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8.16C: Opisthokonts - Animals and Fungi - Biology

8.16C: Opisthokonts - Animals and Fungi - Biology


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Learning Objectives

  • Describe Opistkokonts

The opisthokonts, or “fungi/metazoa group”, are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum choanozoa (previously assigned to the protist “kingdom”). Both genetic and ultrastructural studies strongly support that opisthokonts form a monophyletic group.

One common characteristic of opisthokonts is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the group its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagellae. Most fungi do not produce cells with flagellae, but the primitive fungal chytrids do, suggesting that a common ancestor of current fungal species did have a flagellum.

The close relationship between animals and fungi was suggested by Cavalier-Smith in 1987, who used the informal name opisthokonta (the formal name has been used for the chytrids). The discovery was confirmed by later genetic studies. Early phylogenies placed opisthokonts near the plants and other groups that have mitochondria with flat cristae, but this character varies. Cavalier-Smith and Stechmann argue that the uniciliate eukaryotes such as opisthokonts and Amoebozoa, collectively called unikonts, split off from the other biciliate eukaryotes, called bikonts, shortly after they evolved.

Opisthokonts are divided into Holomycota or Nucletmycea (fungi and all organisms more closely related to fungi than to animals) and Holozoa (animals and all organisms more closely related to animals than to fungi); no opisthokonts basal to the Holomycota/Holozoa split have yet been identified.

Key Points

  • The unifying feature of opisthokonts is the presence of a flagellum, sometimes only ancestrally or at a specific point in the life cycle.
  • Genetic sequencing has confirmed that opisthokonts are genetically related.
  • Opisthokonts are split into two groups: holomycota (includes fungi), and holozoa (includes animals).

Key Terms

  • monophyletic: Of, pertaining to, or affecting a single phylum (or other taxon) of organisms.
  • flagellum: In protists, a long, whiplike membrane-enclosed organelle used for locomotion or feeding.

Amorphea

Amorphea [1] are members of a taxonomic supergroup that includes the basal Amoebozoa and Obazoa. That latter contains the Opisthokonta, which includes the Fungi, Animals and the Choanomonada, or Choanoflagellates. The taxonomic affinities of the members of this clade were originally described and proposed by Thomas Cavalier-Smith in 2002. [2] [4]

The International Society of Protistologists, the recognised body for taxonomy of protozoa, recommended in 2012 that the term Unikont be changed to Amorphea because the name "Unikont" is based on a hypothesized synapomorphy that the ISP authors and other scientists later rejected. [1] [5]


History

The close relationship between animals and fungi was suggested by Thomas Cavalier-Smith in 1987, [3] who used the informal name opisthokonta (the formal name has been used for the chytrids by Copeland in 1956), and was supported by later genetic studies. [13]

Early phylogenies placed fungi near the plants and other groups that have mitochondria with flat cristae, but this character varies. More recently, it has been said that holozoa (animals) and holomycota (fungi) are much more closely related to each other than either is to plants, because opisthokonts have a triple fusion of carbamoyl phosphate synthetase, dihydroorotase, and aspartate carbamoyltransferase that is not present in plants, and plants have a fusion of thymidylate synthase and dihydrofolate reductase not present in the opisthokonts. Animals and fungi are also more closely related to amoebas than to plants, and plants are more closely related to the SAR supergroup of protists than to animals or fungi. Animals and fungi are both heterotrophs, unlike plants, and while fungi are sessile like plants, there are also sessile animals.

Cavalier-Smith and Stechmann [14] argue that the uniciliate eukaryotes such as opisthokonts and Amoebozoa, collectively called unikonts, split off from the other biciliate eukaryotes, called bikonts, shortly after they evolved.


8.16C: Opisthokonts - Animals and Fungi - Biology

The Opisthokonta is a large supergroup of eukaryotes including metazoans and fungi. In addition, the Opisthokonta also includes some flagellate (choanoflagellates), amoeboid (e.g. Nuclearia ) and sporozoan (e.g. Ichthyosporea, Microsporidia) protists. The opisthokonts are phagotrophic or osmotrophic (saprobic, parasitic). Some membrs live symbiotically with land plants or algae (e.g. lichens, mycorrhizal fungi, corals).

One of the most characteristic features of the Opisthokonta is the architecture of flagellate cell, and this feature is origin of the name 'Opistho-konta'. Flagellate cell possesses a single flagellum inserted posteriorly. Mitochondrial cristae are usually flat. The opisthokonts sometimes possess 'filipodia' supported by actin filaments. The filopodia are tapering and branching in fungi and nucleariids, but those (sometimes called 'tentacles') are not in the Filozoa. In the choanoflagellates and sponges (Metazoa), the filopodia form a particle-capturing apparatus, the collor arround flagellum. Metazoa has the most advanced multicellular organization. Some proteins for intercellular junction and interaction are found in Ministeria .

