Spathidium rectitoratum Kahl, 1930

Jang, Seok Won, Vďačný, Peter, Shazib, Shahed Uddin Ahmed & Shin, Mann Kyoon, 2017, Linking morphology and molecules: integrative taxonomy of spathidiids (Protista: Ciliophora: Litostomatea) from Korea, Journal of Natural History 51 (17 - 18), pp. 939-974 : 959-961

Spathidium rectitoratum Kahl, 1930

( Figures 10 (a–i) and 11(a–l); Table 2)

Spathidium amphoriforme var. rectitoratum Kahl, 1930: 166 , fig. S. 160, 27

Epispathidium amphoriforme var. rectitoratum – Foissner, 1987a: 224 (combining author) Spathidium rectitoratum – VďaČný et al., 2014: 108 (18S rRNA gene sequences)

Description of a Korean population

Body size 120–170 × 25–40 µm in vivo, usually about 150 × 35 µm; not contractile but very flexible. Shape spatulate to very narrowly spatulate with a length:width ratio of 3.8– 6.8: 1 in vivo and 2.2–6.6:1 after protargol impregnation; oral region flattened laterally, more or less distinctly set off from oblong trunk by narrowed neck; anterior body end as wide as or wider than mid-body; posterior end rounded ( Figures 10 (a–c,e–g) and 11(a,b, g–i); Table 2). Nuclear apparatus in trunk, distance from anterior end highly variable (i.e. ranging from 20 µm to almost 80 µm after protargol impregnation); nuclear figure about 40–70 µm long ( Table 2). Macronucleus a tortuous strand, somewhat nodulated in vivo; nucleoli numerous, globular and evenly distributed over macronucleus. Micronuclei near or attached to macronucleus at varying positions; exact number difficult to determine due to similar-sized and impregnated cytoplasmic inclusions ( Figures 10 (a,b,e–g) and 11 (c,g–i); Table 2). A single contractile vacuole at posterior body end, about 12 µm across during diastole ( Figure 11 (a,b)). Extrusomes attached to oral bulge and scattered throughout cytoplasm; rod-shaped and with slightly curved ends, 6.5–13 × 0.4–0.7 µm in size; becoming wrinkled after protargol impregnation ( Figures 10 (h) and 11(d,e,j)). Cortex very flexible, contains several rows of colourless granules between adjacent ciliary rows. Cytoplasm colourless, studded with extrusomes and granules about 1–2 µm in diameter ( Figure 11 (a,c,d)). Movement relatively slow, either gliding on microscope slide or swimming by rotation about main body axis.

Cilia 8–11 µm long in vivo, spaced ordinarily except for about five densely spaced basal bodies in the curved anterior kinety portion. On average 24 equidistant, ordinarily spaced meridional ciliary rows curved anteriorly, forming an Epispathidium -like pattern – that is, anterior portion of right side rows curved dorsally and left side rows curved ventrally to parallel the circumoral kinety. Dorsal brush inconspicuous because bristles only up to 3 µm long in vivo; dikinetidal; three-rowed; distinctly heterostichad with middle row being slightly longer than first row; distance between dikinetids about 1 µm in anterior brush portion while 2–2.5 µm in middle and posterior portion ( Figures 10 (b– d) and 11(g,k,l); Table 2).

Oral bulge conspicuous because distinctly set off from body proper, not or only slightly inclined, straight in lateral view ( Figures 10 (a,e–g) and 11(a,e,g–i,k)). Circumoral kinety at base of oral bulge; continuous (i.e. distinctly separated from ciliary rows); composed of narrowly spaced and perpendicularly-oriented dikinetids ( Figure 10 (b– d)). Nematodesmata not recognisable in vivo or after protargol impregnation.

Resting cysts

Cysts colourless, about 40–60 µm across in vivo; without escape apparatus. Cyst wall smooth, only 1–2 µm thick, dotted by refractive granules in optical section. Cytoplasm finely granulated. Macronucleus in cyst centre, slightly shortened but shape unchanged (i.e. strand-like and tortuous). Contractile vacuole, extrusomes, cortical granules and cilia not recognisable ( Figures 10 (i) and 11(f); Table 2).

Comparison with original description and remarks

The Korean population matches Kahl’ s (1930) description with regard to body shape, the tortuous, strand-like macronucleus, the extrusome and contractile vacuole patterns, the inconspicuous dorsal brush, and the straight oral bulge. Moreover, the number of ciliary rows appears similar to what is shown in Kahl’ s drawing (fig. S. 160, 27). According to Kahl (1930), this form is 200–250 µm long, but hungry cells are only 130–170 µm in length. Korean specimens had body lengths of 120–170 µm in vivo, consistent with hungry cells. However, according to morphometric data, they were up to 200 µm long when 15% preparation shrinkage was assumed ( Table 2), which is near the lower limit provided by Kahl (1930). Sequences from Kahl’ s population are needed to decide whether this length difference can be attributed to inter-population variability or whether it justifies species- or subspecies-level separation of the Korean population and Kahl’ s population.

Kahl (1930) considered this taxon to be a variety of Spathidium amphoriforme . According to our analyses it can be morphologically unambiguously separated from the nominotypical form by the inconspicuous dorsal brush as well as from the other variety, securiforme , by the more slender body and the straight oral bulge in lateral view. Moreover, our molecular analyses show that rectitoratum and securiforme form independent lineages in phylogenetic trees ( Figure 16 (a,b)). Because taxa of the S. amphoriforme complex are nested within the S. muscicola cluster and the genus Epispathidium is polyphyletic, we tentatively suggest assigning rectitoratum to the genus Spathidium as a distinct species.

Figures

Figure 10. Spathidium rectitoratum from life (a, h, i) and after protargol impregnation (b–g). (a) Right side view of a representative individual;(b, c) right and left side view of ciliary pattern and nuclear apparatus of a representative specimen; (d) detail of anterior body end showing three-rowed dorsal brush; (e–g) variability of body shape and size as well as of nuclear apparatus; (h) oral extrusomes, 8–13 µm long; (i) resting cyst,50 µm in diameter.CK,circumoral kinety; DB, dorsal brush; MA, macronucleus;OB, oral bulge; SK, somatic kinety/somatic kineties. Scale bars: a–c, e–g = 100 µm; d = 10 µm.

Figure 16. Spathidiid phylogeny based on the 18S rRNA gene (a) and concatenation of the 18S rRNA gene and ITS region sequences (b). Bootstrap values for maximum likelihood (ML) and posterior probabilities were mapped onto the Bayesian inference (BI) tree. A dash indicates bootstrap values below 50%. Newly obtained sequences are in bold. Scale bars indicate numbers of substitutions.