Alveolates: Dinoflagellates, Apicomplexians, and Ciliates

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The illustration shows two dinoflagellates. The first is walnut-shaped, with a groove around the middle and another perpendicular groove that starts at the middle and extends back. Flagella fit in each groove. The second dinoflagellate is horseshoe-shaped, with the body extending from the wide part of the horseshoe toward the narrow end. Like the first dinoflagellate, this one has two perpendicular grooves, each containing a flagellum.
Dinoflagellates. The dinoflagellates exhibit great diversity in shape. Many are encased in cellulose armor and have two flagella that fit in grooves between the plates. Movement of these two perpendicular flagella causes a spinning motion. Source: OpenStax Biology 2e

OpenStax Biology 2e

A large body of data supports that the alveolates are derived from a shared common ancestor. The alveolates are named for the presence of an alveolus, or membrane-enclosed sac, beneath the cell membrane. The exact function of the alveolus is unknown, but it may be involved in osmoregulation. The alveolates are further categorized into some of the better-known protists: the dinoflagellates, the apicomplexans, and the ciliates.

Dinoflagellates exhibit extensive morphological diversity and can be photosynthetic, heterotrophic, or mixotrophic. The chloroplast of photosynthetic dinoflagellates was derived by secondary endosymbiosis of a red alga. Many dinoflagellates are encased in interlocking plates of cellulose. Two perpendicular flagella fit into the grooves between the cellulose plates, with one flagellum extending longitudinally and a second encircling the dinoflagellate. Together, the flagella contribute to the characteristic spinning motion of dinoflagellates. These protists exist in freshwater and marine habitats, and are a component of plankton, the typically microscopic organisms that drift through the water and serve as a crucial food source for larger aquatic organisms.

Dinoflagellates have a nuclear variant called a dinokaryon. The chromosomes in the dinokaryon are highly condensed throughout the cell cycle and do not have typical histones. Mitosis in dinoflagellates is closed, that is, the spindle separates the chromosomes from outside of the nucleus without breakdown of the nuclear envelope.

Some dinoflagellates generate light, called bioluminescence, when they are jarred or stressed. Large numbers of marine dinoflagellates (billions or trillions of cells per wave) can emit light and cause an entire breaking wave to twinkle or take on a brilliant blue color. For approximately 20 species of marine dinoflagellates, population explosions (also called blooms) during the summer months can tint the ocean with a muddy red color. This phenomenon is called a red tide, and it results from the abundant red pigments present in dinoflagellate plastids. In large quantities, these dinoflagellate species secrete an asphyxiating toxin that can kill fish, birds, and marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume these protists may become poisoned.

The breaking wave in this photo is an iridescent blue color.
Dinoflagellate bioluminescence. Bioluminescence is emitted from dinoflagellates in a breaking wave, as seen from the New Jersey coast. (credit: “catalano82”/Flickr)

The apicomplexan protists are named for a structure called an apical complex, which appears to be a highly modified secondary chloroplast. The apicoplast genome is similar to those of dinoflagellate chloroplasts. The apical complex is specialized for entry and infection of host cells. Indeed, all apicomplexans are parasitic. This group includes the genus Plasmodium, which causes malaria in humans. Apicomplexan life cycles are complex, involving multiple hosts and stages of sexual and asexual reproduction.

Illustration A shows an oval cell that has a narrow end and a wide end. The apical complex is located at the narrow end. The three branches of this complex narrow and join at the apical, or narrow, end of the cell. Illustration b shows the life cycle of Plasmodium, which causes malaria. The plasmodium life cycle begins when a mosquito takes a blood meal and injects Plasmodium into the bloodstream. The Plasmodium enters the liver where it multiplies, and eventually reenters the blood. In the blood it enters the ring stage, so called because the cell is curled into a ring shape. The Ring stage may multiply by mitosis or it may undergo meiosis, forming new 1n gametes of male or female sex types. When a mosquito takes a blood meal from an infected host the gametes are ingested. A smaller gamete sex type, called a microgamete, fertilizes a larger sex type, called a macrogamete, producting a 2n zygote. The zygote undergoes mitosis and differentiation. It enters the saliva where it can be injected into another host, completing the cycle.
Apicomplexa. (a) Apicomplexans are parasitic protists. They have a characteristic apical complex that enables them to infect host cells. (b) Plasmodium, the causative agent of malaria, has a complex life cycle typical of apicomplexans. (credit b: modification of work by CDC)

The ciliates, which include Paramecium and Tetrahymena, are a group of protists 10 to 3,000 micrometers in length that are covered in rows, tufts, or spirals of tiny cilia. By beating their cilia synchronously or in waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-based structures that function like paddles, funnels, or fins. Ciliates also are surrounded by a pellicle, providing protection without compromising agility. The genus Paramecium includes protists that have organized their cilia into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria. Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes. Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles, which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell. Ciliates therefore exhibit considerable structural complexity without having achieved multicellularity.

The illustration on the left shows a shoe-shaped Paramecium. Short, hair-like cilia cover the outside of the cell. Inside are food vacuoles, a large macronucleus, and a small micronucleus. The Paramecium has two star-shaped contractile vacuoles. The mouth pore is an indentation located just where the foot narrows. A small opening called the anal pore is located at the wide end of the cell. The micrograph on the right is a Paramecium, which is about 50 microns across and 150 microns long.
Paramecium. Paramecium has a primitive mouth (called an oral groove) to ingest food, and an anal pore to eliminate waste. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. (credit “paramecium micrograph”: modification of work by NIH; scale-bar data from Matt Russell)

Paramecium has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential for sexual reproduction, and is in many ways a typical eukaryotic nucleus, except that its genes are not transcribed. The transcribed nucleus is the macronucleus, which directs asexual binary fission and all other biological functions. The macronucleus is a multiploid nucleus constructed from the micronucleus during sexual reproduction. Periodic reconstruction of the macronucleus is necessary because the macronucleus divides amitotically, and thus becomes genetically unbalanced over a period of successive cell replications. Paramecium and most other ciliates reproduce sexually by conjugation. This process begins when two different mating types of Paramecium make physical contact and join with a cytoplasmic bridge. The diploid micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. Fusion of the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell. This pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform multiple rounds of DNA replication. The copies of the micronuclear chromosomes are severely edited to form hundreds of smaller chromosomes that contain only the protein coding genes. Each of these smaller chromosomes gets new telomeres as the macronucleus differentiates. Two cycles of cell division then yield four new Paramecia from each original conjugative cell.

The illustration shows the life cycle of Paramecium. The cycle begins when two different mating types form a cytoplasmic bridge, becoming a conjugate pair. Each Paramecium has a macronucleus and a micronucleus. The micronuclei undergo meiosis, resulting in four haploid micronuclei in each parent cell. Three of these micronuclei disintegrate. The remaining micronuclei divide once by mitosis, resulting in two micronuclei per cell. The parent cells swap one of these micronuclei. The two haploid micronuclei then fuse, forming a diploid micronucleus. The micronucleus undergoes three rounds of mitosis, resulting in eight micronuclei. The original macronucleus dissolves, and four of the micronuclei become macronuclei. Two rounds of cell division result in four daughter cell per each parent cell, each with one macronucleus and one micronucleus.
Conjugation in Paramecium. The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus. (credit “micrograph”: modification of work by Ian Sutton; scale-bar data from Matt Russell)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

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