Because Corallochytrium (Corallochytrea) possess &alpha-aminoadipate reductase (&alpha-AAR) involved in the &alpha-aminoadipate (AAA) pathway, this organism is sometimes considered to be closely related to fungi than to animals (Sumathi et al. 2006. Protist 157: 363-376). Protistan opisthokonts are sometimes classified into the Choanozoa or Mesomycetozoa. However, these taxa in this sense are not monophyletic. The microsporidia and myxozoans were traditionally classified into protozoa (sporozoa). However, recent studies indicate that they are simplified fungi and animals, respectively.


Opisthokont

The name that I have chosen for this blog is bound to be an unfamiliar term for most people. It probably represents an unfamiliar concept as well, so I will start gently. It is a term that denotes a specific, large, and (to humans) very important group of organisms. The importance of the group is probably understandable once one understands that humans are opisthokonts. So, however, are all other animals. So are all fungi! Rounding out the group are a number of single-celled organisms known to be related to either animals or fungi or both. Notably absent from this group are plants, algae, and quite a few other organisms, including all bacteria.

What can animals and fungi have in common that plants do not? Well, think of a sperm cell. This is a tadpole-like thing, with a roughly spherical cell body and a single, tail-like flagellum trailing behind. The cell swims by wiggling its flagellum, again much like a tadpole swims by wiggling its tail. As it happens, this is a very unusual cell type. Most other flagellated organisms swim with their flagella in front, pulling themselves through the surroinding medium. Only the flagellated cells found in animals, fungi, and related microbes swim with their flagella behind. This gives rise to the name: "opistho-" means "behind", and "-kont" refers to the flagellum.

One could easily be forgiven for finding this a minor difference, given everyday experience. To most of us, animals are things that move around and eat things, and plants and fungi are rooted in the ground. However, there are animals that are rooted to the ground as well (including sponges, corals, and sea squirts), and animals that do not eat (such as some of the worms living near deep sea hydrothermal vents). There are fungi that do not grow in the ground (yeasts are fungi, for instance, and do not root themselves in anything). Everyday experience, it turns out, is insufficient to categorise life science has moved well beyond that.

Science has taken its time to get to where it is today, though. Decades ago, fungi were classed amongst the "lower plants" because their cells are surrounded by rigid cell walls, a characteristic then thought to define a "plant". However, it has since been shown that the materials that make up those cell walls are completely unrelated (they are derived from sugars in plants and from proteins in fungi, for instance). More importantly, the organisms indisputably most closely related to each, which look like single-celled versions of their better-known counterparts, lack any vestige of the cell wall. In other words, the common ancestor of plants and fungi did not have a cell wall this character is a homoplasy, something that evolved more than once in the history of life.

Genetic analysis confirms this. Most hypotheses of evolutionary history (of extant organisms, anyway) are now made by having computers analyse the DNA of comparable genes from different organisms, and these tend to connect animals to fungi, to the exclusion of plants. (There are exceptions -- there are always exceptions -- but those are from genes with a lot of evolutionary "noise". In other words, such genes are either not large enough or evolve too quickly to retain enough information to resolve the animal/plant/fungus relationship with any reliability. There are statistical tests that indicate the trustworthiness of these computer analyses, and those which are judged acceptable almost always support the close relationship between animals and fungi.)

Analysis of genes goes beyond using them to reconstruct evolutionary history directly. For instance, there is an insertion into one of the genes used in the replication of DNA, an extra stretch of about fifty nucleotides (the "letters" of DNA's "alphabet"), which is found in animals, fungi, and their close relatives, and nothing else. This might not seem particularly important, but the gene in question is important enough that it is not prone to change easily (in scientific parlance, it is "evolutionarily conserved"), and perhaps more importantly, the insertion is itself conserved. In other words, the same nucleotides (or some obvious derivation of them) are present in the same place in all opisthokonts.

Non-genetic data helps link the two groups as well. The architecture of individual cells in the single-celled relatives of animals and fungi is strikingly similar, both inside and out. This was not apparent until the advent of electron microscopy many of the features that link the two groups are either too small to be seen with a light microscope (the "regular" kind) or are easily overlooked in favour of other, more striking features, many of which (like the cell walls already mentioned) can be taken to imply connections that do not hold up when investigated through other techniques.

These features include the arrangements of the components of the cytoskeleton, a set of protein-based rods and tubes that gives a cell its shape. The arrangement and replication of the flagella is also thought to be a conserved trait. A substantial part of my graduate work is investigating these things while the coherence of the opisthokonts as a group is nowadays almost beyond question, the uniqueness of some of its defining characteristics is simply not known. Electron microscopy has not been around long enough for much data to have been generated, and most of what has been observed focuses on a few well-known organisms. Those organisms that can tell us the most about the relationships of living things are often obscure and poorly studied, a situation that holds perhaps nowhere more strongly than in this case.

But there is one feature that is readily observed and consistent, and that is the number and position of flagella. Like I mentioned, most organisms have flagella at the front ends of their cells, and pull themselves through their surroundings with them opisthokonts are unusual in pushing their cells through their surroundings. Furthermore, most non-opisthokont cells have flagella that appear in twos, or are obviously derived from ancestors that had flagella in twos, while all opisthokonts' flagella appear without any others associated with them. These may not seem like significant things, but one must bear in mind that, when discussing the divergence of animals and plants and fungi, we are discussing the evolution of single-celled organisms. In that context, seemingly unimportant things like the position and number of flagella can be highly significant.

