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Plant 4, — Estevez, J. Chemical and in situ characterization of macromolecular components of the cell walls from the green seaweed Codium fragile. Glycobiology 19, — Feingold, D. Stumpf and E. Conn New York: Academic Press , — Fry, S. New Jersey: The Blackburn Press. Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells. CrossRef Full Text. Geisler-Lee, J.

Poplar carbohydrate-active enzymes. Gene identification and expression analyses. Plant Physiol. Goubet, F. Polysaccharide analysis using carbohydrate gel electrophoresis: a method to study plant cell wall polysaccharides and polysaccharide hydrolases. Hansen, S.

Plant Sci. Harholt, J. Biosynthesis of pectin. The glycosyltransferase repertoire of the spikemoss Selaginella moellendorffii and a comparative study of the cell wall structure. Hepler, P. Lignification during secondary wall formation in Coleus : an electron microscopy study. Enzymatic treatments reveal differential capacities for xylan recognition and degradation in primary and secondary plant cell walls. Iraki, N. Alteration of the physical and chemical structure of the primary cell wall of growth-limited plant cells adapted to osmotic stress. Cell walls of tobacco cells and changes in composition associated with reduced growth upon adaptation to water and saline stress.

Extracellular polysaccharides and proteins of tobacco cell cultures and changes in composition associated with growth-limiting adaptation to water and saline stress. Kenrick, P. The origin and early evolution of plants on land. Nature , 33— Lee, K. Plant cell wall biology: perspectives from cell wall imaging. Lerouxel, O. Rapid structural phenotyping of plant cell wall mutants by enzymatic oligosaccharide fingerprinting. Mackie, W. The occurrence of mannan microfibrils in the green algae Codium fragile and Acetabularia crenulata. Marcus, S. Restricted access of proteins to mannan polysaccharides in intact plant cell walls.

Matsunaga, T. Occurrence of the primary cell wall polysaccharide rhamnogalacturonan II in pteridophytes, lycophytes, and bryophytes: implications for the evolution of vascular plants. Mitchell, R. A novel bioinformatics approach identifies candidate genes for the synthesis and feruloylation of arabinoxylan. Mohnen, D. Pectin structure and biosynthesis. Moller, I. High-throughput mapping of cell-wall polymers within and between plants using novel microarrays. Mouille, G. Niklas, K. The cell walls that bind the tree of life. BioScience 54, — The evolution of the land plant life cycle.

Parre, E. Pectin and the role of the physical properties of the cell-wall in pollen tube growth of Solanum chacoense. Park, S. A role for CSLD3 during cell-wall synthesis in apical plasma membranes of tip-growing root-hair cells. Pattathil, S. A comprehensive toolkit of plant cell wall glycan-directed monoclonal antibodies. Pauly, M.

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Plant cell wall polymers as precursors for biofuels. Plant Biol. Popper, Z. Evolution and diversity of green plant cell walls. Beyond the green: understanding the evolutionary puzzle of plant and algal cell walls. Evolution and diversity of plant cell walls: from algae to flowering plants. Qu, Y. Roberts, A. Cell 1, — Sarkar, P.

Plant cell walls throughout evolution: towards a molecular understanding of their design principles. Scheible, W. Glycosyltransferases and cell wall biosynthesis: novel players and insights. Scheller, H. Stress-tolerant plants expr-essing mannosylglycerate-producing enzymes. Scherp, P. Occurrence and phylogenetic significance of cytokinesis-related callose in green algae, bryophytes, ferns and seed plants.

Plant Cell Reports 20, — How have plant cell walls evolved? An array of possibilities for pectin. A, Bacic, A. A, Fei, Z. The charophycean green algae provide insights into the early origins of plant cell walls. Spatz, H. Biomechanics of hollow stemmed Sphenopsids: II. Calamites — to have or not to have secondary xylem. Timme, R. Uncovring the evolutionary origin of plant molecular processes: comparison of Coleochaete Coleochaetales and Spirogyra Zygnematales transcriptomes.

BMC Plant Biol. Broad phylogenomic sampling and the sister lineage of land plants. Tsekos, I. The sites of cellulose synthesis in algae: diversity and evolution of cellulose-synthesizing enzyme complexes. Velasquez, S. L, Somerville, C. O -Glycosylated cell wall proteins are essential in root hair growth. Verhertbruggen, Y. Developmental complexity of arabinan polysaccharides and their processing in plant cell walls.

