unicellular charophytes, with the class Zygnematophyceae as a sister group.The clade Streptophyta is formed by the grouping together of the Embryophyta and the Charophyta. The sister group of the Charophytes are the Chlorophyta. In some charophyte groups, such as the Zygnematophyceae or conjugating green algae, flagella are absent and sexual reproduction does not involve free-swimming flagellate sperm. Flagellate sperm, however, are found in stoneworts (Charales) and Coleochaetales, orders of parenchymatous charophytes that are the closest relatives of the land plants, where flagellate sperm are also present in all except the conifers and flowering plants. Fossil stoneworts of Devonian age that are similar to those of the present day have been described from the Rhynie chert of Scotland.Chlorophyta
Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a highly paraphyletic group of all the green algae within the green plants (Viridiplantae) and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. In newer classifications, it refers to the sister of the streptophytes/charophytes. The clade Streptophyta consists of the Charophyta in which the Embryophyta emerged. In this sense the Chlorophyta includes only about 4,300 species. About 90% of all known species live in freshwater.
Like the land plants (bryophytes and tracheophytes), green algae contain chlorophyll a and chlorophyll b and store food as starch in their plastids.
With the exception of Palmophyllophyceae, Trebouxiophyceae, Ulvophyceae and Chlorophyceae, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular. Some members of the group form symbiotic relationships with protozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to form lichens, but the majority of species are free-living. Some conduct sexual reproduction, which is oogamous or isogamous. All members of the clade have motile flagellated swimming cells. While most species live in freshwater habitats and a large number in marine habitats, other species are adapted to a wide range of land environments. For example, Chlamydomonas nivalis, which causes Watermelon snow, lives on summer alpine snowfields. Others, such as Trentepohlia species, live attached to rocks or woody parts of trees. Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.Cyanidiaceae
Cyanidiaceae is a family of red algae, one of two families in the Division Cyanidiophytina.Cyanidiophytina
Cyanidiophytina is a subdivision of red algae.In older texts it was described as an order "Cyanidiales". It was granted division status in the Saunders and Hommersand 2004 classification (as "Cyanidophyta"), but was only elevated to subdivision Cyanidiophytina in the Yoon et al. classification of 2006.Endohelea
Endohelea is a proposed clade of eukaryotes that are related to Archaeplastida and the SAR supergroup.Eumetazoa
Eumetazoa (Greek: εὖ [eu], well + μετά [metá], after + ζῷον [zóon], animal) or Diploblasts, or Epitheliozoa, or Histozoa are a proposed basal animal clade as a sister group of the Porifera.The basal Eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as an Eumetazoan in the Parahoxozoa.
Several other extinct or obscure life forms, such as Iotuba and Thectardis appear to have emerged in the group. Characteristics of eumetazoans include true tissues organized into germ layers, the presence of neurons, and an embryo that goes through a gastrula stage.
Some phylogenists have speculated the sponges and eumetazoans evolved separately from single-celled organisms, which would mean that the animal kingdom does not form a clade (a complete grouping of all organisms descended from a common ancestor). However, genetic studies and some morphological characteristics, like the common presence of choanocytes, support a common origin.Traditionally, Eumetazoans are a major group of animals in the Five Kingdoms classification of Lynn Margulis and K. V. Schwartz, comprising the Radiata and Bilateria — all animals except the sponges. When treated as a formal taxon Eumetazoa is typically ranked as a subkingdom. The name Metazoa has also been used to refer to this group, but more often refers to the Animalia as a whole. Many classification schemes do not include a subkingdom Eumetazoa.Green algae
The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, as well as the potentially more basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia.The land plants, or embryophytes, are thought to have emerged from the charophytes. Therefore, cladistically, embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. The clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and in flatworms. Some species of green algae, particularly of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. Trentepohlia is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is terrestrial, and
Prasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.Horneophytopsida
Horneophytopsida is a class of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue, found from the Late Silurian to the Early Devonian (around 430 to 390 million years ago). They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs (sporangia) on branched stems. They were formerly classified among the rhyniophytes, but it was later found that some of the original members of the group had simple vascular tissue and others did not.In 2004, Crane et al. published a cladogram for the polysporangiophytes in which the Horneophytopsida are shown as the sister group of all other polysporangiophytes. One other former rhyniophyte, Aglaophyton, is also placed outside the tracheophyte clade, as it did not possess true vascular tissue (in particular did not have tracheids), although its conducting tissue is more complex than that of the Horneophytopsida.List of plant orders
This article lists the orders of the Viridiplantae.Lycopodiophyta
The Division Lycopodiophyta (sometimes called lycophyta or lycopods) is a tracheophyte subgroup of the Kingdom Plantae. It is one of the oldest lineages of extant (living) vascular plants and contains extinct plants like Baragwanathia that have been dated from the Silurian (ca. 425 million years ago). These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. Most members of Lycopodiophyta bear a protostele, and the sporophyte generation is dominant. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.Lycopodiopsida
Lycopodiopsida is a class of herbaceous vascular plants known as the clubmosses and firmosses. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia at the bases of the leaves. Traditionally, the group also included the spikemosses (Selaginella and relatives) and the quillworts (Isoetes and relatives) but because these groups have leaves with ligules and reproduce using spores of two different sizes, both are now placed into another class, Isoetopsida that also includes the extinct Lepidodendrales. These groups, together with the horsetails are often referred to informally as fern allies.