So, classifying something as an opisthokont is not a natural thing for most people. It may seem like an obscure and unimportant distinction. Classifying animals and fungi as each others' closest multicellular relatives has (so far) no known consequences to medicine or agriculture or anything else that most people would notice. But the opisthokont hypothesis is, as far as we can tell, an accurate description of the relationships of living things: it is our best understanding of the relevant facts, and the closest that science can come to the truth.


What is Opisthokonta?

The Opisthokonts are named for the single posterior flagellum seen in flagellated cells of the group. The flagella of other protists are anterior and their movement pulls the cells along, while the opisthokonts are pushed. Protist members of the opisthokonts include the animal-like choanoflagellates, which are believed to resemble the common ancestor of sponges and perhaps, all animals. Choanoflagellates include unicellular and colonial forms, and number about 244 described species. In these organisms, the single, apical flagellum is surrounded by a contractile collar composed of microvilli. The collar is used to filter and collect bacteria for ingestion by the protist. A similar feeding mechanism is seen in the collar cells of sponges, which suggests a possible connection between choanoflagellates and animals.

The Mesomycetozoa form a small group of parasites, primarily of fish, and at least one form that can parasitize humans. Their life cycles are poorly understood. These organisms are of special interest, because they appear to be so closely related to animals. In the past, they were grouped with fungi and other protists based on their morphology.


Biology II Chapter 28

Unlike the cells of prokaryotes, eukaryotic cells have a nucleus and other membeane-bounded organelles, such as mitochondria and the Golgi apparatus.

This is despite the fact that most protists are unicellular. Single celled protists are justifiably considered the simplest eukaryotes, but at the cellular level, many protists are very complex.

Unicellular protists carry out the same essential functions, but they do so using subcellular organelles, not multicellular organs.

An organism that is capable of both photosynthesis and heterotrophy.

Some protists exhibit this characteristic.

Chlorarachniophytes likely evolved when a heterotrophic eukaryote engulfed a green alga, which still carried out photosynthesis with its plastids and contains a tiny vestigial nucleus of its own.

Consistent with the hypothesis that chlorarachinophytes evolved from a eukaryote that engulfed another eukaryote, their plastids are surrounded by four membranes. The two inner membranes originated as the inner and outer membranes of the ancient cyanobacterium. The third membrane is derived from the engulfed alga's plasma membrane, and the outermost membrane is derived from the heterotyophic eukaryote's food vacuole. In other protists, plastids acquired by secondary endosymbiosis are surrounded by three membranes, indicating that one of the original four membranes was lost during the course of evolution.

A group lacking conventional mitochondria and having fewer membrane bounded organelles than other protists.

Many of the "amitochondriate protists" do infact have mitochondria - though reduced onces.

Because the root of the eukaryotic tree is not known, all five supergroups are shown as diverging simultaneously from a common ancestor - something that is not correct but it is unknown which organisms were first diverge from the others.

  • Red, Green, secondary endosymbiosis

Fungi and Animals are within the Unikonta supergroup.

Excavates include parasites such as Giardia, as well as many predatory and photosynthetic species.

  • Some members of this supergroup have an excavated groove on one side of the cell body. Two major clades (the parabasalids and diplomanads) have modified mitochondria others (the euglenozoans) have flagella that differ in structure from those of other organisms.

CChromalveolates include some of the most important photosynthetic organisms on Earth. This group includes the brown algae that form underwater kelp "forests" as well as important pathogens such Plasmodium, which causes malaria, and Phytophthora, which caused the devastating potato famine in 19th century Ireland.

  • This group may originated by an ancient secondary endosymbiosis event.

This group consists of species of amoebas, most of which have pseudopodia that are threadlike in shape.

  • Pseudopodia are extensions that can bulge from any portion of the cell they are used in movement an in the capture of prey.

This group of eukaryotes includes red algae and green algae, along with land plants. Red algae and green algae include unicellular species, colonial species (such as the green alga Volvox), and multicellular species. Many of the large algae known informally as "seaweeds" are multicellular red or green algae. Protists in Archaeplastida include key photosynthetic species that form the base of the food web in some aquatic communities.

Green and red algae are included in the supergroup Archaeplastida.

Golden and brown algae are included in the supergroup Chromalveolata.

This group of eukaryotes includes amoebas that have lobe- or tube-shapped pseudopodia, as well as animals, fungi, and protists that are closely related to animals or fungi.

According to one current hypothesis, this unikonts may have been the first groups of eukaryotes to diverge from other eukaryotes however, hypothesis has yet to be widely accepted.

(T/F) Most diplomonads and parabasalids of the supergroup Excavata are found in anaerobic environments.

(T/F) These two groups also lack plastids and have modified mitochondria (until recently, they were thought to lack mitochondria altogether).
True False

Diplomanads lack functional electron transport chains and hence cannot use oxygen to help extract energy from carbohydrates and other organic molecules. Instead, diplomanads have to get the energy they need from anaerobic biochemical pathways.