Vogel, J. Genome sequencing and analysis of the model grass Brachypodium distachyon. Nature , — Vreeland, V. Monoclonal antibodies as molecular probes for cell wall antigens of the brown alga Fucus. Lycophytes were tall trees during the Carboniferous period. Today, the only lycophytes that exist are small, herbaceous plants. What happened? Answer: Early land plants evolved cuticles, which are waxy layers that keep water from evaporating.

Since air is not as wet as water, plants that had a mechanism for keeping water in were well-situated to live on land. The earliest land plants were still limited in their growth because they did not have well-developed roots or leaves. Later, seedless vascular plants evolved roots that anchored them and helped them draw nutrients from the soil.

Answer: Although we can never predict what a plant will do in a new environment or predict invasions, the plant from Arizona has a better chance of becoming invasive. In fact, it has a better chance of surviving period, since it is probably already adapted to desert conditions.

Mini Review ARTICLE

Here we give an update of the current understanding of the primary endosymbiotic event that gave rise to the Archaeplastida. In addition we provide an overview of the diversity in the Rhodophyta, Glaucophyta and the Viridiplantae excluding the Embryophyta and highlight how genomic data is enabling us to understand the relationships and characteristics of algae emerging from this primary endosymbiotic event.

The origin of oxygenic photosynthesis has changed the face of our planet in all aspects. Estimates based on geological and geochemical evidence, and molecular phylogenetic analyses calibrated with the fossil record agree on a minimum age of 2. Because oxygenic photosynthesis involves the photolysis of water into electrons, protons, and free oxygen, Cyanobacteria are singularly responsible for oxygenating the atmosphere and transforming a once reducing environment into an oxidising one Holland, With oxygen becoming gradually available as a very potent electron acceptor, the path lay open for aerobic organisms to evolve.

Aerobes soon managed to maintain much more productive ecosystems as more energy per electron transfer could be harvested. The rising atmospheric oxygen is thought to have directly triggered cellular compartmentalization and eukaryogenesis Fig 1E. When oxygen levels rose the constraint likely decreased permitting larger and more communication-related transmembrane proteins opening the door for subsequent compartmentalization.

The fossil record Knoll et al. The cyanobacterium was gradually enslaved and integrated into the cellular machinery as a new organelle: the plastid. This event has been termed primary endosymbiosis. The cyanobacterial origin of plastids is supported by overwhelming genetic evidence and ultrastructural similarities between plastids and their cyanobacterial relatives Box 1. The original cyanobacterial genome underwent a drastic reduction with most genes either lost or transferred to the host nucleus, termed endosymbiotic gene transfer EGT.

Genes that have been transferred to the host nucleus are transcribed and translated in the host cytosol or endoplasmic reticulum and are targeted back to the chloroplast using a protein import system Bhattacharya et al. In contrast to what might be intuitively expected, also gene products of host origin can be plastid-targeted and only a subset of cyanobacterial genes takes up a function in the organelle Deusch et al. Next to sequence divergence, amelioration and modularity of transferred genes are thought to be additional complicating factors to detect horizontal gene transfer Chan et al.

Remarkably, some phylogenomic analyses, with the exception of the glaucophyte study of Reyes-Prieto et al. Three extant groups of photosynthetic eukaryotes have primary plastids: the green plants, red algae, and the glaucophytes. Together they make up the Archaeplastida. Even though the cyanobacterial origin of the plastids in these groups is beyond dispute, the number of endosymbiotic events and the relationships among the three lineages is more contentious Delwiche, ; Delwiche, For a long time, variation in plastid structure and light-harvesting pigments has given credit to a polyphyletic origin of primary plastids, i.

Recent evidence points toward a single origin of primary plastids, which implies a single ancestor of the plastid as well as the monophyly of the three lineages that make up the Archaeplastida Rodriguez-Ezpeleta et al. As pointed out by Larkum et al. There is at least one exception to this rule: Paulinella chromatophora , a cercozoan amoeba with photosynthetic inclusion of cyanobacterial origin Marin et al. Even though several analyses provide moderate to strong support for a monophyletic Archaeplastida Burki et al. The incongruence between analyses is likely caused by systematic biases including endosymbiotic gene transfer as suggested by the high instability of resultant topologies of photosynthetic clades with varying levels of taxon sampling and missing data Parfrey et al.