The class Lycopodiopsida as interpreted here contains a single living order, the Lycopodiales, and a single extinct order, the Drepanophycales.Mesostigma
Mesostigma viride is a species of freshwater green algae. It is now considered to be one of the earliest diverging members of green plants/algae (Viridiplantae).
Earlier studies were unable to resolve the position of the species, and it was often placed as a sister to all other green algae, as one of the basal members of the Streptophyta, or as close to Chaetosphaeridium. More recent studies agree that Mesostigma and Chlorokybus form a clade, being the earliest diverging green plants.Opisthosporidia
Opisthosporidia (granting True Fungi) or Fungi are a sister clade of the Cristidiscoidea together forming the Holomycota. A basal group is the 'basal clone group 2' (BCG2). The other basal Opisthosporidan clade are the Aphelida together with the True Fungi, joined with the 'basal clone group 1' (BCG1) together with the Rozellomyceta. Historically, the True Fungi were not considered to have emerged in the Opisthosporidia, i.e. was not considered to be a descendant of the last common ancestor of Aphelida and Cryptomycota.Palmophyllophyceae
The Palmophyllophyceae are a proposed basal Chlorophyte clade consisting of the Palmophyllales and Prasinococcales.Palpitea
Palpitea is a proposed clade of eukaryotes that are related to Archaeplastida and the SAR supergroup.Phragmoplastophyta
The Phragmoplastophyta (Lecointre & Guyader 2006) or Streptophytina (Lewis & McCourt 2004, incl. Coleochaetophyceae, Zygnematophyceae and Mesotaeniaceae) are a proposed sister clade of the Klebsormidiaceae, with which they form the Streptophyte/Charophyte clade. The Phragmoplastophyta consist of the Charophycaea and another unnamed clade which contains the Coleochaetophyceae, Zygnematophyceae, Mesotaeniaceae, and Embryophytes (land plants). It is an important step in the emergence of land plants within the green algae. It is equivalent to the ZCC clade/grade, cladistically granting the Enbryophyta.The mitosis of Phragmoplastophyta takes place via a phragmoplast.
Another synapomorphy of this clade is the synthesis of cellulose microfibrils by a complex of octameric cellulose synthetases. This complex crosses the plasma membrane and polymerizes molecules from the cytoplasm into cellulose microfibrils, which, together with each other, form fibrils, necessary in the formation of the wall. The Phragmoplastophyte wall is also formed of phenolic compounds.
Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.Spirotaenia
Spirotaenia is a genus of basal green algae that may be sister to the Chlorokybophyceae. It was previously considered to be part of the Zygnemataceae. It is sexually conjugating, a mode of reproduction that was previously only known in the Zygnemataceae/Mesotaeniaceae, the sister groups to the land plants. This is surprising, as Spirotaenia is much more basal. The conjugating process is substantially aberrant. Spirotaenia may actually be more than one distinct lineage which may not be closely related.Streptophyta
Streptophyta, informally the streptophytes (from the Greek strepto, for twisted, i.e., the morphology of the sperm of some members), is a clade of plants. The composition of the clade varies considerably between authors, but the definition employed here includes land plants and all green algae except the Chlorophyta and possibly the more basal Mesostigmatophyceae, Chlorokybophyceae, and Spirotaenia.Viridiplantae
Viridiplantae (literally "green plants") are a clade of eukaryotic organisms made up of the green algae, which are primarily aquatic, and the land plants (embryophytes), which emerged within them. Green algae traditionally excludes the land plants, rendering them a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. They have cells with cellulose in their cell walls, and primary chloroplasts derived from endosymbiosis with cyanobacteria that contain chlorophylls a and b and lack phycobilins. More than 350,000 species of Viridiplantae exist.In some classification systems, the group has been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional plant kingdom to include the green algae. Adl et al., who produced a classification for all eukaryotes in 2005, introduced the name Chloroplastida for this group, reflecting the group having primary chloroplasts with green chlorophyll. They rejected the name Viridiplantae on the grounds that some of the species are not plants, as understood traditionally. The Viridiplantae are made up of two clades: Chlorophyta and Streptophyta as well as the basal Mesostigmatophyceae and Chlorokybophyceae. Together with Rhodophyta and glaucophytes, Viridiplantae are thought to belong to a larger clade called Archaeplastida or Primoplantae.
A taxonomic evaluation of eukaryotes based on myosin distribution showed the Viridiplantae lost class-I myosins.