Parabasalids generate some energy anaerobically, releasing hydrogen gas as a by-product.

Protists that have a single, large mitochondrion that contains an organized mass of DNA called a kinetoplast.

They are within the supergroup Excavata and group euglenozoans. These organisms have a spiral or crystalline rod of unknown function within their flagella.

These protists include spcies that feed on prokaryotes in freshwater, marine and moise terrestrial ecosystems, as well as species that parasitize animals, plants and other protists. Such as Trypanosoma which causes sleeping sickness in humans.

Protists that has a pocket at one end of the cell from which one or two flagella emerge. Many species of the euglenid Euglena are mixotrophs: in sunlight they are autotrophic, but when sunlight is unavailable, they can become heterotrophic, absorbing organic nutrients from their environment.

Many other euglenids engulf prey by phagocytosis.

Their mitochondria do not have an electron transport chain and so cannot function in aerobic respiration.

A very large and diverse supergroup of extremely diverse protists and has recently been proposed based on two lines of evidence.

1) Some (though not all) DNA sequence data suggest that the chromaveolates form a monophyletic group.

2) Some data support the hypothesis that the chromalveolates originated more than a billion years ago, when a common ancestor of the group engulfed a signle-celled, photosynthetic red alga. Because red alae are thought to have originated by primary endosymbiosis, such an origin for the chromalveolates is ferred to as secondary endosymbiosis.

A group of protists within the supergroup Chromalveolates (the other group is stramenopiles) whose monophyly is well supported by molecular systematics (forming a clade). Structurally, species in this group have membrane-bound sacs (alveoli) just under the plasma membrane. The function of the alveoli is unknown but it is hypothesized that they may help stablize the cell surface or regulate the cell's water and ion content.

The alveolates include three subgroups: a group of flagellates (the dinoflagellates), a group of parasites (the apicomplexans) and a group of protists that move using cilia (the ciliates).

Protists within the supergroup Chromalveolates and the group alveolates.

These are characterized by cells that are reinforced by cellulose plates. Two flagella located in perpendicular grooves in this "armor" make dinoflagellates sprin as they move through the water. Dinoflagellates are abundant components of both marine and freshwater plankton, communities of microorganisms that live near the water's surface.

  • These dinoflagellates include some of the most important photosynthetic species. However, many photosynthetic dinoflagellates are mixotrophic, and roughly half of the dinoflagellates are purely hterotrophic.
  • Dinoflagellate blooms - episodes of explosive population growth- sometimes cause a phenomenon called "red tide" in coastal waters.

Dinoflagellate blooms may have originated from secondary endosymbiosis of red alga. Red alga are believed to have originated from primary endosymbiosis.

Dinoflagellate blooms are episodes of explosive population growth that appear brownish red or pink because of the presence of carotenoids, the most common pigments in dinoflagellate plastids.

Toxins produced by certain dinoflagellates have caused massive kills of invertebrates and fishes. Humans who eat molluscs that have accumulated the toxins are affected as well, sometimes fatally.

Protists within the supergroup Chromalveolates and the group alveolates.

Nearly all apicomplexans are parasites of animals and some cause serious human diseases. The parasites spread through their host as tiny infectious cells called sporozoites.

Apicomplexans are so named because one end (the apex) of the sporozoite cell contains a complex of organelles specialized for penetrating host cells and tissues.

Although apicomplexans are not photosynthetic, recent data has shown that they retain a modified plastid (apicoplast), most likely of red algal origin.

Most apicomplexans have intricate life cycles with both sexual and asexual stages.

Protists within the supergroup Chromalveolates and the group alveolates.

Ciliates are large, varied group of protists named for their use of cilia to move and feed. The cilia may completely cover the cell surface or may be clustered in a few rows or tufts. In certain species, rows of tightly packed cilia function collectively in locomotion. Other ciliates scurry about on leg-like structures constructed from many cilia bonded together.

A distinctive feature of ciliates is the presence of two types of nuclei: tiny micronuclei and large macronuclei. A cell has one or more nuclei of each type.

Genetic variation results from conjugation, a sexual process in which two indivudals exchange haploid micronuclei. Ciliates generally reproduce asexually by binary fission, during which the existing macronucleus disintegrates and a new one is formed from the cell's micronuclei. Each macronucleus typically contains multiple copies of the ciliate's genome.

Genes in the macronucleus control the everyday functions of the cell, such as feeding, waste removal and maintaing water balance.

Genetic variation results from conjugation, a sexual process in which two indivudals exchange haploid micronuclei. Ciliates generally reproduce asexually by binary fission, during which the existing macronucleus disintegrates and a new one is formed from the cell's micronuclei. Each macronucleus typically contains multiple copies of the ciliate's genome.

A group of protists within the supergroup Chromalveolates (the other group is Alveolates). The stramenopiles are a group of marine algae that include some of the most important photosynthetic organisms on the planent, as well clades of heterotrophs.

Their name (stramen - straw pilos - hair) refers to their characeristic flagellum, which has numerous fine, hairlike projections. In most stramenopiles, this "hairy" flagellum is paired with a shorter "smooth" (nonhairy) flagellum.

