Rhogostoma
Rhogostoma Belar, 1921 (ref. ID; 7130) or (Schussler, 1921) (ref. ID; 7582)
Class Flosea (Leidy, 1879): Order Gromiida (Claparede & Lachmann, 1859): Family Gromiidae (ref. ID; 7582)
Order Cryomonadida Cavalier-Smith, 1993; Family Rhizasididae Skuja, 1948 (ref. ID; 7130)
ref. ID; 7130
We isolated six new strains of this thecate amoeba from soil or freshwater (only four were used in this study because two were identical to existing strains), videoed them and sequenced their 18S and ITS2 rDNA; they belong to R. schuessleri or R. minus, the only two known species, or to a new species. First we show that a previously sequenced strain misidentified as Lecythium sp. (Nikolaev et al. 2003) is also R. minus. (ref. ID; 7130)
Type species
Rhogostoma schuessleri (ref. ID; 7130)
- Rhogostoma micra Cavalier-Smith & Scoble sp. n. (ref. ID; 7130)
- Rhogostoma minus Belar, 1921 (ref. ID; 7130)
- Rhogostoma schuessleri Belar, 1921 (ref. ID; 7130)
Rhogostoma micra Cavalier-Smith & Scoble sp. n. (ref. ID; 7130)
Diagnosis
18S rDNA sequence, GenBank HQ121436; ITS2 rDNA GenBank HQ176327; cell size: 7.5 µm (6.5-10 µm). Morphology as R. minus, from which reliably distinguishable only by distinct 18S rDNA sequence. Test more often associated with debris and bacteria. Behaviour very similar, but pseudopodia more often, but not restricted to, trailing and leading bifurcating strands extruded from beneath the cell; cells can be less regular in shape than in R. minus. Finger-like pseudopodia more frequently exhibited, and lamellar pseudopodia less frequent, than in R. minus; when travelling, cell occasionally tilts to point cleft in direction of travel, perhaps to facilitate or as a result of extension of leading pseudopodia for locomotion - cells then appear slightly narrowed at the aperture end (light-bulb-shaped; compare figue G3 of Belar (1921) for R. minus). Freshwater. Division by binary fission. (ref. ID; 7130)
Comments
R. minus/micra clade is a complex of closely related small filose or sometimes reticulose organisms distinct from the larger lamellar/filose R. schuessleri. The fact that R. minus has both nonreticulose strains (like the other two species) and a reticulose strain CCAP1966/4 suggests that the latter evolved its ability to fuse pseudopodia relatively recently, suggesting that in this group it is a fairly easy evolutionary transition (yet one rarely made in Filosa, apart from in Chlorarachnea and very few cercomonads, unlike in the probably ancestrally reticulose Endomyxa and Retaria: Bass et al. 2009). (ref. ID; 7130)
Etymology
micros Gk small. (ref. ID; 7130)
Type strain
Rhog PML2A CCAP1966/2 (2009; still stream sediment, Port Meadow, Oxford, UK. JMS). (ref. ID; 7130)
Descriptions
Flexible, smooth inorganic test, with aperture from which radiating lamellar, finger-like, non-granular, branching filopodia constantly extend and retract at all angles; shape a strongly laterally flattened prolate ellipsoid projecting vertically from substratum; cell broader diameter seen from dorsal end: 7.3 µm (6.5-8.5 µm) in our living cultures when rapidly growing (can be 10-12 µm in old stationary cultures). Nikolaev et al. (2003) do not make it clear whether their measurements were from its oral to aboral end (what length should mean) or at right angles to that axis as ours are. Cell fills the completely. Cleft-like aperture beneath the cell hard to see in life, opening towards substratum; pseudopodia emerge through the cleft, can extend many times body length and pull the cell along the substratum; cell pulled over leading pseudopodia as trailing pseudopodia retract, progress smooth. Refractile granules; several small vesicles; large lateral cv; n usually central. Form groups of several cells. Divide by binary fission. Freshwater. Heterotrophic. Freshwater species of Lecythium are 3-10 times larger. ITS2 rDNA sequence: GenBank HQ176325.
