Micrometopion

Micrometopion Cavalier-Smith (ref. ID; 7130 original paper)

Infraphylum Monadofilosa Cavalier-Smith 1997: Class Metromonadea Cavalier-Smith, 2007 (ref. ID; 7130)

[ref. ID; 7130]
Diagnosis; Rigid non-amoeboid, non-metabolic marine zooflagellates that glide on the substratum by a non-acronematic posterior flagellum, which emerges from a ventrolateral groove (on cell's left) a fifth of body length from anterior. Typically uniflagellate, strongly compressed, with a slightly beaked anterior rostrum that is less obvious and less bulbous than in Metopion and lacks its two rows of granules. (ref. ID; 7130)
Etymology; micro Gk + Metopion, the zooflagellates described by Larsen and Patterson (1990), as its resembles but is smaller than Metopion fluens. (ref. ID; 7130)
Type species; Micrometopion nutans (ref. ID; 7130)

Micrometopion nutans Howe et al., 2011 (ref. ID; 7130 original paper)

Micrometopion nutans Howe et al., 2011 (ref. ID; 7130 original paper)

Diagnosis

18S rDNA sequence, GenBank HQ121441; cell size 6.1 um (5-7.5 um). PF longer than in M. fluens (Larsen and Patterson, 1990) at 11.9 um (8.5-14 um) 2-2.75X BL, not acronematic; second flagellum stubby rarely visible or rarely present, its posterior-pointing and position in the same groove as PF is best distinction from Planomonas (where AF comes from anterior tunnel and is usually very thin: Cavalier-Simith et al. 2008). Both flagella equally thick. Cells flattened, quasi-oval; less convex on left, which is slightly indented at flagellar insertion, making left anterior appear as a slightly projecting rostrum. Cells compressed laterally; rigid, non-metabolic; lie on thier side flush with substratum. PF trails alongside, from point of attachment under rostrum. Cell pivots in nodding, knocking motion, raising and lowering its posterior while gliding along substratum. Cells usually not constantly and energetically, often travelling fairly directly at variable speed. Bud-like projections, approximately a quarter of cell size, seen on several cells near posterior; probably result of misdivision. Culture medium half-strength artificial sewwater; consumes eukaryotes; fed on Procryptobia sorokini. Flagellated resting stage; no cysts. Large cv, usually central, occasionally in posterior half. Cells divide parallel to plane of compression. (ref. ID; 7130)

Comments

The second short flagellum that projects laterally from the groove in some cells and beats actively might be a growing posterior flagellum prior to division. Mylnikov gave us this strain as "Metopion fluens small form". Earlier he had given us a larger one designated simply Metopion fluens, described ultrastructurally by Mylnikov et al. (1999) without light micrographs. Its 18S rDNA sequenced previously (Cavalier-Smith and Chao 2003) so radically differs from that now found for Micrometopion nutans that we cannot treat them as the same genus, despite morphological similarity. That larger-celled culture was isolated from a marine aquarium in the Zoology of Invertebrates Department, Moscow State University (salinity 35 0/00), but it is long dead and no high quality light micrographs exist. However, Mylnikov kindly supplied micrographs of that culture from his files. Comparison with the type micrograph (figure 52(f) of Larsen and Patterson) gives no strong reason to question their identification as M. fluens, but their lack of clarity makes identification somewhat uncertain, so we now call that strain Metopion aff. fluens. Comparison of the text of Larsen and Patterson (1990) and Mylnikov et al. (1999) initially raised doubts about their identity, as the former gave 10 um and the latter 15-17 um for PF length; however, in the nine micrographs Mylnikov sent us the length is 11-12 um, not significantly different from our measurements of the micrographs in Larsen and Patterson (11-12.5 um). Cell length for the Moscow strain (Mylnikov et al. 1999) and in the type diagnosis were both 6-9 um: so no measurement discrepancies really exist. Descriptions of behavior and morphology are also concordant. The angle of PF to the cell may differ subtyl, but they are obviously much more similar than either is to Micrometopion. Micrometopion differs in five ways: (1) cells on average are smaller than M. fluens and M. aff. fluens; (2) PF length is >2X BL, but closer to 1.5X BL in M. fluens and M. aff. fluens; (3) the position of the PF and the angle at which it trails with respect to the cell differ subtyl; (4) it has a less bulbous rostrum with no obvious granules; (5) the cell appears narrower than M. fluens. These slight phenotypic differences would not require a different genus were it not that they differ so greatly in sequence that some trees do not even group them, one or other often grouping distantly with Metromonas. Micrometopion nutans usually occupies somewhat more basal positions in 18S rDNA phylogenies than our Metopion aff. fluens sequence, differing among methods and taxon samples (none significantly supported). It is not associated with any known environmental sequence, having only 89% similarity to any 18S rDNA sequence in GenBank; this lack of close neighbours using 18S rDNA probably exacerbates its positional instability. However, 28S rDNA analyses suggest a closer relationship to Metromonas. Polyubiquitin was sequenced from the Moscow Metopion aff. fluens strain (Bass et al. 2005). We place Micrometopion in Metromonadea for reasons given in the discussion. (ref. ID; 7130)

Etymology

nutans L. nodding. (ref. ID; 7130)

Type strain

CCAP 1904/1 (2002, littoral sample from White Sea (salinity 12 0/00), nr Kartesh, Russia; A.P. Mylnikov). (ref. ID; 7130)