The group consists of diatoms, golden algae, brown algae, and oomycetes.

Protists within the supergroup Chromalveolates and the group stramenopiles.

  • Diatoms are unicellular algae that have unique glass-like wall made of hydrated silica (silicon dioxide) embedded in an organic matrix. The wall consists of two parts that overlap like a shoe box and its lid. The walls provide effective protection from the crushing jaws of predators.
  • Much of the diatom's strength comes from the delicate lacework of holes and grooves in their walls if the wallers were smooth, it would take 60% less force to crush them.

Diatoms reproduce asexually by mitosis with each daughter cell receives half of the parental cell wall and generates a new half that fits inside it. Some species form cysts as resistant stages. Sexual reproduction occurs but is not common in diatoms.

The first sentence is true but the second sentence is false. Diatom blooms are benefitial in reducing global warming as their bodies incorporate carbon into their bodies from carbon dioxide in the air during photosynthesis. The bodies sink to the ocean floor and are less likely to be broken down by bacteria and other decomposers than those that are eaten.

Protists within the supergroup Chromalveolates and the group stramenopiles.

Golden algae are typically biflagellated, with both flagella attached near one end of the cell. The characteristic color results from yellow and brown carotenoids.

Many golden algae are components of freshwater and marine plankton. While all golden algae are photosynthetic, some species are mixotrophic. These mixotrophs can absorb dissolved organic compounds or ingest food particles, including living cells (eukaryotes and prokaryotes) by phagocytosis.

Most species are unicellular, but some are colonial.

Protists within the supergroup Chromalveolates and the group stramenopiles.

Brown algae are the most complex and largest algae. All are multicellular and most are marine.

Brown algae include species have specialized tissues and organs that resemble those in plants. But morphological and DNA evidence indicates that the similarities evolved independently in the algal and plant lineages, and thus are analogous, not homologous.

A brown algae that is plantlike is known as thallus ( thalli, thallous - sprout). Unlike the body of a plant, a typical thallus consists of a rootlike holdfast, which anchors the alga, and a stemlike stipe, which supports the leaflike blades.

Brown algae reproduce by alternation of generations - the alternation of multicellular haploid and diploid forms.

Brown algae belong to the supergroup of Chromalveolata whereas land plants belong to the supergroup of Archaeplastida.

Brown algae include species have specialized tissues and organs that resemble those in plants. But morphological and DNA evidence indicates that the similarities evolved independently in the algal and plant lineages, and thus are analogous, not homologous.

The diploid individuals is called the sporophyte because it produces spores. The spores are haploid and move by means of flagella the flagellated spore are called zoospores. The zoospores develop into haploid male and female gametophytes, which produce gametes. The union of two gametes (fertilization or syngamy) results in a diploid zygote, which matures and gives rise to a new sporophyte.

In the two generations, for heteromorphic, the sporophytes and gametophytes are structurally different.

For ismorphic generations, the sporophytes and gametophytes look similar to each other but they differ in chromosome number.

Protists within the supergroup Chromalveolates and the group stramenopiles.

Oomycetes include the water molds, the white rusts and the downy mildews. Based on morphology, they were previously classified as fungi but they are not related to fungi and is most likely due to convergent evolution. In both oomycetes and fungi, the high surface-to-volume ratio of filamentous structures enhances the uptake of nutrients from the environment.

One of the differences includes that oomycetes typically have cell walls made of cellulose, whereas the walls of fungi consist mainly of another polysaccharide, chitin.

Although oomycetes descended from plastid-beating ancestors, they no longer have plastids and do not perform photosynthesis. Instead, they typically acquire nutrients as decomposers or parasites.

Most water molds are decomposers. White rusts and downy mildews generally live on land as plant parasites.

Algae and plants with alternation of generations have a multicellular haploid stage and a multicellular diploid staage.

In the other two life cycles, either the haploid stage or the diploid stage is unicellular.

The supergroup Rhizaria that includes the groups chlorarachniophytes, foraminiferans and radiolarians.

Many species in Rhizaria are among organisms referred to as amoebas. Amoebas were formerly defined as protists that move and feeb by means of pseudopodia, extensions that may bulge from almost anywhere on the cell surface.

An amoeba moves by extending a psudopodium and anchoring the tip. Amoebas are not only confined within the supergroup Rhizaria and are dispersed across many distantly related eukaryotic taxa. Most that belong to the clade Rhizaria are distinguished morphologically from other amoebas by having threadlike pseudopodia.

Protists within the supergroup Rhizarians. These are also known as forams - named for their porous shells, called tests.

Foram tests consist of a single piece of organic material hardened with calcium carbonate. The pseudopodia that extend through the pores function in swimming, test formation and feeding. Many forams also derive nourishment from photosynthesis of symbiotic algae that live within the tests.

90% of all identified species of forams are known from fossils.

Protists within the supergroup Rhizarians.

These protists have delicate, intricately symmetrical internal skeletons that are generally made of silica. The pseudopodia of these protists radiate from the central body and are reinforced by bundles of microtubules. The microtubules are covered by a thin layer of cytoplasm, which engulfs smaller microorganisms that become attached to pseudopodia.

