Morphological and molecular phylogeny of dileptid and tracheliid ciliates: Resolution at the base of the class Litostomatea (Ciliophora, Rhynchostomatia)

Abstract

Dileptid and tracheliid ciliates have been traditionally classified within the subclass Haptoria of the class Litostomatea. However, their phylogenetic position among haptorians has been controversial and indicated that they may play a key role in understanding litostomatean evolution. In order to reconstruct the evolutionary history of dileptids and tracheliids, and to unravel their affinity to other haptorians, we have used a cladistic approach based on morphological evidence and a phylogenetic approach based on 18S rRNA gene sequences, including eight new ones. The molecular trees demonstrate that dileptids and tracheliids represent a separate subclass, Rhynchostomatia, that is sister to the subclasses Haptoria and Trichostomatia. The Rhynchostomatia are characterized by a ventrally located oral opening at the base of a proboscis that carries a complex oral ciliature. We have recognized two orders within Rhynchostomatia. The new order Tracheliida is monotypic, while the order Dileptida comprises two families: the new, typically bimacronucleate family Dimacrocaryonidae and the multimacronucleate family Dileptidae. The Haptoria evolved from the last common ancestor of the Litostomatea by polarization of the body, the oral opening locating more or less apically and the oral ciliature simplifying. The Trichostomatia originated from a microaerophylic haptorian by further simplification of the oral ciliature, possibly due to an endosymbiotic lifestyle.

Constraints on Phylogenetic Interrelationships among Four Free-living Litostomatean Lineages Inferred from 18S rRNA gene-ITS Region sequences and Secondary Structure of the ITS2 molecule

“…This result corresponds well with some recent molecular studies based on the 18S rRNA gene (e.g. Strüder- Kypke et al 2006;Gao et al 2008;Vďačný et al 2010Vďačný et al , 2011aVďačný et al , 2011bVďačný et al , 2014Rajter and Vďačný 2016) and is also corroborated by the specific length of stems and loops of helices II and III of the ITS2 molecule. Specifically, there are eight and 15 nucleotide pairs in helix II and III in rhynchostomatians, while seven and 14 nucleotide pairs in helix II and III in haptorians.…”

“…Within the Rhynchostomatia, the orders Tracheliida and Dileptida were uncovered with full or high statistical support only in 18S rRNA gene and concatenated 18S-ITS region trees. The branching pattern within the order Dileptida was congruent with results of our previous phylogenetic studies (Vďačný et al 2011bJang et al 2014;. The family Dileptidae was, however, revealed to be monophyletic with high statistical support only in Bayesian trees inferred from the concatenated datasets, while the family Dimacrocaryonidae was depicted monophyletic with full or high support in all 18S rRNA gene and 18S-ITS region trees.…”

“…haptorians) to be independent lineages (Strüder- Kypke et al 2006;Gao et al 2008;Vďačný et al 2010Vďačný et al , 2011aVďačný et al , 2011b. Indeed, members of the subclass Rhynchostomatia exhibit some important ciliate plesiomorphic features: the aforementioned complex oral ciliature where circumoral kinety corresponds to paroral membrane and preoral kineties to adoral membranelles of other ciliates, the ventral cytostome located at the base of the proboscis, and anarchic fields produced during stomatogenesis (Vďačný and Foissner 2009;Vďačný et al 2010Vďačný et al , 2011aVďačný et al , 2011b. In contrast, haptorians display several derived morphological and morphogenetical features: a simple oral ciliature, an apical cytostome, and a non-complex stomatogenesis without formation of anarchic fields (Foissner 1996).…”

“…The majority of recent phylogenetic studies focused mainly on the internal evolutionary relationships within the subclass Rhynchostomatia (Vďačný et al 2011bJang et al 2014;, the order Pleurostomatida (Lin et al 2007Gao et al 2008;Pan et al 2010Pan et al , 2013Wu et al 2015Wu et al , 2016, and the orders Spathidiida and Haptorida (Vďačný et al 2011aVďačný and Foissner 2013;Jang et al 2015Jang et al , 2017Rajter and Vďačný 2016). In spite of the great effort and considerable increase in taxon sampling during the last decade, phylogenetic relationships among these main monophyletic groups have been left almost unresolved.…”

“…According to Vďačný et al (2011a), they are classified in two subclasses, the Rhynchostomatia Jankowski, 1980 and the Haptoria Corliss, 1974. The former subclass is characterized by a prominent proboscis and a ventrally located cytostome, and includes two orders, the Tracheliida Vďačný et al, 2011b andthe Dileptida Jankowski, 1978. The latter subclass contains five well-defined groups: (i) the order Pleurostomatida Schewiakoff, 1896 with laterally flattened body and ventrally extending, slit-like cytostome; (ii) the order Didiniida Jankowski, 1978 with barrel-shaped to globular body carrying an anterior oral cone; (iii) the order Lacrymariida Lipscomb and Riordan, 1990 with teardrop-shaped cell whose anterior part is differentiated into a head-like structure; (iv) the order Spathidiida Foissner and Foissner, 1988 with cylindroidal to spatulate body equipped with a typically three-rowed dorsal brush; and finally (v) the order Haptorida Corliss, 1974 with bursiform body and usually two-rowed brush.…”