Abandoning sex: multiple origins of asexuality in the ciliate Tetrahymena

The genus Tetrahymena (Ciliophora, Oligohymenophorea) probably represents the best studied ciliate genus. At present, more than forty species have been described. All are colorless, i.e. they do not harbor symbiotic algae, and as aerobes they need at least microaerobic habitats. Here, we present the morphological and molecular description of the first green representative, Tetrahymena utriculariae n. sp., living in symbiosis with endosymbiotic algae identified as Micractinium sp. (Chlorophyta). The full life cycle of the ciliate species is documented, including trophonts and theronts, conjugating cells, resting cysts and dividers. This species has been discovered in an exotic habitat, namely in traps of the carnivorous aquatic plant Utricularia reflexa (originating from Okavango Delta, Botswana). Green ciliates live as commensals of the plant in this anoxic habitat. Ciliates are bacterivorous, however, symbiosis with algae is needed to satisfy cell metabolism but also to gain oxygen from symbionts. When ciliates are cultivated outside their natural habitat under aerobic conditions and fed with saturating bacterial food, they gradually become aposymbiotic. Based on phylogenetic analyses of 18S rRNA and mitochondrial cox1 genes T. utriculariae forms a sister group to Tetrahymena thermophila.

Section: Discussionsupporting

confidence: 64%

The Green Tetrahymena utriculariae n. sp. (Ciliophora, Oligohymenophorea) with Its Endosymbiotic Algae (Micractinium sp.), Living in Traps of a Carnivorous Aquatic Plant

Pitsch

Adamec

Dirren

et al. 2016

“…For both TMA and TMB, the sequence similarities are consistent with cox 1 and SSU phylogenies (Chantangsi et al. ; Doerder ) which show that T. thermophila and its amicronucleates are most closely related to nsp25, followed by nsp15, T. malaccensis and more distantly T. elliotti . All of these species form a well‐supported clade in the larger “borealis” clade, but are not closely related to T. borealis (Chantangsi et al.…”

Section: Discussionsupporting

confidence: 78%

Molecular Polymorphism in the MTA and MTB Mating Type Genes of Tetrahymena thermophila and Related Asexual Species

Booth

Wolfe

Doerder

2015

Each of the seven mating types of Tetrahymena thermophila is determined by a pair of large genes, MTA and MTB, whose expression peaks at early conjugation. Each protein consists of a mating-type specific domain and a common transmembrane domain. To assess variation in natural populations, regions of both domains from wild isolates expressing mating types V and VII were analyzed. Corresponding regions of amicronucleates incapable of mating also were examined. MTA and MTB showed high haplotype diversity, with greater sequence variation in MTB. Mating type VII was less variable than mating type V, suggesting more recent origin. No polymorphism distinguished between mat1- and mat2-like alleles encoding different arrays of mating types, nor did polymorphisms give evidence of population structure. MTA and MTB variants have different phylogenies, suggesting independent rather than concerted evolution, and are under weak purifying selection. Codon usage is less biased than for housekeeping genes, and reassigned glutamine encoding stop codons are preferentially used. Amicronucleate T. thermophila and closely related nsp15 and nsp25 have higher levels of nucleotide and amino acid substitution, consistent with cox1 distances. The results suggest that complete sequencing of mating type genes of wild isolates coupled with functional analysis will be informative.

“…In other instances, however, there are no close haplotypes, for example, T. thermophila and its relatives T. alphathermophila and T. betathermophila , or a species is represented solely by amicronucleate isolates. For reasons discussed by Doerder (), some of these could be ancient, their micronucleate progenitors having become extinct. T. pyriformis , for example, is on an SSUrRNA branch (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Evidence of population structure suggests dispersal is rare, and but population intrafertility, though rarely studied (Nyberg ), suggests it is sufficiently common to maintain species integrity, as also observed for widely dispersed and intrafertile species of Paramecium (Przybos and Surmacz ). As for dispersal itself, if some species are indeed tens of millions of years old (Doerder ; Wright and Lynn ), continental drift may be important. On a more recent time scale, dispersal may have been facilitated by an abundance of wetlands following glaciation.…”

Section: Discussionmentioning

confidence: 99%

Barcodes Reveal 48 New Species ofTetrahymena,Dexiostoma, andGlaucoma: Phylogeny, Ecology, and Biogeography of New and Established Species

Doerder

2018

Tetrahymena mitochondrial cox1 barcodes and nuclear SSUrRNA sequences are particularly effective at distinguishing among its many cryptic species. In a project to learn more about Tetrahymena natural history, the majority of >1,000 Tetrahymena-like fresh water isolates were assigned to established Tetrahymena species with the remaining assigned to 37 new species of Tetrahymena, nine new species of Dexiostoma and 12 new species of Glaucoma. Phylogenetically, all but three Tetrahymena species belong to the well-established "australis" or "borealis" clades; the minority forms a divergent "paravorax" clade. Most Tetrahymena species are micronucleate, but others are exclusively amicronucleate. The self-splicing intron of the LSUrRNA precursor is absent in Dexiostoma and Glaucoma and was likely acquired subsequent to the "australis/borealis" split; in some instances, its sequence is diagnostic of species. Tetrahymena americanis, T. elliotti, T. gruchyi n. sp., and T. borealis, together accounted for >50% of isolates, consistent with previous findings for established species. The biogeographic range of species found previously in Austria, China, and Pakistan was extended to the Nearctic; some species show evidence of population structure consistent with endemism. Most species were most frequently collected from ponds or lakes, while others, particularly Dexiostoma species, were collected most often from streams or rivers. The results suggest that perhaps hundreds of species remain to be discovered, particularly if collecting is global and includes hosts of parasitic forms.