Humans and fungi are more closely related than humans and plants or fungi and plants.

A monophyletic supergroup that descended from the ancient protist that engulfed a cyanobaterium.

The group consists of red algae, green algae and land plants.

More than a billion years ago, a heterotrophic protist acquired a cyanobaterial endosymbiont, and the photosynthetic descendants of this ancient protist evolved into red algae and green algae. The lineage that produced green algae, gave rise to land plants.

Prostis within the supergroup Archaeplastida.

Red algae are the most abundant large algae in the warm coastal waters of tropical oceans. Their accestory pigments allow them to absorb blue and green light, which penetrate relatively far into the water.

Most red algae are multicellular, although none are as large as the giant brown kelps.

The thalli of many red algae are filamentous, often branched and interwoven in lacy patterns. The base of the thallus is usually differentiated as a simple holdfast.

Red algae have especially diverse life cycles, and alternation of generations is common. Unlike other algae, they have no flagellated stages in their life cycle and depend on the water currents to bring gametes together for fertilization.

Protists within the supergroup Archaeplastida.

Green algae have an ultrastructure and pigment composition much like the chloroplasts of land plants.

Some systematists advocate including green algae in an expanded "plant" kingdom, Viridiplantae. Phylogenetically, this change makes sense, since otherwise the green algae are a paraphyletic group.

Green algae are divided into two main groups: chlorophytes and charophyceans.

Larger size and gereater complexity evolved in chlorophytes by three different mechanisms:

1) The formation of colonies of individual cells, as seen in Volvox and in filamentous forms that contribute to the stringy masses known as pond scum.

2) The formation of true multicellular bodies by cell division and differentiation, as seen in the seaweed Ulva

3) The repeated division of nuclei with no cytoplasmic division, as seen in multinucleate filaments of Caulerpa.

1) The formation of colonies of individual cells, as seen in Volvox and in filamentous forms that contribute to the stringy masses known as pond scum.

2) The formation of true multicellular bodies by cell division and differentiation, as seen in the seaweed Ulva

3) The repeated division of nuclei with no cytoplasmic division, as seen in multinucleate filaments of Caulerpa.

Nearly all species of chlorophytes reproduce sexually by means of biflagellated gametes that have cup-shaped chloroplasts.

Alternation of generations have evolved in life cycles of chlorophytes, including Ulva, in which the alternate generations are isomorphic.

Ulva's thallus contains many cells and is differentiated into lifelike blade and roolike holdfast. Caulerpa's thallus is composed of multinucleate filaments without cross-walls, so it is essentially one large cell.

1) The formation of colonies of individual cells, as seen in Volvox and in filamentous forms that contribute to the stringy masses known as pond scum.

2) The formation of true multicellular bodies by cell division and differentiation, as seen in the seaweed Ulva

3) The repeated division of nuclei with no cytoplasmic division, as seen in multinucleate filaments of Caulerpa.

A recently proposed, extremely diverse supergroup of eukaryotes that includes animals, fungi and some protists.

There are to major clades of unikonts, the amoebozoans and the opisthokonts (animals, fungi and closely related protist groups).

The root of the eukaryote phylogenetic tree is unknown but it has been proposed that the unikonts were the first eukaryotes to diverge from eukaryotes. Stechmann and Smith proposed that the supergroup Unikonta did not have DHFR-TS gene fusion while all other supergroups experienced the fusion.

A group of protists within the supergroup Unikonta and the clade amoebozoan.

Slime molds, or mycetozoans, were once thought to be fungi because they produce fruiting bodies that aid in spore dispersal. However, the resemblance between slime molds and fungi appears to be an example of evolutionary convergence.

Slime molds have diverged into two main branches, plasmodial slime molds and cellular slime molds - distinguished in part by their unique life cycles.

Plasmodial slime molds, the diploid condition is the predominant part of the life cycle, whereas cellular slime molds are haploid organisms. (only the zygote is diploid).

Both are categorized in the Unikonata supergroup under amoebozoan.

In most plasmodial slime molds, the diploid condition is the predominant part of the life cycle. At one stage of their life cycle, they form a mass called a plasmodium, which is a single mass of cytoplasm that is undivided by plasma membranes and contains many diploid nuclei.

In cellular slime molds, they are haploid organisms (only the zygote is diploid). Although the mass of cells resembles a plasmodial slime mold, the cells remain separated by their individual plasma membranes.

They also have fruiting bodies that function in asexual, rather than sexual reporduction.

Protists within the supergroup Unikonta and clade amoebozoans that are not free-lving (unlike the others), that belong to the genus Entamoeba serving as parasites. They infect all classes of vertebrates as well as some invertebrates.

Humans are host to at least six species of Entamoeba, but only one, E. histolytica is known to be pathogenic.

The clade within Unikonta that includes an extremely diverse group of eukaryotes such as animals, fungi, and several groups of protists.

Two groups to mention are choanoflagellates which are more closely related to animals than to other protists and nucleariids that are more closely related to fungi than other protists.

Excavates - include protists with modified mitochondria and protists with unique flagella.