Indeed gene sampling has been shown to account for at least some of the incongruence among the relationships of primary plastid lineages Inagaki et al. The persistent incongruence of large concatenated datasets shows that a solution may not be found by increasing sequence length Baurain et al. Instead when relaxing the assumption of vertical gene transfer by abandoning concatenation and choosing for a gene-by-gene approach, Chan et al. In the light of the persistent uncertainty on the monophyly of Archaeplastida, it may not come as a surprise that the relationships between green plants, red algae and glaucophytes are still unclear.

Phylogenetic gene analyses are unfortunately not conclusive on the relationships between the major clades of the Archaeplastida Rodriguez-Ezpeleta et al. Furthermore, several studies point towards an early diverging red algal lineage Burki et al. Therefore analyses concentrating on EGT genes of cyanobacterial origin only might be more trustworthy. Under the assumption of a single origin of primary plastids, the question remains what kind of cyanobacterium participated in the origin of plastids.

Unfortunately, due to the large divergence times and the considerable extent of horizontal gene transfer between cyanobacteria Deusch et al. In addition, it is difficult to determine when this primary endosymbiosis occurred. Estimates based on fossil evidence and biomarkers are widely divergent Knoll, A recent calibrated phylogeny of Parfrey et al.

Following the origin of Archaeplastida, photosynthesis spread widely among diverse eukaryotic groups via secondary and tertiary endosymbiotic events Archibald, ; Gould et al. Overviews of the intricate histories of plastid acquisition are provided in Archibald The red algae or Rhodophyta are a distinct lineage of eukaryotic algae, containing about 5,—6, species of mostly multicellular, marine algae. The red algae are distinguishable amongst eukaryotic lineages by a combination of biochemical and ultrastructural features, some of which they share with Glaucophyta and Cyanobacteria.

First, red algal plastids lack chlorophyll accessory pigments. Instead light energy is directed to the reaction centre by phycobiliproteins phycocyanin, allophycocyanin, and phycoerythrin. Light harvesting antennae pigments are grouped in hemispherical protein complexes, phycobilisomes, anchored to the thylakoids. These are not stacked in grana like in the Viridiplantae, but lie singly and more or less equidistant in the plastid stroma. From the early twentieth century until very recently red algae were classified in two distinct groups, most commonly treated as classes, Bangiophyceae and Florideophyceae, within a single phylum, Rhodophyta.

This dichotomy in the classification is reflected in the morphological complexity that characterizes the red algae, with the Bangiophyceae uniting the morphologically simple forms unicells, or undifferentiated filaments and blades and the Florideophyceae containing the more complex growth forms.

Growth in the Florideophyceae is essentially filamentous but individual filaments may aggregate to form a pseudoparenchymatous tissue. Growth forms include filaments, blades, elaborately branched thalli as well as calcified crusts coralline algae. The structurally simple Bangiophyceae is composed of a series of radiations that define the ancestral lineages of the red algae, and part of the traditionally circumscribed Bangiophyceae is more closely related to the Florideophyceae.

The phylum Rhodophyta is now subdivided into two subphyla, Cyanidiophytina and Rhodophytina, and seven classes, Cyanidiophyceae, Bangiophyceae, Compsopogonophyceae, Florideophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae Fig. The diversity contained in the Compsopogonophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae is still ill-defined as can be witnessed by the many unnamed lineages that typically adorn phylogenetic trees Scott et al. The taxonomic affinity of Bangiomorpha was long contested, with some authors e. Cavalier-Smith, advocating that Bangiomorpha is a blue-green alga or a mixture of at least two species of blue-green algae, possibly related to the Stigonematales.

Bangiomorpha being a eukaryotic fossil would indicate a Mesoproterozoic origin of red algae and by extension all major lineages of the eukaryotes, which contradicts the hypothesis of Cavalier-Smith that places eukaryogenesis at Ma Cavalier-Smith, A red algal nature of Bangiomorpha , however, is not in conflict with the most recent timing of eukaryotic diversification using calibrated phylogenies Parfrey et al.

