Actinophryid: Difference between revisions
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The actinophryids are an order of heliozoa, a polyphyletic array of stramenopiles, having a close relationship with pedinellids and Ciliophrys. They are common in fresh water and occasionally found in marine and soil habitats. Actinophryids are unicellular and roughly spherical in shape, with many axopodia that radiate outward from the cell body. Axopodia are a type of pseudopodia that are supported by hundreds of microtubules arranged in interlocking spirals and forming a needle-like internal structure or axoneme. Small granules, extrusomes, that lie under the membrane of the body and axopodia capture flagellates, ciliates and small metazoa that make contact with the arms.<ref name="Suzaki1980">Template:Cite journal</ref><ref name="Ando1989">Template:Cite journal</ref>
Description
[edit]Actinophryids are largely aquatic protozoa with a spherical cell body and many needle-like axopodia. They resemble the shape of a sun due to this structure, which is the inspiration for their common name: heliozoa, or "sun-animalcules". Their bodies, without arms, range in size from a few tens of micrometers to slightly under a millimeter across.<ref name="CavSmith2013">Template:Cite journal</ref>
The outer region of cell body is often vacuolated. The endoplasm of actinophryids is less vacuolated than the outer layer, and a sharp boundary layer may be seen by light microscopy.<ref name="Barrett1958">Template:Cite journal</ref> The organisms can be either mononucleate, with a single, well defined nucleus in the center of the cell body, or multinucleate, with 10 or more nuclei located under the outer vacuolated layer of cytoplasm. The cytoplasm of actinophryids is often granular, similar to that of Amoeba.<ref name="Anderson1960">Template:Cite journal</ref>
Actinophryid cells may fuse when feeding, creating larger aggregated organisms. Fine granules that occur just under the cell membrane are used up when food vacuoles form to enclose prey.<ref>Patterson, D. J. & Hausmann, K. 1981. Feeding by Actinophrys sol (Protista, Heliozoa): I. Light microscopy. Microbios 31: 39–55.</ref> Actinophryids may also form cysts when food is not readily available. A layer of siliceous plates is deposited under the cell membrane during the encystment process.<ref>Patterson, D.J. 1979. On the organization and classification of the protozoon Actinophrys sol Ehrenberg, 1830. Microbios 26: 165–208.</ref>
Contractile vacuoles are common in these organisms, which are presumed to use them to maintain body volume by expelling fluids to compensate for the entry of water by osmosis. Contractile vacuoles are visible as clear bulges from the surface of the cell body that slowly fill then rapidly deflate, expelling their contents into the environment.
Axopodia
[edit]The most distinctive characteristic of the actinophryids is their axopodia. These axopodia consist of a central, rigid rod which is coated in a thin layer of ectoplasm. In Actinophrys the axonemes end on the surface of the central nucleus, and in the multicellular Actinosphaerium they end at or near nuclei.<ref name="Anderson1960" /> The axonemes are composed of microtubules arranged in a double spiral pattern characteristic of the order.<ref>Template:Cite book</ref> Due to their long, parallel construction, these microtubules demonstrate strong birefringence.<ref name="Tilney1967">Template:Cite journal</ref><ref name="Suzaki1994">Template:Cite journal</ref>
These axopodia are used for prey capture, in movement, cell fusion and perhaps division.<ref name="Suzaki1980" /><ref name="Ando1989" /> They are stiff but may flex especially near their tips,<ref name="Barrett1958" /> and are highly dynamic, undergoing frequent construction and destruction. When used to collect prey items, two methods of capture have been noted, termed axopodial flow and rapid axopodial contraction.<ref name="Suzaki1980" /> Axopodial flow involves the slow movement of a prey item along the surface of the axopod as the ectoplasm itself moves, while rapid axopodial contraction involves the collapse of the axoneme's microtubule structure.<ref name="Suzaki1994" /> This behavior has been documented in many species, including Actinosphaerium nucleofilum, Actinophrys sol, and Raphidiophrys contractilis.<ref name="Suzaki1994" /><ref name="Kinoshita2001">Template:Cite journal</ref><ref name="Kinoshita1995">Template:Cite journal</ref> The rapid axopodial contraction occurs at high speed, often in excess of 5mm/s or tens of body lengths per second.<ref name="Shigenaka1982">Template:Cite book</ref>