Chromalveolates - may have originated by secondary endosymbiosis

Rhizarians - a large diverse group of protists defined by DNA similarities

Archaeplastida - Red algae and green algae are the closest relative of land plants

Unikonts - include protists that are closely related to fungi and animals.

Diplomonads and parabasalids - modified mitochondria.

Euglenozoans - spiral or crystalline rode inside flagella

Alveolates - Membrane-bounded sacs (alveoli) beneath plasma membrane

Stramenopiles - Hairy and smooth flagella

Forams - Amoebas with threadlike pseudopodia and a porous shell

Radiolarians - Amoebas with threadlike pseudopodia radiating from central body

Red algae - Phycoerythrin (accessory pigment)

Green algae - Plant-type chloroplasts

Amoebozoans - Amoebas with lobe shapped pseudopodia

Opisthokonts - Tons. (includes fungi and animals.)

Plastids that are surrounded by more than two membranes are evidence of

a) evolution from mitochondria
b) fusion of plastids
c) origin of plastids from archaea
d) secondary endosymbiosis
e) budding of the plastids from the nuclear envelope.

Biologists suspect that endosymbiosis gave rise to mitochondria before plastids partly because

A) mitochondrial DNA is less similar to prokaryotic DNA than is plastid DNA
B) the products of photosynthesis could not be metabolized without mitochondrial enzymes
C) mitochondrial proteins are synthesized on cytosolic ribosomes, whereas plastids utilize their own ribosomes.
D) without mitochondrial CO2 production, photosynthesis could not occur.
E) all eukaryotes have mitochondria (or their remnants) whereas many eukaryotes do not have plastids

Which grroup is incorrectly paired with its description?

A) apicomplexans - parasites with intricate life cycles
B) diatoms - important producers in aquatic communities
C) rhizarians - morphologically diverse group defined by DNA similarities
D) diplomonads - protists with modified mitochondria
E) red algae - acquired plastids by secondary endosymbiosis

Based on the phylogenetic tree in the figure below, which of the following statements is correct?

A) The most recent common ancestor is older than that of Chromalveolata
B) The most basal (first to diverge) eukaryotic supergroup cannot be determined
C) Excavata is the most basal eukaryotic supergroup.
D) The most recent ancestor of Chromalveolata is older than that of Rhizaria
E) The most recent common ancestor of red algae and land plants is older than that of nucleariids and fungi.

Which protists are in the same eukaryotic supergroup as land plants?

A) green algae
B) dinoflagellates
C) red algae
D) brown algae
E) both a and c

In life cycles with alternation of generations, multicellular haploid forms alternate with

A) multicellular diploid forms.
B) unicellular diploid forms.
C) multicellular polypoid forms.
D) multicellular haploid forms
E) unicellular haploid forms.


Abbreviations

Cell Division Cycle 3 - Group 2 septin from the yeast Saccharomyces cerevisiae, often used as a reference septin

C-Terminal Extension: The carboxy terminal end of a septin protein that is highly variable across septins groups

N-Terminal Extension: The amino terminal end of a septin protein that is highly variable across septin groups

Polybasic Domain: Region of a septin protein found amino-terminal to the GTP binding domain, largely composed of basic residues. Also referred to as the Phosphoinositol Binding domain due to its requirement for septins to bind phosphoinositol


Opisthokont

The name that I have chosen for this blog is bound to be an unfamiliar term for most people. It probably represents an unfamiliar concept as well, so I will start gently. It is a term that denotes a specific, large, and (to humans) very important group of organisms. The importance of the group is probably understandable once one understands that humans are opisthokonts. So, however, are all other animals. So are all fungi! Rounding out the group are a number of single-celled organisms known to be related to either animals or fungi or both. Notably absent from this group are plants, algae, and quite a few other organisms, including all bacteria.

What can animals and fungi have in common that plants do not? Well, think of a sperm cell. This is a tadpole-like thing, with a roughly spherical cell body and a single, tail-like flagellum trailing behind. The cell swims by wiggling its flagellum, again much like a tadpole swims by wiggling its tail. As it happens, this is a very unusual cell type. Most other flagellated organisms swim with their flagella in front, pulling themselves through the surroinding medium. Only the flagellated cells found in animals, fungi, and related microbes swim with their flagella behind. This gives rise to the name: "opistho-" means "behind", and "-kont" refers to the flagellum.

One could easily be forgiven for finding this a minor difference, given everyday experience. To most of us, animals are things that move around and eat things, and plants and fungi are rooted in the ground. However, there are animals that are rooted to the ground as well (including sponges, corals, and sea squirts), and animals that do not eat (such as some of the worms living near deep sea hydrothermal vents). There are fungi that do not grow in the ground (yeasts are fungi, for instance, and do not root themselves in anything). Everyday experience, it turns out, is insufficient to categorise life science has moved well beyond that.