In addition to the fossils indicative of a Mesoproterozoic origin of red algae, the remarkably well-preserved multicellular red algae from the Doushantuo Formation in southern China ca. The updated classification of the red algae also better reflects the ultrastructural and ecological diversity of the group. Of special interest are the Cyanidiophyceae, a group of unicellular and presumably asexual algae, which live in thermo-acidophilic conditions that are detrimental to most eukaryotic live on earth Barbier et al.

Yoon et al. Even though a reinterpretation of this event is necessary now that the chromalveolate hypothesis is being increasingly challenged Archibald, ; Baurain et al. Because the Cyanidiophyceae are one of few eukaryotic groups that thrive in environments that are otherwise dominated by Archaea and Bacteria, their enzymes are of special interest to the biotechnology and pharmaceutical industry.

It is, therefore, not surprising that Cyanidioschyzon merolae was the first eukaryotic alga for which a full genome sequence was available Matsuzaki et al. The small and compact genome of C. Only seven Cyanidiophyceae species are currently described but this number severely underestimates the diversity. Applying environmental sequencing from a single locality in Italy, Ciniglia et al. The great majority of red algae are multicellular, marine seaweeds, with an enormous range of morphologies and complex haplo-diploid life histories which involve additional zygote amplification stages resulting in large numbers of spores from a single fertilization Verbruggen et al.

Red seaweeds belong nearly exclusively to two classes, Bangiophyceae in the narrow sense of the new classification and Florideophyceae Fig. Traditionally, two genera belonging to a single family, Bangiaceae, have been recognized in the Bangiophyceae. Regrettably, this taxonomic vigour also resulted in the fact that the Porphyra , which has wrapped sushi for decades, has now become a Pyropia Zuccarello, The near-morphological stasis that characterizes the Bangiophyceae, contrasts sharply with the wealth of growth forms that is encountered in its sister taxon, the Florideophyceae.

DNA sequence data have progressively refined an ordinal classification e. Choi et al. Ordinal relationships, however, remain at least partly unresolved Verbruggen et al. Genomic data of red seaweeds are currently limited to a number of organelle genomes and EST libraries of commercially important species such as Chondrus , Gracilaria , and Porphyra Asamizu et al. The green plant clade Viridiplantae includes green algae and embryophytic land plants, and is one of the main groups of photosynthetic eukaryotes.

Green plants are diverse in terms of species number, morphology, biochemistry and ecology. Monophyly of the group is well established based on ultrastructural, biochemical and molecular data Leliaert et al. Green plants share a number of unique characteristics. The chloroplasts are surrounded by a double membrane, have thylakoids grouped in lamellae, and contain chlorophyll a and b along with some accessory pigments including carotenoids and xanthophylls.

Pyrenoids when present are embedded within the chloroplast and are surrounded by starch, which is the main reserve polysaccharide.

Cell walls when present are generally composed of cellulose. Many green algae are flagellates or have flagellate cells in some stage of the life cycle. The flagella generally two or four on a cell are isokont, which means that they are similar in structure, although they may differ in length.

The region between the flagellar axoneme and the basal body is characterized by a stellate structure Graham et al. Apart from these unifying ultrastructural and biochemical features, green plants are extremely diverse morphologically. They range from unicells with sizes comparable to bacteria to large and complex multicellular or siphonal life forms. Although the described species diversity of land plants including over , species exceeds that of green algae about 15, named species , green algae encompass a greater cytomorphological, biochemical and reproductive diversity, which reflects their old evolutionary age Leliaert et al.

The progenitor of green plants was likely a unicellular flagellate or at least had flagellate stages in its life cycle. Colonial and multicellular forms have evolved multiple times in several lineages, including the Streptophyta, Ulvophyceae, Chlorophyceae, Trebouxiophyceae and Palmophyllales Fig.

Green plants are also ecologically very diverse. Embryophytes have dominated terrestrial habitats for millions of years; some land plants have adapted secondarily to freshwater or marine environments. The ecological importance of green algae has been mainly in marine and freshwater environments. The origin of land plants from a green algal ancestor was a key event in the evolution of life on earth.

An early split in the evolution of green plants gave rise to two main clades: the Chlorophyta and Streptophyta Leliaert et al. The Chlorophyta probably diversified as unicellular algae in the Meso- and Neoproterozoic. This early radiation of Chlorophyta was important to the eukaryotic greening that shaped the geochemistry of our planet Worden et al.

