Malawimonas
Malawimonas O'Kelly & Nerad, 1999 (ref. ID; 7492 original paper)
Family Malawimonadidae (ref. ID; 7492 original paper)
[ref. ID; 7492]
Diagnosis; Malawimonads without loricas or other morphologically distinctive covering on cell or flagellar surfaces. Flagellate cells bacterivorous, ingesting prey near the posterior end of the ventral groove; permanently differentiated cytostome lacking. (ref. ID; 7492)
Notes; O'Kelly (O'Kelly 1993) argued that the jakobid flagellates are more similar to the retortamonads, amitochondriate flagellates normally found as hindgut commensals in animals (Brugerolle and Mignot 1990; Kulda and Nohynkova 1978), that to any other taxon of eukaryotes, with or without mitochondria. This assessment was based on those ultrastructural features, especially those associated with the flagellar vanes and the kinetids, that could be ascertained from the existing ultrastructural studies on retortamonads (Brugerolle 1991; Brugerolle and Mignot 1990).
New information on retortamonad kinetid architecture has since been obtained for the free-living species Chilomastix cuspidata (Bernard, Simpson and Patterson 1997). Also, the genus Trimastix has been rediscoverd and found to have many retortamonad-like features (Brugerolle and Patterson 1997; O'Kelly, Farmer and Nerad 1999). Comparisons of these features (O'Kelly, Farmer and Nerad 1999) further support the idea that retortamonads, Trimastix and jakobids are similar, and may be closely-related taxa. Of greatest interest is the observation of shared features between Malawimonas and Trimastix on the one hand, and the Jakobidae, Histionidae and retortamonads on the other. Malawimonas and Trimastix possesss a prominent anterior root, lacking or very reduced in the other jakobids and retortamonads. Also, in the left ventral root of Malawimonas and Trimastix, microtubules are added to the inside of the ventral groove instead of the outside as in Jakoba, Histiona, Reclinomonas, and the retortamonads. The multilayered structure associated with the left ventral root in jakobids, histionids, and retortamonads is absent in Malawimonas and Trimastix. These observations suggest that jakobid flagellates belong to two clades, each of which includes metamonad, or metamonad-like, flagellates as well as jakobids. (ref. ID; 7492)
Etymology; "Malawi flagellate" (Malawi-, the African nation from which the type species was isolated; monas, Gk. "wanderer"). Malawimonas is a third declension feminine Latin noun. (ref. ID; 7492)
Type species; Malawimonas jakobiformis (ref. ID; 7492)
- Malawimonas jakobiformis O'Kelly & Nerad, 1999 (ref. ID; 7492 original paper) reported author and year? (ref. ID; 4980)
Malawimonas jakobiformis O'Kelly & Nerad, 1999 (ref. ID; 7492 original paper) reported author and year? (ref. ID; 4980)
Diagnosis
Freshwater protists with characteristics of genus, associated with lake sediments. Cell shape plastic, typically elongate, tapered at both ends, ventral surface planar to concave, dorsal surface convex. Flagellate cells 4.0-8.5 um in length, 2.0-4.5 um in width. Cysts 3.5-4.9 um diameter, spherical, with smooth wall, attached to substrate by pad to adhesive material. (ref. ID; 7492)
Descriptions
- Trophic cells: Trophic cells of Malawimonas jakobiformis n.gen., n.sp. were uninucleate, biflagellate heterotrophic nanoflagellates. The cells were "naked"; nether scales nor lorica were present, and there was little ultrastructural evidence for a substantial glycocalyx or other surface coat. Cells were usually slender, approximately twice as long as wide, with tapered anterior and posterior end; the shape was plastic, easily deformed by influences such as coverslip pressure of food content. The ventral surface was concave to planar, while the dorsal surface was convex. Protargol-stained cells, fixed late in exponential phase growth, measured 4.0-8.5 um in length (mean+/-standard deviation 5.5+/-0.95 um, N=30) and 2.0-4.5 um across at the widest point (3.2+/-0.56 um, N=30). Recently-fed living cells, distended with ingested bacteria, were slightly larger, especially in width, whereas starved cells were shorter and thinner. Spheroidal "monsters" up to 50 um in diameter with several nuclei and pairs of flagella were observed occasionally after heavy feeding.
