Planilamina ovata

Jin, Didi, Qu, Zhishuai, Wei, Bojue, Montagnes, David J. S., Fan, Xinpeng & Chen, Xiangrui, 2021, Two parasitic ciliates (Protozoa: Ciliophora: Phyllopharyngea) isolated from respiratory-mucus of an unhealthy beluga whale: characterization, phylogeny and an assessment of morphological adaptations, Zoological Journal of the Linnean Society 191, pp. 941-960 : 955-958

COMMENTS ON PLANILAMINA OVATA

Planilamina ovata was first collected from Atlantic bottlenose dolphins and a false killer whale in the USA, and was described by Ma et al. (2006) using the protargol staining method. The Ningbo population closely matches the original description in body shape, living and stained morphological features, except for some minor differences: the Ningbo population has a larger cell size in vivo (50–80 × 32–47 μm vs. 28–65 × 20–43 μm) and has a wider range number of right kineties (41–58 vs. 41–51). These are minor differences, and the ranges overlap, so we conclude that our identification of the Ningbo population is correct.

PARASITES AROSE FROM A DYSTERIIDAE ANCESTOR

Sniezek & Coats (1996) established the family Kyaroikeidae with Kyaroikeus as the type genus, placed this family in the order Cyrtophorida and, according to their morphogenesis, suggested this family to be closely related to the family Dysteriidae , which is currently composed only of free-living species. Later, Ma et al. (2006) erected the parasitic genus Planilamina and assigned it to Kyaroikeidae . Our SSU rDNA-based phylogenetic results support that the two genera belong to the subclass Cyrtophoria ( Fig. 8), and the general topologies of the subclass match the results of others (e.g. Gao et al., 2012; Chen et al., 2016; Qu et al., 2017). However, addition of our new gene sequences questions the validity of the Kyaroikeidae , because the clade represented by this family includes both parasitic and free-living genera and falls within Dysteriidae ( Fig. 8). This molecular clustering is also reflected by morphological characteristics; i.e. the kyaroikeids and dysteriids share a similar ciliary pattern in that they both include highly degenerated left kineties in the frontleft of the cell and short post-oral kineties in midbody ( Figs 1A, 6A; Lynn, 2008).

The two parasitic genera, representing the current Kyaroikeidae , do exhibit unique features (i.e. a large number of right kineties, large ciliated regions and dense cilia; Figs 1H, I, 6E). However, the free-living genus Trochilia , which clusters with the two parasitic ciliates, is morphologically more like other free-living members of the Dysteriidae (see fig. 4K in Liu et al., 2017). We suggest that the unique structures of the parasites (i.e. dense cilia, pellicular fold contained five to six layers, prominent oral cavity and pellicular pores) are convergent and arose through adaptation to their novel environment (see next section). Thus, we propose that, as a family, Kyaroikeidae is superfluous and suggest that, for the time being, it should be treated as a subfamily of Dysteriidae .

Regardless of the formal position of Kyaroikeidae , our phylogenetic analysis clearly indicates that the parasitic genera Planilamina and Kyaroikeus evolved from a free-living Dysteriidae-like ancestor. Furthermore, the close association of the free-living genus Trochilia to the parasitic genus Planilamina ( Fig. 8) implies that parasitism may have arisen more than once.

The free-living Dysteriidae tend to occupy periphytic environments, including sediments, sea ice and associations with marine algae ( Petz et al., 1995; Song & Wilbert, 2000; Meng et al., 2018). Marine mammals and, specifically, beluga whales will roll in sediments and rub against hard surfaces to remove dead skin and ectoparasites ( Smith et al., 1992). This may have allowed invasion of free-living Dysteriidae into their respiratory system, where they evolved to live permanently. Undoubtedly, when more parasitic and free-living taxa in these clades are recognized, our predictions may be more rigorously evaluated.

MORPHOLOGICAL MODIFICATIONS FOR A PARASITIC LIFE

Kyaroikeus paracetarius and Planilamina ovata appear to be obligatory parasites, as they could not live freely in water (see Methods). We suggest that they have morphologically adapted to this life by evolving structures that: (1) increase movement through viscous mucus; (2) improve ingestion of cellular material; and (3) adhere to flocs of mucus and facilitate food uptake. We outline these below and suggest they are worthy of further investigation.

Increased movement through viscous mucus: The free-living dysteriid species have few, fragmented right kineties (at most 13 rows in Dysteriidae spp. and only four in Trochilia spp. ), and these are constrained in a narrow, ventral groove with sparsely distributed cilia and weak microtubule structure ( Qu et al., 2015). In contrast, the two parasitic species have many non-fragmented right kineties that occupy a substantial part of the cell surface ( Figs 1H–J, 6E); they also are densely ciliated. We suggest that these modifications contribute to the motility of the organisms in viscous mucus. Moreover, the cortex of the dorsal surface is compressed into stripes and, under these pellicular folds, there is a unique microtubular structure (outlined below and described by Sniezek et al., 1995). In several groups of ciliates, microtubules that run longitudinally under the pellicle allow cells to maintain and change cell shape ( Lynn, 2008). Generally, there are only one or two layers and several bundles of these microtubules ( Calvo et al., 1986; Wirnsberger-Aescht et al., 1989; Kurth & Bardele, 2001). However, in K. paracetarius , each pellicular fold contains five to six layers and multiple bundles ( Figs 4H, 5), suggesting a greater role in movement, possibly allowing cells to penetrate the mucus. (Supporting information, Supplementary Video S1).

Improved ingestion of cellular material: Compared to the free-living dysteriids, the two parasitic genera have a pronounced oral cavity. The oral region reflects functional diversity among ciliates ( Eisler, 1992). For members of the free-living dysteriids, their oral region is prominent with strong nematodesmal rods, allowing them to capture particulate food ( Foissner et al., 1991; Qu et al., 2015). In contrast, the two parasitic species have densely arranged cilia near the oral area that are likely used to transport large volumes of liquid, moving large food particles (exfoliated epithelial cells) towards the cytostome into their deep oral cavity ( Figs 1E, F, 6A).

Adhering to mucus and improved food uptake: Pellicular pores that occur in the pellicle of sessile peritrich ciliates (e.g. Lom & Corliss, 1968; Finley et al., 1972) are considered to be sites of mucus material secretion, lorica-formation and stalk-production ( Bauer-Nebelsick et al., 1996; Lynn, 2008). To our knowledge, such pores are not reported in free-living dysteriids. However, they are also found in the nonciliated area and the podite of an ectoparasitic ciliate ( Brooklynella hostilis Lom & Nigrelli, 1970 ) of marine fishes ( Lom & Corliss, 1971). Similar structures occur in Cryptocaryon irritans Brown, 1951, a parasitic ciliate causing white spot disease of marine fishes, where pellicular openings are connected to small vesicles and may serve in enzyme excretion or food uptake ( Matthews et al., 1993). We observed pellicular pores in K. paracetarius ( Figs 3E, F, 4H, 5) and suggest that they may function in secretion of mucus material (for adhesion) or secretion of enzymes (aiding in feeding).