The axopodial contractions have been shown to be highly sensitive to environmental factors such as temperature and pressure<ref name="Tilney1967" /><ref name="Tilney1969">Template:Cite journal</ref> as well as chemical signals like Ca2+ and colchicine.<ref name="Kinoshita2001" /><ref name="Tilney1968">Template:Cite journal</ref>
Reproduction
[edit]Reproduction in actinophryids generally takes place via fission, where one parent cell divides into two or more daughter cells. For multinucleate heliozoa, this process is plasmotomic as the nuclei are not duplicated prior to division.<ref name="Barrett1958" /> It has been observed that reproduction appears to be a response to food scarcity, with an increased number of divisions following the removal of food and larger organisms during times of food excess.<ref name="Johnson1894">Template:Cite journal</ref>
Actinophryids also undergo autogamy during times of food scarcity. This is better described as genetic reorganization than reproduction, as the number of individuals produced is the same as the initial number. Nonetheless, it serves as a way to increase genetic diversity within an individual which may improve the likelihood of expressing favorable genetic traits.<ref name="Grell1973">Template:Cite book</ref>
Plastogamy has also been extensively documented in actinophryids, especially in multinucleate ones. Actinosphaerium were observed to combine freely without the combination of nuclei, and this process sometimes resulted in more or less individuals than originally combined. This process is not caused merely by contact between two individuals but can be caused by damage to the cell body.<ref name="Johnson1894" />
Cyst function and formation
[edit]Under unfavourable conditions, some species will form a cyst. This is often the product of autogamy, in which case the cysts produced are zygotes.<ref name="Grell1973" /> Cells undergoing this process withdraw their axopodia, adhere to the substrate, and take on an opaque and grayish appearance.<ref>Template:Cite journal</ref> This cyst then divides until only uninucleate cells remain. The cyst wall is thickly layered 7–8 times and includes gelatinous layers, layers of silica plates, and iron.<ref name="Patterson1981">Template:Cite journal</ref>
Taxonomy
[edit]Originally placed in Heliozoa (Sarcodina), the actinophryids are now understood to be part of the stramenopiles. They are unrelated to centrohelid and desmothoracid heliozoa with which they had been previously classified.
There are several genera included within this classification.<ref name="Heliozoa III"/> Actinophrys are smaller and have a single, central nucleus.<ref name="Kinoshita2001" /> Most have a cell body 40–50 micrometer in diameter with axopods around 100 μm in length, though this varies significantly. Actinosphaerium are several times larger, from 200 to 1000 μm in diameter, with many nuclei<ref name="Kinoshita2001" /> and are found exclusively in fresh water.<ref>Template:Cite web</ref> A third genus, Camptonema, has a debated status. It has been observed once and was treated as a junior subjective synonym of Actinosphaerium by Mikrjukov & Patterson in 2001,<ref name="Heliozoa III"/> but as a valid genus by Cavalier-Smith & Scoble (2013).<ref name="CavSmith2013" /> Heliorapha is a further debated taxon, it being a new generic vehicle for the species azurina that was initially assigned to the genus Ciliophrys.<ref name="CavSmith2013" />
Classification
[edit]According to the latest review of actinophryid classifications, they are organized into two suborders, three families and three genera.<ref name="CavSmith2013"/><ref name="Heliozoa III"/>
- Order Actinophryida Hartmann 1913 [Actinophrydia Kühn 1926; Actinophrydea Hartmann 1913]
- Suborder Actinosphaerina Template:Au
- Family Actinosphaeriidae Template:Au
- Actinosphaerium Ritter von Stein 1857 [Echinosphaerium Hovasse 1965] — 4 species.
- Family Actinosphaeriidae Template:Au
- Suborder Actinophryina Template:Au
- Family Actinophryidae Dujardin 1841
- Actinophrys Ehrenberg 1830 [Trichoda Müller 1773 nomen oblitum; Peritricha Bory de St.Vincent 1824 nomen dubium non Stein 1859] — 4 species.
- Family Helioraphidae Template:Au
- Heliorapha Template:Au — 1 species.
- Family Actinophryidae Dujardin 1841
- Suborder Actinosphaerina Template:Au
Gallery
[edit]References
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