During the Mesozoic, the dominance of marine green algae in the phytoplankton gradually decreased as they were largely displaced by the red-plastid-containing dinoflagellates, coccolithophores and diatoms Falkowski et al. These ancestral green unicells gave rise to modern prasinophytes and the core Chlorophyta that diversified as unicellular and multicellular organism in marine, freshwater and terrestrial habitats.

Ancestral charophytes invaded the land during the mid-Ordovician and early Silurian million years ago , giving rise to the land plants Delaux et al. Molecular phylogenetic studies have drastically reshaped our views of green plant evolution and continue to do so Leliaert et al. However, many uncertainties remain, especially about the deepest branches of the green plants.

Phylogenetic hypotheses are critical in providing an evolutionary framework for comparative genomic studies. In the following section, we give a brief overview of the major green plant lineages and their relationships. The Chlorophyta form a large and morphologically diverse clade of marine, freshwater and terrestrial green algae. The orientation of this flagellar root system has been an important character for defining the main groups of Chlorophyta. Molecular data has revealed several major chlorophytan clades. Several early diverging clades of unicellular algae, collectively termed the prasinophytes, form a paraphyletic assemblage at the base of the chlorophytan tree.

These clades are relatively species-poor compared to the three principal clades of the core Chlorophyta: Ulvophyceae, Trebouxiophyceae and Chlorophyceae Fig. Prasinophytes form a heterogeneous assemblage of mostly unicellular algae with diverse cell shapes that are naked, covered by walls or organic body scales; flagella are present or absent Leliaert et al. Prasinophytes are predominantly found in marine environments, although several species also occur in freshwater. About ten distinct prasinophyte lineages have been identified, but their phylogenetic affinities remain largely unresolved Leliaert et al.

The Nephroselmidophyceae includes flagellates with complex scale covering, and is possibly one of the earliest diverging chlorophytan lineages Turmel et al. Future population genomic studies may enable us to estimate the prevalence of sexual recombination in algae Toulza et al. The Mamiellales include marine and freshwater flagellates and coccoid forms. Species of Ostreococcus and Micromonas are among the smallest eukaryotes known, with cell sizes of 0. We refer to Toulza et al. The Pyramimonadales includes large flagellates covered by complex body scales, found in marine and freshwater environments.

The Picocystis clade includes the coccoid Picocystis from saline lakes. Several other prasinophytic groups have uncertain phylogenetic affinities. These include the Pycnococcaceae, a clade of marine flagellate and coccoid species Nakayama et al. The Palmophyllales includes green algae from dimly lit benthic marine habitats. These algae feature a unique type of multicellularity, forming well-defined macroscopic bodies composed of small spherical cells embedded in a firm gelatinous matrix.

Phylogenetic analysis either places the Palmophyllales as the sister clade to all other Chlorophyta or allies it with the Prasinococcales Leliaert et al. The core Chlorophyta evolved from one of the ancestral prasinophytic lineages probably somewhere in the Neoproterozoic Herron et al. The core Chlorophyta includes the species-poor and early-diverging Pedinophyceae marine and freshwater uniflagellates and Chlorodendrophyceae marine and freshwater quadriflagellates , and the large and diverse clades, Trebouxiophyceae, Ulvophyceae and Chlorophyceae TUC Leliaert et al.

The TUC clades include a wide variety of morphological forms and eco-physiological features. Unlike the prasinophytes, where sexual reproduction has rarely been observed, the core chlorophytes encompass a large diversity of life cycle strategies, many of which involve sexual reproduction. Marine members of the Ulvophyceae generally have life cycles involving an alternation between two free-living multicellular phases a haploid gametophyte and diploid sporophyte.

Many freshwater Chlorophyceae and Trebouxiophyceae have a haploid vegetative phase and a single-celled, often dormant zygote as the diploid phase. Conversely, terrestrial members of the core chlorophytes are mainly asexual Rindi, We refer Umen and Olson for a review on the evolution of sex in the chlorophycean green algae Chlamydomonas and Volvox.

A new mode of cell division likely evolved in the clade uniting the Chlorodendrophyceae and the TUC clade and was subsequently lost in the Ulvophyceae Leliaert et al. This type of cell division is mediated by a phycoplast, which is an array of microtubules oriented parallel to the plane of cell division, determining the formation of a new cell wall Graham et al. Furthermore, some phylogenetic studies showed that at least the Trebouxiophyceae and Ulvophyceae might not be monophyletic e.