Science has taken its time to get to where it is today, though. Decades ago, fungi were classed amongst the "lower plants" because their cells are surrounded by rigid cell walls, a characteristic then thought to define a "plant". However, it has since been shown that the materials that make up those cell walls are completely unrelated (they are derived from sugars in plants and from proteins in fungi, for instance). More importantly, the organisms indisputably most closely related to each, which look like single-celled versions of their better-known counterparts, lack any vestige of the cell wall. In other words, the common ancestor of plants and fungi did not have a cell wall this character is a homoplasy, something that evolved more than once in the history of life.

Genetic analysis confirms this. Most hypotheses of evolutionary history (of extant organisms, anyway) are now made by having computers analyse the DNA of comparable genes from different organisms, and these tend to connect animals to fungi, to the exclusion of plants. (There are exceptions -- there are always exceptions -- but those are from genes with a lot of evolutionary "noise". In other words, such genes are either not large enough or evolve too quickly to retain enough information to resolve the animal/plant/fungus relationship with any reliability. There are statistical tests that indicate the trustworthiness of these computer analyses, and those which are judged acceptable almost always support the close relationship between animals and fungi.)

Analysis of genes goes beyond using them to reconstruct evolutionary history directly. For instance, there is an insertion into one of the genes used in the replication of DNA, an extra stretch of about fifty nucleotides (the "letters" of DNA's "alphabet"), which is found in animals, fungi, and their close relatives, and nothing else. This might not seem particularly important, but the gene in question is important enough that it is not prone to change easily (in scientific parlance, it is "evolutionarily conserved"), and perhaps more importantly, the insertion is itself conserved. In other words, the same nucleotides (or some obvious derivation of them) are present in the same place in all opisthokonts.

Non-genetic data helps link the two groups as well. The architecture of individual cells in the single-celled relatives of animals and fungi is strikingly similar, both inside and out. This was not apparent until the advent of electron microscopy many of the features that link the two groups are either too small to be seen with a light microscope (the "regular" kind) or are easily overlooked in favour of other, more striking features, many of which (like the cell walls already mentioned) can be taken to imply connections that do not hold up when investigated through other techniques.

These features include the arrangements of the components of the cytoskeleton, a set of protein-based rods and tubes that gives a cell its shape. The arrangement and replication of the flagella is also thought to be a conserved trait. A substantial part of my graduate work is investigating these things while the coherence of the opisthokonts as a group is nowadays almost beyond question, the uniqueness of some of its defining characteristics is simply not known. Electron microscopy has not been around long enough for much data to have been generated, and most of what has been observed focuses on a few well-known organisms. Those organisms that can tell us the most about the relationships of living things are often obscure and poorly studied, a situation that holds perhaps nowhere more strongly than in this case.

But there is one feature that is readily observed and consistent, and that is the number and position of flagella. Like I mentioned, most organisms have flagella at the front ends of their cells, and pull themselves through their surroundings with them opisthokonts are unusual in pushing their cells through their surroundings. Furthermore, most non-opisthokont cells have flagella that appear in twos, or are obviously derived from ancestors that had flagella in twos, while all opisthokonts' flagella appear without any others associated with them. These may not seem like significant things, but one must bear in mind that, when discussing the divergence of animals and plants and fungi, we are discussing the evolution of single-celled organisms. In that context, seemingly unimportant things like the position and number of flagella can be highly significant.

So, classifying something as an opisthokont is not a natural thing for most people. It may seem like an obscure and unimportant distinction. Classifying animals and fungi as each others' closest multicellular relatives has (so far) no known consequences to medicine or agriculture or anything else that most people would notice. But the opisthokont hypothesis is, as far as we can tell, an accurate description of the relationships of living things: it is our best understanding of the relevant facts, and the closest that science can come to the truth.


Taxonomy

Opisthokonts are divided into Holomycota or Nucletmycea (fungi and all organisms more closely related to fungi than to animals) and Holozoa (animals and all organisms more closely related to animals than to fungi) no opisthokonts basal to the Holomycota/Holozoa split have yet been identified. Holomycota and Holozoa are composed of the following groups.

  • Holomycota
      • including chytrids (previously thought to be protists)
      • including microsporidia (previously thought to be protists)
      • including Hyaloraphidium (previously thought to be a green alga, now thought to be a chytrid)
      • excluding oomycetes (water molds) (now thought to be stramenopiles)
      • excluding labyrinthulomycetes (slime nets) (now thought to be stramenopiles)
      • excluding myxomycetes (now thought to be amoebozoans)
          (capsasporid amoebae) (ministeriid amoebae)
    • including Myxozoa (previously thought to be protists)

    The paraphyletic taxon Choanozoa includes either non-animal holozoans, or non-animal, non-fungi opisthokonts.

    The choanoflagellates have a circular mitochondrial DNA genome with long intergenic regions. This is four times as large and contains two times as many protein genes as do animal mitochondrial mitochondria.

    Corallochytrium seem likely to be more closely related to the fungi than to the animals on the basis of the presence of ergosterol in their membranes and being capable of synthesis of lysine via the AAA pathway.

    The ichthyosporeans have a two amino acid deletion in their elongation factor 1 α gene that is considered characteristic of fungi.

    The ichthyosporean mitochondrial genome is >200 kilobase pairs in length and consists of several hundred linear chromosomes that share elaborate terminal-specific sequence patterns.