Morphology, morphogenesis, and molecular phylogeny of a novel saline soil ciliate, Heterourosomoida sinica n. sp. (Ciliophora, Hypotrichia)

Abstract

The living morphology, infraciliature, morphogenesis and phylogenetic position of a new hypotrichous ciliate, Heterourosomoida sinica n. sp., discovered in saline soil in Northwest of China, were investigated. Heterourosomoida sinica n. sp. is characterized as follows: body size about 70–95 × 20–30 μm in vivo, ellipsoid or elongate; cortical granules dark green, 0.5 μm across, arranged around dorsal cilia and in short irregular rows on ventral side; consistently 18 frontoventral-transverse cirri; left and right marginal rows composed of 17–24 and 13–21 cirri, respectively; two macronuclear nodules and one or two micronuclei. The detailed morphogenesis of a species of Heterourosomoida is reported for the first time. The ontogenesis on dorsal side is basically similar to that in Urosomoida-species and the process on ventral side is characterized by: 18 frontoventral-transverse cirri develop from six anlagen with the anlagen V and VI formed in the primary mode. Phylogenetic analyses based on SSU rDNA sequence data reveal that Heterourosomoida sinica n. sp. clusters with Kleinstyla dorsicirrata and forms a clade with the type species H. lanceolata.

Introduction

Recent faunistic studies have revealed numerous new taxa of hypotrichous ciliates, suggesting that this group is even more diverse than previously supposed (e.g., Berger 1999, 2006, 2008, 2011; Chen et al. 2018; Foissner 2016; Jung et al. 2018; Kim et al. 2019; Kaur et al. 2019; Li et al. 2019; Luo et al. 2019). Furthermore, much work has been carried out on the morphogenesis and molecular phylogeny of hypotrichs, which has led to a better understanding of their systematics and evolutionary relationships (Li et al. 2018; Luo et al. 2016; Huang et al. 2016; Lyu et al. 2018; Zhang et al. 2019).

The taxon Oxytrichidae Ehrenberg, 1838, mainly characterized by dorsal kinety fragmentation, is one of the most diverse and complex groups of hypotrichs. Taxonomy of this kind of organism are always challenging. The family Urosomoididae, established by Foissner (2016) with Urosomoida Hemberger in Foissner, 1982 as name-bearing type genus, is diagnosed by less than 18 frontoventral-transverse cirri and lacking dorsal kinety fragmentation. The genus Heterourosomoida, established by Singh and Kamra (2015) with H. lanceolata (original combination Oxytricha lanceolata Shibuya, 1930) as type species, is mainly defined as follows: usually 18 frontoventral-transverse cirri, three dorsal kineties with one caudal cirrus each, usually one dorsomarginal row; dorsal morphogenesis basically in Urosomoida pattern. Hence, hitherto, the family attribution of Heterourosomoida remains unknown. Berger (2006, 2008) suggested to place taxa like Urosomoida, which has a dorsomarginal kinety, but lack dorsal kinety 3 fragmentation to the non-monophyletic assemblage “non-oxytrichid Dorsomarginalia” and considered these taxa outside the Oxytrichidae, but within Dorsomarginalia regardless of whether or not the taxon has 18 frontoventral-transverse cirri. At present, two valid species are assigned to Heterourosomoida, i.e. H. lanceolata (Shibuya, 1930) Singh and Kamra, 2015 and H. salinarum Foissner, 2016 (Foissner 2016; Singh and Kamra 2015).

In May 2017, an unknown hypotrichous ciliate was isolated from saline soil in Dingbian, Yulin, Shaanxi Province, China. Observations of its morphology both in vivo and after protargol staining demonstrate that it represents a novel species within the genus Heterourosomoida. In this paper, its morphology and morphogenesis are described. The small subunit ribosomal DNA (SSU rDNA) of the new isolate was sequenced and analyzed in order to estimate its phylogenetic position.