The Trebouxiophyceae includes flagellates, coccoids, colonies, and multicellular filaments and blades. The group is predominantly freshwater or terrestrial; some members occur in brackish or marine habitats. Analysis of the complete genome of Chlorella variabilis NC64A an endosymbiont of the ciliate Paramecium bursaria has provided insights into the genetic facilitation of an endosymbiotic lifestyle Blanc et al.

In particular expansion of protein families containing protein—protein interaction domains and adhesion domains could have been involved in adaptation to symbiosis. Although Chlorella and many other members of Trebouxiophyceae has been assumed to be asexual and nonmotile, meiosis- and flagella-specific proteins have been found in its genome, suggestive of cryptic sex and involvement of a flagella-derived structure in sexual reproduction Blanc et al.

The Chlorophyceae includes flagellates, coccoids and various colonial and multicellular forms. The group occurs mainly in freshwater and to a lesser extent in terrestrial habitats; some are marine Klochkova et al. Five main lineages have been recognized: the speciose and diverse Sphaeropleales and Chlamydomonadales including some of the most common freshwater phytoplankters, and the smaller clades, Chaetophorales, Oedogoniales and Chaetopeltidales Leliaert et al.

The unicellular flagellate Chlamydomonas has been extensively studied as a model for photosynthesis, chloroplast biogenesis, flagellar assembly and function, cell-cell recognition, circadian rhythm and cell cycle control Grossman et al.

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Analysis of the complete genomes of Chlamydomonas reinhardtii and Volvox carteri has provided important genetic insights into the evolution of multicellularity and sex Merchant et al. The Ulvophyceae includes unicells and multicellular algae, as well as giant-celled forms with unique cellular characteristics Cocquyt et al. Ulvophytes are generally known as macro-algae growing along marine coasts green seaweeds. Species in the Ulvales, Bryopsidales and Cladophorales frequently dominate rocky shores, tropical lagoons and reefs.

Some species of Ulva can form extensive, free-floating blooms, known as green tides Ye et al. Several ulvophytes e. Ulva and Cladophora have secondarily adapted to freshwater environments. Some early diverging lineages Oltmannsiellopsidales and Ignatius include microscopic organisms occurring in freshwater or terrestrial habitats, indicating that the ancestral ulvophytes may have been freshwater or terrestrial unicells Cocquyt et al. The Streptophyta include a paraphyletic assemblage of green algae charophytes and the land plants.

Charophytes range in morphology from unicellular to complex multicellular organisms, and occur in freshwater or moist terrestrial habitats. Streptophyta share a number of unique traits, including motile cells when present with two subapically inserted flagella and an asymmetrical flagellar apparatus that contains a distinctive multilayered structure and parallel basal bodies; open mitosis with a persistent mitotic spindle; and several unique enzymes Leliaert et al.

There are six main lineages of charophytes: Mesostigmatophyceae, Chlorokybophyceae, Klebsormidiophyceae, Zygnematophyceae, Charophyceae and Coleochaetophyceae McCourt et al. Many phylogenetic studies have aimed to resolve the relationship among these lineages, and in particular to determine the origins of land plants Karol et al.

Mesostigma Mesostigmatophyceae and Chlorokybus Chlorokybophyceae form the earliest-diverging streptophytic lineages Timme et al. Mesostigma is a flagellate covered with diverse organic scales, and is found in freshwater habitats. Chlorokybus forms packets of a few non-motile cells, and grows in moist terrestrial environments McCourt et al. The freshwater or terrestrial filamentous Klebsormidiophyceae diverged after the Mesostigmatophyceae and Chlorokybophyceae. In contrast to these three early-diverging lineages that undergo cell division by furrowing, the remaining lineages Charophyceae, Zygnematophyceae, Coleochaetophyceae and the land plants evolved a new mechanism of cell division involving a phragmoplast, which consists of an array of microtubules oriented perpendicularly to the plane of cell division, determining the formation of the cell plate and new cell wall.

Species in the three early-diverging lineages have never been observed to reproduce sexually, in contrasts to the remaining streptophytes where sex is widespread McCourt et al. The Zygnematophyceae conjugating green algae is a species-rich and morphologically diverse clade, including non-motile unicells, filaments and small colonial forms.