Other strains: A slightly larger strain (RhogFloat CCAP1966/4 (GenBank 18S rDNA HQ121431): isolated in 2009 by DB from soil from Oxford University Harcourt Arboretum, Nuneham Courtenay, UK; cell size: width 7-14; length 9-15 µm) had an identical ITS2 sequence to CCAP1966/3; so we treat it as the same species, even though its filopodia often fuse into a definite reticulum or reticula form by lacunae opening within lamellipodia (unlike R. schuessleri, minus, and micra). Given this ITS2 identity, the apparent differences in 18S DNA sequences of CCAP1966/3 (AJ514867) and CCAP1966/4 (HQ121431) are all attributable to PCR and/or sequencing errors. The errors are probably all in the CCAP1966/3 sequence (AJ514867: Nikolaev et al. 2003) making it a longer branch on our trees, because we independently isolated three other strains that all yielded identical 18S rDNA sequences to HQ121431 of CCAP1966/4. As these came from California, USA (freshwater stream sediment near Monterey), freshwater stream sediment near Porlock Harbour, Somerset (JMS), and Hinksey Lake, Oxford, UK (DB) this R. minus genotype is widespread in north temperate continents in both freshwater and soil. (ref. ID; 7130)
Comments
Freshwater strain (CCAP1966/3) isolated by A.P. Mylnikov (his strain code Le-2) from a waste treatment plant at Borok, Yaroslavskaya Oblast, Russia, in 1993 (as 'Lecythium sp.'), whose actin and 18S rDNA sequences were reported (Nikolaev et al. 2003) without photographic documentation of its morphology. We restudied it to check its identity because we are also sequencing other genes from it (Lewis et al. in prep.) and isolated six related strains. Belar (1921) described two Rhogostoma species that differed from freshwater Lecythium in four respects: (1) having a narrow crack-like, not a large round, aperture; (2) being filose not reticulose; (3) being several times smaller (R. minus 10-12 µm, R. schuessleri 15 µm); and (4) in thecae having bilateral not radial symmetry. In all four respects Mylnikov's strain matches R. minus and differs from L. hyalinum. It was thus misidentified and is Rhogostoma minus not a Lecythium. Its correct identity may have been overlooked as it was unaccountably omitted from the standard work (De Saedeleer 1934). Nikolaev et al. (2003) noted cell length as 8-10 µm and width as 5-8 µm, but wrongly wrote that it 'closely resembles the type species, Lecythium hyalinum (Hertwig et Lesser, 1874), as illustrated by Cash and Wailes (1915; Plates 49 and 51).' In fact, the original description and illustration of Hertwig and Lesser (1874) shows pseudopods sometimes anastomosing (probably not so in this strain) and that the nucleus alone is 10-15 µm across, which would make cell width (seen end on from aboral pole) ~30-45 µm, i.e. 3-4 times Mylnikov's strain. Moreover L. hyalinum (Ehrenberg) Hertwig and Lesser is depicted with perfect radial symmetry; all these features are clear in Cash and Wailes (1915) except that they do not describe or depict anastomosis, so perhaps studied a different species from Hertwig and Lesser (1874). The other freshwater species in Cash and Wailes (1915) (Lecythium spinosum (= Trinema spinosum Penard, 1890); Lecythium granulatum (= Gromia granulata Schulze, 1875); Lecythium mutabile (= Pamphagus mutabile Bailery, 1853)) are comparably large or even larger and do not resemble Mylnikov's strain. De Saedeleer (1934) described a marine Lecythium minutum (3-11 µm) that is similar to R. minus except for its obvious circular aperture. (ref. ID; 7130)
Descriptions
Cell size 15 µm. Indistinguishable from the original description (Belar 1921). 18S rDNA sequence, GenBank HQ121430; ITS2 rDNA GenBank HQ176326. Larger than the three preceding species. Forms conspicuous lamellipodium broader than the cell, from which filopodia extend. (ref. ID; 7130)
Type strain
LArb CCAP1966/1. (2009; isolated from Oxford University Harcourt Arboretum soil, Nuneham Coutenay, UK. JMS). (ref. ID; 7130)