A new view on the morphology and phylogeny of eugregarines suggested by the evidence from the gregarineAncora sagittata(Leuckart, 1860) Labbé, 1899 (Apicomplexa: Eugregarinida)

Abstract

BackgroundGregarines are a group of early branching Apicomplexa parasitizing invertebrate animals. Despite their wide distribution and relevance to the understanding the phylogenesis of apicomplexans, gregarines remain understudied: light microscopy data are insufficient for classification, and electron microscopy and molecular data are fragmentary and overlap only partially.MethodsScanning and transmission electron microscopy, PCR, DNA cloning and sequencing (Sanger and NGS), molecular phylogenetic analyses using ribosomal RNA genes (18S (SSU), 5.8S, and 28S (LSU) ribosomal DNAs (rDNAs)).Results and DiscussionWe present the results of an ultrastructural and molecular phylogenetic study on the marine gregarine Ancora sagittata from the polychaete Capitella capitata followed by evolutionary and taxonomic synthesis of the morphological and molecular phylogenetic evidence on eugregarines. The ultrastructure of Ancora sagittata generally corresponds to that of other eugregarines, but reveals some differences in epicytic folds (crests) and attachment apparatus to gregarines in the family Lecudinidae, where Ancora sagittata has been classified. Molecular phylogenetic trees based on SSU (18S) rDNA reveal several robust clades (superfamilies) of eugregarines, including Ancoroidea superfam. nov., which comprises two families (Ancoridae fam. nov. and Polyplicariidae) and branches separately from the Lecudinidae; thus, all representatives of Ancoroidea are here officially removed from the Lecudinidae. Analysis of sequence data also points to possible cryptic species within Ancora sagittata and the inclusion of numerous environmental sequences from anoxic habitats within the Ancoroidea. LSU (28S) rDNA phylogenies, unlike the analysis of SSU rDNA alone, recover a well-supported monophyly of the gregarines involved (eugregarines), although this conclusion is currently limited by sparse taxon sampling and the presence of fast-evolving sequences in some species. Comparative morphological analyses of gregarine teguments and attachment organelles lead us to revise their terminology. The terms “longitudinal folds” and “mucron” are restricted to archigregarines, whereas the terms “epicystic crests” and “epimerite” are proposed to describe the candidate synapomorphies of eugregarines, which, consequently, are considered as a monophyletic group. Abolishing the suborders Aseptata and Septata, incorporating neogregarines into the Eugregarinida, and treating the major molecular phylogenetic lineages of eugregarines as superfamilies appear as the best way of reconciling recent morphological and molecular evidence. Accordingly, the diagnosis of the order Eugregarinida Léger, 1900 is updated.

Long‐read metabarcoding of the eukaryotic rDNA operon to phylogenetically and taxonomically resolve environmental diversity

“…Comparisons to the 18S region alone of the queries (~1,200 bp) provided a similar overall topology to the combined tree, but with lower overall resolution (Figure S2). Furthermore, some key groups in the apicomplexan phylogeny were either missing or not supported by the 18S‐only tree, a pattern that was also recovered by previous analyses (Simdyanov et al, , ). Together, our phylogenetic comparisons revealed that 18S and 28S together provide increased resolution compared to 18S alone, but the differences between single‐ and two‐gene trees vary across groups.…”

Why the –omic future of Apicomplexa should include gregarines

“…Biological and morphological studies have established that in gregarines, this apical complex is used for host cell attachment to allow the parasite to feed from its host cell by a process known as myzocytosis (Schrével et al., 2016; Simdyanov and Kuvardina, 2007). The host cell penetration by the parasite is not complete as the gregarine remains extracellular with only its apical end intimately engaged in a host–parasite interplay that has been studied at microscopic level but whose molecular actors are poorly defined (Valigurova et al., 2007; Schrével et al., 2016; Simdyanov et al., 2017). See also (Desportes and Schrével, 2013) for exhaustive descriptions of several additional examples.…”

“…It is important here to indicate that there are alternative invasive modes in apicomplexan parasites such as Theileria and Cryptosporidium that differ from the better described Toxoplasma / Plasmodium mode (Gubbels and Duraisingh, 2012). What are the molecular similarities between the T. gondii and P. falciparum parasitophorous vacuole make up and the food vacuole of gregarines, that forms at the gregarine–host cell interface (Valigurova et al., 2007; Schrével et al., 2016; Simdyanov et al., 2017)? Or is the similarity stronger to the feeder organelle of epicellular Cryptosporidium ?…”

“…Regarding the first point, there are currently ∼1770 formally described gregarine species, unequally distributed between archigregarines (∼20), eugregarines (∼1700) and neogregarines (∼50) (Portman and Slapeta, 2014). In parallel, taxonomic and phylogenetic revisions concerning gregarines are a currently very active – but quite unstable – field with a diversity of successive proposals regarding their phylogenetic inter‐relations as well as with other apicomplexan parasites (Cavalier‐Smith, 2014; Schrével et al., 2016; Rueckert and Horak, 2017; Simdyanov et al., 2017; Simdyanov et al., 2018). Molecular phylogenies are nowadays mostly based on usage of SSU rDNA marker, more rarely complete ribosomal loci (Diakin et al., 2016; Diakin et al., 2017; Simdyanov et al., 2018).…”

“…A close examination of the recent taxonomy of Apicomplexa clearly mentions that most groups are currently polyphyletic or paraphyletic, notably: Aconoidasida, Coccidia, Gregarinasina, Archigregarinorida and Eugregarinorida (Adl et al., 2018). This is mostly due to lack of sufficiently informative and resolving data concerning these organisms, still mostly based on SSU rRNA phylogenies (see (Cavalier‐Smith, 2014; Simdyanov et al., 2017; Simdyanov et al., 2018) to give few examples) and recent, emerging, phylogenomic analyses (Janouskovec et al., 2019; Kwong et al., 2019; Mathur et al., 2019). Certainly, insufficient sampling also prevents a solid and integrated vision of phylogenetic inter‐relationships for all of these species (Del Campo et al., 2019; Janouskovec et al., 2019; Kwong et al., 2019; Mathur et al., 2019).…”