Sexual reproduction occurs by a unique process of conjugation, involving fusion of non-motile gametes. Flagellate stages are completely absent. The Charophyceae stoneworts include freshwater algae with complex macroscopic bodies composed of a main axis with whorled branches. Growth is by an apical meristematic cell. Sexual reproduction is oogamous with oogonia and antheridia surrounded by sterile cells.

Charophyceae are well represented in the fossil record, which a large diversity extending back to the Silurian McCourt et al. The Coleochaetophyceae is a small clade of branched filaments that sometimes form discoid parenchymatous thalli Graham, Based on morphological similarities with embryophytes, Coleochaete has traditionally been put forward as the closest relative of land plants Graham, ; Graham et al.

For example, some species of Coleochaete have corticated zygotes that are retained on the mother plant from which they receive nourishment via placental transfer cells with wall ingrowths. Also, cytokinesis and phragmoplast formation are similar to land plants Graham et al. Identifying the closest living relative of land plants has proven to be a difficult task Cocquyt et al.

Two recent studies based on broad phylogenomic sampling have suggested the Zygnematophyceae as sister lineage of the land plants Timme et al. Although the two lineages share few morphological characteristics, the sister relationship between Zygnematophyceae and land plants is supported by some cellular and molecular features, including similarities in auxin signalling De Smet et al.

The importance of the group lies mainly in its critical phylogenetic position, branching deeply within the Archaeplastida Fig. Similar to red algae, plastids have unstacked thylakoids and light-harvesting proteins organized into phycobilisomes. About 13 species have been described in five genera: Glaucocystis , including coccoid cells with a cellulosic wall and two short rudimentary flagella; Cyanophora and Peliaina are wall-less flagellates with two heterokont flagella; and Gloeochaete and Cyanoptyche , including non-motile cells in a gelatinous matrix, reproducing by motile or non-motile spores Schenk, To date, the genome of a single glaucophyte has been sequenced Price et al.

Genomic data and a better understanding of the phylogenetic position of glaucophytes will provide valuable insights into the endosymbiotic origin and evolution of plastids in eukaryotes. Genomic data are rapidly accumulating. Whole-genome data provide a great resource for analysis of eukaryotic genome evolution and user friendly online platforms for exploring this genome information is becoming increasingly available e.

Acquisti, C. Oxygen content of transmembrane proteins over macroevolutionary time scales. Nature , Allen, J. Evolutionary biology — Out of thin air. Archibald, J. The evolution of algae by secondary and tertiary endosymbiosis. Advances in Botanical Research, 64, 87— Asamizu, E.

Journal of Phycology 39, Baldauf, S. An overview of the phylogeny and diversity of eukaryotes. Journal of Systematics and Evolution 46, Barbier, G. Comparative genomics of two closely related unicellular thermo-acidophilic red algae, Galdieria sulphuraria and Cyanidioschyzon merolae , reveals the molecular basis of the metabolic flexibility of Galdieria sulphuraria and significant differences in carbohydrate metabolism of both algae.

Plant Physiology , Baurain, D. Molecular Biology and Evolution 27, Becker, B.

Evolution and Diversity

Streptophyte algae and the origin of embryophytes. Annals of Botany , Berney, C. A molecular time-scale for eukaryote evolution recalibrated with the continuous microfossil record. Bhattacharya, D. How do endosymbionts become organelles? Understanding early events in plastid evolution. BioEssays 29, Comparisons of nuclear-encoded small-subunit ribosomal RNAs reveal the evolutionary position of the Glaucocystophyta.

Molecular Biology and Evolution 12, Division Glaucocystophyta. Plant Systematics and Evolution Supplement 11, Blanc, G. The Chlorella variabilis NC64A genome reveals adaptation to photosymbiosis, coevolution with viruses, and cryptic sex. Plant Cell 22, Buick, R. When did oxygenic photosynthesis evolve?

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Burki, F. Large-scale phylogenomic analyses reveal that two enigmatic protist lineages, Telonemia and Centroheliozoa, are related to photosynthetic Chromalveolates. Genome Biology and Evolution 1, The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins. Doi: Phylogenomics reshuffles the eukaryotic supergroups. Plos One 2. Butterfield, N. Bangiomorpha pubescens n. Paleobiology 26, Canfield, D. Early anaerobic metabolisms.