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. 2021 Apr;172(2):125803.
doi: 10.1016/j.protis.2021.125803. Epub 2021 Mar 11.

Macronuclear Plasticity in Two South American Populations of Spirostomum (Ciliophora, Heterotrichea) Warns About its Use for Species Classification: Revision and New Insights

Pedro H Campello-Nunes  1 , Leonardo D Fernández  2 , Thiago S Paiva  1 , Carlos A G Soares  3 , Inácio D Silva-Neto  1 , Noemi M Fernandes  4
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Macronuclear Plasticity in Two South American Populations of Spirostomum (Ciliophora, Heterotrichea) Warns About its Use for Species Classification: Revision and New Insights

Pedro H Campello-Nunes et al. Protist. 2021 Apr.

Abstract

Spirostomum is a widely distributed heterotrichean genus composed of well-known species with described ecology and phylogenetic affinities. The morphological classification of Spirostomum species is mostly based on the body size/shape, number of cortical granule rows and macronuclear characteristics. These features along with molecular phylogenies based on ribosomal genes divide the genus into two phylogroups, one including species with a compact macronucleus, and another including species with a moniliform macronucleus. Here, we present our observations on atypical Spirostomum specimens with unusually two distinct macronuclei and shortened adoral zone of membranelles. These atypical forms appeared in the cultures of S. minus and S. yagiui, sampled at different sites in South America (Chile and Brazil) and associated with unrelated substrate types. Morphological observations of living and stained cells, 18S rRNA gene analyses, and a thorough investigation of the literature suggest that the atypical phenotype may be a result of uncommon pathways during the conjugative process. Thus, we demonstrate that studies of ciliate natural populations and their morphological variations, especially from undersampled biogeographical regions, can reveal the boundaries of widely used morphological characters for Spirostomum taxonomy and species identification.

Keywords: Ciliates; Heterotrichea; South America; macronuclear retention; macronucleus; morphogenesis.

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Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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