Apsiktrata gracilis Foissner et al. 1994
Creators: name: Jiang, Limin
name: Wang, Congcong
name: Liao, Lijian
name: Al-Farraj, Saleh A.
name: Warren, Alan
name: Hu, Xiaozhong
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Chromista
Ciliophora
Prostomatea
Prostomatida
Apsiktratidae
Apsiktrata
Apsiktrata gracilis

description: Apsiktrata gracilis (Penard, 1922) Foissner et al., 1994
(Fig. 5; Table 2)
Description of Qingdao population: Cell size in vivo ~40–70 μm × ~15–30 μm, on average 50 μm × 20 μm, with a length-to-width ratio of 2–3:1 (Fig. 5A, F, I, J). Body generally obovate, circular in apical view, anterior end broadly rounded, posterior half significantly narrowed; well-fed individuals more rounded in mid-region (Fig. 5A, F, I, J). Cytostome apical, circular in outline; oral basket funnel shaped, ~10–14 μm × ~2–4 μm, with slender fibres attached to anterior end; collar-like bulge around anterior end of body (Fig. 5A, J, N). Surface of body with shallow longitudinal furrows, along which somatic kineties run (Fig. 5K). Silverline system comprises irregular parallelograms connected by slanting fibrils to form a grid (Fig. 5G, M). Cytoplasm colourless, usually with several large food vacuoles coloured vividly blue–green by algal food and with several small brown or colourless globules (Fig. 5I). Extrusomes rod shaped, ~2–3 μm long after protargol staining, distributed along somatic kineties, lying orthogonal to pellicle (Fig. 5E, H, K, L, Q). One macronucleus, ellipsoidal, ~9–13 μm × ~8–11 μm after protargol staining, with several large spherical nucleoli, located slightly above equatorial region near cell mid-line; one micronucleus, globular, ~1 μm × ~1.5 μm after protargol staining, adjacent to macronucleus (Fig. 5A, P). Contractile vacuole in posterior third of body, ~11 μm in diameter when fully expanded (Fig. 5A, F, J). Locomotion by swimming slowly in upper layer of water, with an irregular trajectory.
Abbreviations:CV, coefficient of variation (as a percentage); Mean,arithmetic mean; N, number of specimens examined.
Oral ciliature consisting only of a ring of radially arranged circumoral dikinetids, each kinetosome bearing a cilium ~5 μm long (Fig. 5B, D, O). Most somatic cilia ~8–10 μm long; single caudal cilium ~20–30 μm long (Fig. 5A). In total, 19–25 longitudinal somatic kineties composed of monokinetids. Kinetids uniformly spaced in anterior half of body, forming seven to nine regular transverse rings (Fig. 5B–D).
Molecular data and phylogenetic analyses
The SSU rRNA gene sequences of the three isolates obtained in the present study have been deposited in the GenBank database with the length (in base pairs),GC content and GenBank accession numbers as follows: Platina marina (1616 bp, 42.57%, OQ538389); Parametacystis pulchra (1715 bp, 39.65%, OQ538388); and Apsiktrata gracilis (1542 bp, 42.15%, OQ538387). The SSU rRNA gene sequence-based tree was constructed as shown in Figure 6. Given that the topologies of the ML and BI trees were basically concordant, only the ML tree is presented, with support values from both algorithms indicated on the branches.
Our phylogenetic analyses revealed that the class Prostomatea was not monophyletic, owing to the class Plagiopylea nesting within it (Fig. 6). The orders Prorodontida and Prostomatida were also non-monophyletic. The order Prorodontida was divided into four major clades: Placidae, Colepidae + Prorodontida e + Holophryidae, Urotrichida + Plagiocampidae + Cryptocaryon idae + LagynusidaeandBalanionidae + Hexasterias.Thepossibility of the order Prorodontida being monophyletic was rejected by the AU test (log likelihood: 1.41818184; AU value: 0.015).
Even with the newly obtained data from the present study, only five 18S rRNA gene sequences of prostomatids were available,and these were clustered in three distantly separated clades. Platina marina (OQ538389) and Parametacystis pulchra (OQ538388) clustered together with high support (ML 92%, BI 1.00) and then grouped with Foissnerophrys alveolata (MW481206) with full support (ML 100%, BI 1.00; Fig. 6, marked with #). This clade nested deep within the class Prostomatea and was sister to the class Plagiopylea with maximal support (ML 100%, BI 1.00). Metacystis similis Zhang et al., 2015 (KT27408), the only available sequence of the genus Metacystis, clustered with species assigned to the families Holophryidae and Prorodontidae within the order Prorodontida with high support (ML 99%, BI 1.00) and occupied the basal position within this assemblage. Apsiktrata gracilis (OQ538387), the only species of the family Apsiktratidae, was sister to the family Placidae, although with no or low support (ML 48%, BI 0.92) The possibility of the order Prostomatida being monophyletic was rejected by the AU test (log likelihood: 1.41818184, AU value: 4 × 10 −16).
The putative secondary structures of the SSU rRNA gene V9 region of 18 species (16 prostomateans and two plagiopyleans) were predicted and compared (Fig. 7A). These taxa shared a similar secondary structure; the general model is formed by a long helix, with some median loops and a terminal loop. Species of the order Prostomatida shared the same structure as plagiopyleans, i.e. at a relatively conserved position near the 3ʹ end, they had a bulge consisting of two nucleotides (G and A) at bases 10 and 11. However, in the same position, Apsiktrata and other species of the order Prorodontida had a bulge consisting of three nucleotides (G, A and U) at bases 10, 11 and 12, respectively, and in some of these species there was also a bulge with two nucleotides (C and G) at bases 15 and 16, respectively.
The taxa that were the focus of the present study were divided into two groups, i.e. group 1 and group 2. The species of group 1 (Prostomatida + Trimyemidae) shared unique nucleotides at 31 sites in the SSU rRNA gene sequences (Fig. 7B). In contrast, the species of group 2 (Apsiktratidae + some representative prorodontids) shared unique nucleotides at 40 sites (Fig. 7B).
description: Published as part of Jiang, Limin, Wang, Congcong, Liao, Lijian, Al-Farraj, Saleh A., Warren, Alan & Hu, Xiaozhong, 2024, Re-evaluation of the systematic position of the order Prostomatida (Protista: Ciliophora), with the establishment of two new genera and two new species, pp. 849-864 in Zoological Journal of the Linnean Society 200 (4) on pages 855-857, DOI: 10.1093/zoolinnean/zlad116, http://zenodo.org/record/11241025