The two flagella were of approximately equal length, being 1-1.5 times as long as the cell body and lacking hairpoints (acronemata) or surface decoration. The anterior flagellum extended at an angle to the long axis of the cell, and at rest assumed a characteristic "crook" shape, bent near the base. The posterior flagellum was appressed, but not attached to the ventral cell surface. Cells swam well and in straight lines, turning about their longitudinal axes as they swam. They could also "squirm" about clumps of bacteria and other obstacles. No sessile feeding stages were observed.
A conspicuous ventral groove was present, typically in the anterior 1/2-2/3 cell. The posterior flagellum, numbered "1", resided in this groove. A protruding flange was present on the left side of the groove, near the anterior end of the cell, while a cytoplasmic lip defined the right margin of the groove more posteriorly. Cells were otherwise approximately oval in cross-section.
The single nucleus resided in the anterior half of the cell. Its anterior end was not closely associated with, or visibly connected to, any component of the kinetid. Alongside and posterior to the nucleus, a single sausage-shaped mitochondrion was present. The mitochondrial cristae were determined to be discoidal through examination of numerous cells. The anterior end of the mitochondrion was not closely associated with the basal body complex. No kinetoplast or otherwise prominent nucleoidal region was observed.
Presumed extrusomes were spherical, membrane bound, and with uniformly dense content. Cells had 0-10 of these organelles scattered haphazardly along the cell surface everywhere except in the ventral groove.
A single Golgi stack (dictyosome), consisting of 3-5 loosely-associated cisternae and numerous associated vesicles, was located in the anterior end of the cell, just posterior to the basal bodies but anterior to the nucleus and mitochondrion. Typically, it lay just to the left of the cell's median longitudinal axis.
The two flagella were inserted at the anterior end of the cell. Both flagella constained a 9+2 axoneme. The anterior flagellum (flagellum 2) had a round cross-section throughout, but the posterior flagellum (flagellum 1) had a single vane with differentiated content that arose near the proximal end of the flagellum and was present through most of its length. The vane projected ventrally at its anterior end, but to the right side of the cell for most of the rest of its length.
The kinetid consisted of two basal bodies, four microtubular roots, secondary cytoskeletal microtubules (the"dorsal fan") emanating from one of the roots, and a number of components linking the basal bodies to each other and to the root microtubules.
The basal bodies were unusual in that the transition region of each was located ca. 250 nm from the proximal end of the basal body, and about 300 nm from the plane of the plasmalemma. The transition region was marked by a central axosome into which the central microtubules of the flagellar axoneme were inserted, and by the loss of C tubules from the basal body triplets.
The basal bodies formed a pair at the anterior end of the cell. The proximal end of the posteriorly-directed basal body (basal body 1) approached the transition region of the anteriorly-directed basal body (basal body 2), but was always separated from it by at least the width of a basal body, ca. 250 nm. The long axes of the two basal bodies were askew, that of basal body 1 being displaced ca. 150 nm (0.5-0.7 basal body diam.) to the left side of the cell. The angle between the basal bodies was variable, but rarely greater than 90 degrees.
The basal bodies were linekd by a single, apparently unstriated distal fiber, attached to discrete sites on each basal body. A single microtubular root, the anterior root, containing two or three tubules, arose from the anterior surface of basal body 2, near the transition region. This root extended along the left side of the cell, and was ca. 1.0 um long. Secondary cytoskeletal microtubules arose along the entire length of this root, and extended posteriorly along the dorsal surface of the cell, forming a dorsal fan.
Three microtubular roots arose from the posterior basal body. The right ventral root arose near the proximal end of the posterior basal body. At its proximal end, the root contained ca. three microtubules, but additional microtubules were added distally to the right side of the spline, so that the root eventually contained ca. 20 tubules. At about the level of posterior flagellar emergence, the right root split into two splines. The leftmost spline consisted of five microtubules, which extended posteriorly as a coherent bundle along the left side of the ventral groove. The rightmost spline, containing the remaining ca. 15 microtubules, also enxtended posteriorly along the ventral groove. The 8-10 rightmost microtubules of this spline formed a group that, together, defined the margin of the cytoplasmic lip, while the remaining microtubules were sparsely distributed along the floor of the ventral groove.
