Unveiling the phylogenetic position of the type species of the syndinean genus <i>Amoebophrya</i> and broad divergence of the <i>A. ceratii</i> complex

The syndinean genus Amoebophrya comprises endoparasitic dinoflagellates that infect various marine protists. Since the description of the type species, A. sticholonchae, a parasite of the radiolarian Sticholonche zanclea, an additional six Amoebophrya species have been formally described. Over the past 2 decades, intensive high-throughput environmental sequencing has discovered widespread and extensive diversity of novel marine alveolate group II (MALV II), generally considered the Amoebophryidae. Of the large number of uncultured environmental sequences, only several clades of MALV II are associated with a few sequences representing the Amoebophrya ceratii complex (ACC) infecting free-living dinoflagellates. In this study, we examined developmental traits and molecular signatures of A. sticholonchae and nine members of the ACC infecting diverse dinoflagellates from Korean coastal waters. Molecular phylogeny inferred from 18S rRNA gene sequences revealed that A. sticholonchae clustered with radiolarian-associated sequences, forming a basal lineage to the ACC, which comprised eight distinct clades (ACC1-8). Developmental traits were phylogenetically structured. Of the eight clades, ACC1-4 infected host nuclei and lacked temporary polyploidy, except that cytoplasmic infection occurred in Karlodinium veneficum cells containing multiple parasites; ACC5 and ACC8 exhibited cytoplasmic infection with temporary polyploidy, except for Prorocentrum micans, showing a similar frequency of cytoplasmic and nuclear infections, and ACC6-7 showed nuclear infection accompanied by temporary polyploidy. In contrast, A. sticholonchae uniquely combined cytoplasmic infection with the absence of temporary polyploidy. Although complex, these results suggest that phylogenetic divergence of the ACC group is generally consistent with developmental stage-specific characteristics, which is potentially useful in future taxonomic classification of this diverse lineage.