Abstract
Nuclear ribosomal and mitochondrial genes have been utilized individually or in combination to identify known species and discriminate closely related species. However, compared with metazoans, genetic diversity within the ciliate order Euplotida is poorly known. The aim of this study is to investigate how much nucleotide sequence divergence occurs within Euplotes. A total of 14 new gene sequences, comprising four SSU rDNA and 10 CO1 (including three species for the first time) were obtained. Phylogenetic analyses were carried out based on sequences of two DNA fragments from the same 27 isolates. We found that CO1 revealed a larger interspecific divergence than the SSU rRNA gene, thus demonstrating a higher resolution for separating congeners. Genetic distances differ significantly at the species level. Euplotes balteatus was revealed to have a large intraspecific variation at two loci, while E. vannus showed different levels of haplotype variability, which appeared as a polyphyletic cluster on the CO1 tree. These high genetic divergences suggest the presence of more cryptic species. By contrast, the CO1 gene showed low variability within E. raikovi, appearing as monophyletic clusters, which indicates that this species could be identified based on this gene. Conclusively, CO1 is a suitable marker for the study of genetic diversity within Euplotes, and increased taxon sampling gives an opportunity to screen relationships among members of this genus. Additionally, current data present no clear biogeographical pattern for Euplotes.
Highlights
Traditional taxonomy of protozoans relies heavily on high expertise
The SSU rDNA sequences of ten populations (Ev8-11, Er1-3, Eb1-2, En) were between 1480 and 1905 nucleotides long, and GC content was in the range of 43.31–46.15%
The SSU rDNA sequences of all 27 populations assigned to the ten species were analyzed here
Summary
Traditional taxonomy of protozoans relies heavily on high expertise. Numerous studies have suggested a higher protozoan diversity than originally proposed [1], which brings a great challenge for taxonomy. Ciliates (phylum Ciliophora), as a diverse group of protozoans, have a wide geographical distribution and play a vital ecological role as trophic links in microbial food webs [3,4,5]. Despite this significance, our knowledge of ciliate genetic diversity is neither complete nor equal across the entire phylum, especially when compared with the huge amount of morphological information. Recent studies have shown the limitations of SSU rDNA for discriminating closely related congeners [15,16,17,18]
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