Fibrous material, designated the I (for "intermediate") fiber, was associated with the ventral surface of the root near its proximal end. The I fiber disappeared at the point of the separation of the two splines. Dense material, interpreted as a very short A fiber, was present on the dorsal side of some of the root microtubules at the proximal end of the root. The B fiber originated at the junction between the distal fiber and basal body 1. It extended posteriorly ventral to the right root and along the plasmalemma. This fiber eventually merged with the rightmost spline. It was not visible in the cytoplasmic lip. A prominently-striated band linked the proximal end of the right root to two axonemal triplets of basal body 2 at the level of the transition region.
The left ventral root arose on the ventral surface of basal body 1, just proximal to the transition region. At its origin, two microtubules were present in the root, but immediately distally the number increased to six by addition of microtubules to the left side of the root. At about the level of posterior flagellar emergence, the root split into two three-microtubule units. The leftmost three microtubules terminated near the anterior end of the cell, while the remaining three microtubules extended posteriorly, along the ventral surface of a broad, low ridge alongside the left margin of the ventral groove. The left ventral root was subtended by an unstriated fiber, the C fiber, that originated ventral to the root origin on basal body 1.
An intermediate singlet root originated dorsal to basal body 1, near its transition region. The single microtubule of this root extended posteriorly, eventually becoming associated with the rightmost three microtubules of the left ventral root. At its proximal end, the root microtubule was attached to the posterior basal body by a single thin fiber.
The microtubular roots, with the exception of the leftmost three microtubules of the left ventral root, reunited at the posterior end of the ventral groove, itself at the posterior end of the cell. No clearly differentiated cytostomal region was discerned. (ref. ID; 7492)
- Cell division: Cells entering mitosis became spherical. Paired flagella were present at presumed spindle poles throughout mitosis (it proved impossible to capture images of these flagella). An organized chromsomal plate appeared at metaphase, separating into two at anaphase. No nucleolus was apparent. Chromosomal separation at anaphase was accompanied by increases in the distance between presumed spindle poles and decreases in the distance between spindle poles and their associated chromosomes. Cytokinesis occured at the completion of mitosis. (ref. ID; 7492)
- Cysts: Senescent cultures produced numerous walled cysts. The cysts were spherical, without decoration or protuberances. Living cysts measured 3.5-4.9 um in diameter (3.9+/-0.06 um, N=30). The cyst wall was thin and lacked visibly differentiated layers. Cysts were attached to substrate by a pad of fibrillar material, which concealed a small aperture in the wall. Cysts each contained a single nucleus and mitochondrion, as well as basal bodies and microtubular root from the trophic cell cytoskeleton. Flagellar axonemes were not present in mature cysts, although TEM images indicated that retraction of flagella was a feature of early cyst formation. Cysts remained viable for several months in culture flasks storted at 15-30 degrees C. Excystment occurred with in 2 days upon addition of fresh culture medium and bacteria. (ref. ID; 7492)
Remarks
Malawimonas jakobiformis has many features in common with other described members of the jakobid grade (Flavin and Nerad 1993; Mylnikov 1989; O'Kelly 1993, 1997; Patterson 1990). These organisms are biflagellate, heterotrophic, bacterivorous nanoflagellates characterized by a prominent ventral groove, two heterodynamic flagella of which the posteriormost resides in the ventral groove and has a single flagellar vane, and the lack of a persistent cytostome/cytopharynx complex. The flagellar vane character serves to separate jakobids, including M. jakobiformis, from all other protists. Only Colponema loxodes Stein has a single vane similar to that of jakobids (Mignot and Brugerolle 1975). This latter species is a large flagellate that preys on other eukaryotes rather than bacteria, and has prominent cortical alveolae, large structured extrusomes, a perinuclear Golgi body with its cis face adjacent to the nuclear envelope, and secondary cytoskeletal microtubules associated with at least one of the ventral roots (Mignot and Brugerolle 1975). All of these features separate C. loxodes from the jakobids (O'Kelly 1993). Retortamonads (Bernard, Simpson and Patterson 1997; Brugerolle 1991; Brugerolle and Mignot 1990) and Trimastix (Brugerolle and Patterson 1997; O'Kelly, Farmer and Nerad 1999) have two or three vanes on the posterior flagellum.
