Allas
Allas Sandon, 1927 (ref. ID; 7130)
Family Thaumatomonadidae Hollande, 1952 (ref. ID; 7130)
[ref. ID; 5694]
Free-living heterotrophic flagellates. (ref. ID; 5694)
[ref. ID; 7130]
Nomenclatural confusion surrounds the morphologically similar genera Allas Sandon, 1927 and Thaumatomonas De Saedeleer, 1931. The main difference between Al. diplophysa of Sandon (1927) and Thaumatomonas lauterborni of De Saedeleer (1931), who was probably unaware of Sandon's work, is the larger size of Allas (10-24 um not 7-11 um for the flagellate phase (De Saedeleer 1931); later authors gave 8.3-16.5 um as the range for the T1 Ts. lauterborni clone: Mylnikov and Karpov (1993) and the fact that Sandon did not observe syncytia in older cultures. However, the presence or absence of syncytia is insufficient on its own for generic distinction, as in cercomonads some species within the same genus readily make syncytia whereas close relatives do not (Bass et al. 2009) and these is no reason to think that thaumatomonads differ in this respect. Thus syncytia (not of uniform morphology) were reported in Ts. lauterborni, Ts seravini (Mylnikov and Karpov, 1993), and Ts. zhukovi (Mylnikov and Mylnikov, 2003), but not is Ts. coloniensis (Wylezich et al., 2007). The redrawing of De Saedeleer's Thaumatomonas lauterborni in Patterson et al. (2002) is at the wrong scale and made the cell much too big and also the anterior flagellum proportionally far too long (~12 um instead of the correct 1.8-3.1 um), thus exaggerating the relatively small differences between it and Allas and the Thaumatomonas species described here. Thus there are no ligth microscopic morphological grounds for relating Thaumatomonas as a genus distinct from Allas, which agrees with their close similarity in 18S rDNA sequence.
The tight clustering and interspersion of sequences previously published for strains identified as Allas and Thaumatomonas also calls into question their distinctness (Cavalier-Smith and Chao 2003; Wylezich et al. 2007). Previously we thought that this interspersion, and the light microscopic differences between Allas aff. diplophysa and Thaumatomonas being not significantly greater that those among previously named species of Thaumatomonas, meant that no valid distinction could be made between Thaumatomonas and Allas, and that the younger name Thaumatomonas must be suppressed as a synonym. Supression would require a change to the family but not the ordinal name, and as Thaumatomonas is much more often used than Allas it would be ideal if it can be avoided. We achieve this by using the novel scale type here discovered in ATCC50365 Allas diplophysa (sequenced by Cavalier-Smith and Chao 2003) as a distinguishing feature for Allas. We do not know what kinds of scales were present in the type species of either genus, as both were described before electron microscopy (the 'delicate cuticle' sometimes seen by Sandon (1926) was probably the scale layer). But purely for nomenclatural stability, we now treat ATCC50365 as a reference strain of Allas and likewise treat strain T-1 of Mylnikov (scales described by Karpov and Zhukov 1987 and Mylnikov and Karpov 1992) as correctly identified as Thaumatomonas lauterborni (the type species) even though its cells were slightly larger and its anterior flagellum slightly shorter than in De Saedeleer (1931). Accepting both avoids having to change the younger name Thaumatomonas to Allas by using scale morphology to distinguish the genera. Though we do not know for sure that the original Thaumatomonas lauterborni even had scales, the possibility that it was a Protaspa, some of which are superficially similar, is excluded by its clearly parallel centrioles (De Saedeleer 1931) - divergent in Protaspa (Hoppenrath and Leander 2006), making it most likely to be genuinely related to later described Thaumatomonas species, all of which have scales; unfortunately Sandon (1927) did not show whether centrioles are parallel in Al. diplophysa or not, they apparently are in Al. aff. diplophysa. Allas aff. diplophysa scales are all oval as in Thaumatomonas coloniensis (Wylezich et al., 2007), but unlike them come in two forms; a few are bilaterally symmetric like those of Thaumatomonas, but most are a novel type of oval scale with a single dense strut at the middle of one side only. Thus we retain both genera even though they are very closely related and can be distinguished only by scale ultrastructure or sequences. (ref. ID; 7130)
- Allas aff. diplophysa Sandon, 1927 (ref. ID; 7130)
Allas aff. diplophysa Sandon, 1927 (ref. ID; 7130)
Descriptions
18S rDNA sequence, GenBank AF411262, ITS2 rDNA sequence HQ176331. Distinguished from Thaumatomonas oxoniensis, Ts. vancouveri and Reckertia filosa by larger cell-size: 16 um (13-20 um, our observations, though 10-24 um in Sandon 1927). In contrast to Ts. oxoniensis and R. filosa, PF is much shorter, usually <1-1X BL, rarely extends beyond cell. AF short, but longer than in R. filosa and Ts. oxoniensis. Motile cell shape oval, flattened, disc-like. Long, branching filose and lamellar pseudopodia produced from beneath stationary cells; project in all directions. Scales mostly oval, usually with 2 (rarely 3) terminal struts at each end and one lateral strut with U-shaped cross section midway on one side; 5-6 lateral perforations on that side and 3-4 on the opposite side. Some scales lack the lateral strut and then have the same number of holes on each side (3-4). Those with struts may be bilaterally symmetric in overall shape (especially the lower plate, the upper one typically being flatter and less convex on the side away from the strut) but usually bulge out more on the strutted side making the whole shape asymmetric; rarely the bulge may be more pronounced making the scale somewhat triangluar. Bacterivorous. Anterior cv. Cysts smooth, round, ~10 um. Freshwater, cultured in Volvic with grain. (ref. ID; 7130)
Comments
The type species, Allas diplophysa Sandon 1927 (p.113, Plate III fig.15) was originally described as "Sp. epsilon" (Sandon 1924). Allas diplophysa was described as "a flattened, more or less oblong organism with rounded ends, rather metabolic and sometimes with regged pseudopodia especially at the hind end; length 10-24 um" though usually it is 18 um, its "width about half the length; two flagella arising at the anterior end of the body, one, slightly longer than the body, being trailed passively behind, and the other which is very short (about 2 to 3 um long) moving actively. The animal glides rapidly forwards very like Allantion tachploon, but sometimes with a slight vibration owing to the active lashing of the short flagellum." ATCC50365 is close to Sandon's description. We therefore designated it Al. aff. diplophysa Sandon. We did not make it a neotype as its cell size is smaller (average 16 um, with an upper limit of 20 um). Although cell size can depened on culture conditions, the PF of this strain is usually 1X BL and sometimes not seen extending beyond the cell, whereas Sandon's was just over 1X BL. Therefore it is not strictly concordant with Sandon's description. Accepting it as a genuine Allas (unlike the other strains previously called Allas which are even smaller and thus still less like Al. diplophysa) its novel scale type becomes the first clear morphological distinction from Thaumatomonas; accordingly, in Figure 15 we renamed as Thaumatomonas the other 'Allas' strains that lack this scale type. We do not know if another sequenced 'Allas' strain (formerly Allas sp. SA) has scales, but its position, on the tree makes that likely; from its position, with no contrary evidence, we treat it also as a Thaumatomonas. (ref. ID; 7130)
Type strain
ATCC50365 "Allas diplophysa" (1991; light brown sandy soil, Tuscon, AZ, USA; T.K. Sawyer). (ref. ID; 7130)