Trichostomatid Ciliates (Alveolata, Ciliophora, Trichostomatia) Systematics and Diversity: Past, Present, and Future

Abstract

The gastrointestinal tracts of most herbivorous mammals are colonized by symbiotic ciliates of the subclass Trichostomatia, which form a well-supported monophyletic group, currently composed by ∼1,000 species, 129 genera, and 21 families, distributed into three orders, Entodiniomorphida, Macropodiniida, and Vestibuliferida. In recent years, trichostomatid ciliates have been playing a part in many relevant functional studies, such as those focusing in host feeding efficiency optimization and those investigating their role in the gastrointestinal methanogenesis, as many trichostomatids are known to establish endosymbiotic associations with methanogenic Archaea. However, the systematics of trichostomatids presents many inconsistencies. Here, we stress the importance of more taxonomic works, to improve classification schemes of this group of organisms, preparing the ground to proper development of such relevant applied works. We will present a historical review of the systematics of the subclass Trichostomatia highlighting taxonomic problems and inconsistencies. Further on, we will discuss possible solutions to these issues and propose future directions to leverage our comprehension about taxonomy and evolution of these symbiotic microeukaryotes.

Cryptic Diversity in Paramecium multimicronucleatum Revealed with a Polyphasic Approach

“…The issue of whether “morphology or molecules” should primarily be taken into account within systematics of different groups of protists [ 43 , 44 ], and in particular, in ciliates [ 45 , 46 , 47 ] has been under debate for the last decade. Currently, molecular data are favored and seem to give more detailed results, although morphological traits are still extremely valuable and provide important complementary data [ 48 , 49 , 50 , 51 ]. While general appearance (i.e., cell shape and size) allows for quick and rough species assignment in Paramecium , type and number of MICs, along with the morphology of contractile vacuoles, are considered the most important discriminating fine traits of these ciliates [ 16 ].…”

Insights into the systematics of the family Ophryoscolecidae (Ciliophora, Entodiniomorphida)

“…In the following years, new ophryoscolecids were discovered in several wild and domestic ruminant host species [revised in Cedrola et al, 2020]. Based on these first studies, Dogiel (1927) proposed the first classification for the family Ophryoscolecidae (Table S1).…”

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They are mainly associated with ruminant herbivorous mammals and participate in several digestive processes of their hosts (Firkins et al, 2020). Accordingly, questions concerning the metabolism of these ciliates have been intensively studied (Firkins et al, 2020;Newbold et al, 2015), while questions about their systematics still remain poorly understood (Cedrola et al, 2020).Ophryoscolecids were discovered in the XIX century when Gruby and Delafond (1843) reported the occurrence of several "animalcules" inhabiting the stomach and intestines of herbivorous mammals. The authors did not describe or draw any of the species found.

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“…The family Ophryoscolecidae Stein, 1859 currently comprises 225 species (Table ) of trichostomatid ciliates (Ciliophora, Litostomatea, and Trichostomatia), distributed among three subfamilies: Entodiniinae Lubinsky, 1957, Diplodiniinae Lubinsky, 1957, and Ophryoscolecinae Lubinsky, 1957 (Cedrola et al, 2020). They are mainly associated with ruminant herbivorous mammals and participate in several digestive processes of their hosts (Firkins et al, 2020).…”

“…They are mainly associated with ruminant herbivorous mammals and participate in several digestive processes of their hosts (Firkins et al, 2020). Accordingly, questions concerning the metabolism of these ciliates have been intensively studied (Firkins et al, 2020; Newbold et al, 2015), while questions about their systematics still remain poorly understood (Cedrola et al, 2020).…”