Kinetid comparisons further reinforce the identification of M. jakobiformis with the jakobids. In particular, the ventral root microtubule configuration (a bifurcating left root, a bifurcating right root, and an intermediate singlet root, all proceeding alone the ventral groove parallel to the long axis of the cell to a juxtaposition point at the cell posterior) occurs elsewhere only in the amitochondriate retortamonad flagellates and Trimastix (Bernard, Simpson and Patterson 1997; Brugerolle 1991; Brugerolle and Patterson 1997; O'Kelly, Farmer and Nerad 1999), as described in more detail below. The pattern of cell division conforms to what has been observed in Reclinomonas and Jakoba (O'Kelly 1993), and preservation of kinetid elements in the M. jakobiformis cyst is consistent with what has been observed in Reclinomonas americana (O'Kelly 1997).
In terms of external morphology, M. jakobiformis most closely resembles Jakoba libera; both species lack cell coverings and sessile trophic stages, swim in a similar manner, and share the tendency for the anterior flagellum to form a "crook". However, M. jakobiformis cannot be assigned to Jakoba, nor to any other genus of jakobids. Malawimonas jakobiformis has discoidal mitochondiral cristae; in Reclinomonas and Histiona, mitochondrial cristae are tubular, and in Jakoba they are irregularly flattened (O'Kelly 1993). The organelles tentatively identified as extrosomes in M. jakobiformis lack the structured contents of extrusomes (discobolocysts) in Jakoba (Patterson 1990), Reclinomonas (O'Kelly 1993, 1997), and Histiona (Mylnikov 1989). The posterior flagellar vane arises from a clearly defined point on the ventral surface of the flagellum in M. jakobiformis; in other jakobids, the vane has a diffuse origin along the dorsal surface (O'Kelly 1997). In M. jakobiformis, the dorsal fan microtubules arise in the vicinity of a two-stranded root emanating from the anterior basal body; in other jakobids, this root is absent, and the dorsal fan microtubules arise from the vicinity of the basal body itself (O'Kelly 1993, 1997).
In M. jakobiformis, microtubules from the right root tend to be predominant in the floor of the ventral groove; in other jakobids, microtubules from the left root predominate (O'Kelly 1997). In M. jakobiformis, a few microtubules are added to the inside of the left ventral root, and the associated C fiber is short and lacks a lamellar substructure; in other jakobids, numerous microtubules are added to the outside of the left ventral groove, and the associated C fiber is longer and lamellate, forming a multilayered structure (O'Kelly 1993, 1997). These differences are at least as significant as are those that separate the jakobid families Jakobidae and Histionidae. Therefore, Malawimonas is placed in the Malawimonadidae n. fam. (ref. ID; 7492)
Etymology
"Resembling Jakoba" (jakobi-, the jakobid genus Jakoba, from Jakoba Ruinen, who described the species now called J. liera; formis, Gk. "form of"). (ref. ID; 7492)
Type locality
Chirombo Bay, Lake Nyasa, Malawi (14 degrees 5'S, 34 degrees 5'W). (ref. ID; 7492)
Type material
Holotype: Cryopreserved living material, conserved at the American Type Culture Collection (ATCC) as strain 50310. Protargol-stained slides and resin-embedded cells derived from strain 50310 conserved at ATCC and the Natural History Museum, Smithsonian Institution, accession number USNM 51468. (ref. ID; 7492)
Internet data dissemination
Updated summaries of the morphology, taxonomy, and biology of M. jakobiformis appear on the Protist Image Data Web site (http://megasun.bch.umontreal.ca/protists/malawi/). (ref. ID; 7492)