Taxonomy and phylogeny of Aphanomycopsis bacillariacearum, a holocarpic oomycete parasitoid of the freshwater diatom genus Pinnularia

Abstract

Investigations into simple holocarpic oomycetes are challenging, because of the obligate biotrophic nature of many lineages and the periodic presence in their hosts. Thus, despite recent efforts, still, the majority of species described remains to be investigated for their phylogenetic relationships. One of these species is Aphanomycopsis bacillariacearum, the type species of the genus Aphanomycopsis. Species of Aphanomycopsis are endobiotic holocarpic parasites of diverse hosts (e.g., diatoms, desmids, dinoflagellates). All species classified in this genus were assigned to it based on the presence of branching hyphae and the formation of two generations of zoospores, of which the first one is not motile. Originally, Aphanomycopsis with its type species, A. bacillariacearum, had been classified in the Saprolegniaceae. However, the genus has undergone multiple taxonomic reassignments (to Ectrogellaceae, Lagenidiaceae, and Leptolegniellaceae) in the past. To settle the taxonomy and investigate the phylogenetic placement of Aphanomycopsis, efforts were undertaken to isolate A. bacillariacearum from its original host, Pinnularia viridis and infer its phylogenetic placement based on nrSSU (18S) sequences. By targeted isolation, the diatom parasitoid was rediscovered from Heiðarvatn lake, Höskuldsstaðir, Iceland. Phylogenetic reconstruction shows that A. bacillariacearum from Pinnularia viridis is embedded within the Saprolegniales, and largely unrelated to both diatom-infecting oomycetes in the Leptomitales (Ectrogella, Lagenisma) and those placed within the early-diverging lineages (Miracula, Diatomophthora) of the Oomycota.

Lagena—an overlooked oomycete genus with a wide range of hosts

“…It is noteworthy that phylogenetic investigations on diatom-infecting oomycetes have revealed that, contrary to the assumption of Dick (2001), these were not forming a monophyletic assemblage, but found to be spread throughout the phylogeny of oomycetes (Thines et al 2015;Buaya and Thines 2020b;Buaya et al 2017Buaya et al , 2021. The genus Ectrogella was revealed to indeed belong to the Saprolegniomycetes (Buaya and Thines 2019), which was also the case for Aphanomycopsis (Buaya et al 2021) and for Lagenisma (Thines et al 2015). In addition to this, it was also found that some diatom parasitoids formed a distinct, early-diverging lineage of oomycetes and were described as members of a new genus, Miracula (Buaya et al 2017).…”

“…However, recent phylogenetic investigations have shown that Olpidiopsis is an early diverging lineage restricted to oomycete hosts, leading to the transfer of species infecting red algae to the genus Pontisma (Buaya et al 2019a). It is noteworthy that phylogenetic investigations on diatom-infecting oomycetes have revealed that, contrary to the assumption of Dick (2001), these were not forming a monophyletic assemblage, but found to be spread throughout the phylogeny of oomycetes (Thines et al 2015;Buaya and Thines 2020b;Buaya et al 2017Buaya et al , 2021. The genus Ectrogella was revealed to indeed belong to the Saprolegniomycetes (Buaya and Thines 2019), which was also the case for Aphanomycopsis (Buaya et al 2021) and for Lagenisma (Thines et al 2015).…”

Miracula einbuarlaekurica sp. nov., a new holocarpic endoparasitoid species from pennate freshwater diatoms in Iceland

“…Samples collected and preserved in 70% ethanol in the same manner were deposited in the herbarium collection of the Senckenberg Museum of Natural History (Herbarium Senckenbergianum, FR), Cryptogams Section, Frankfurt am Main (accessions: FR-0046131 for isolate 1, FR-0046132 for isolate 2, both isolated from the same raw sample). Morphological characterisation of the parasite was done as described earlier (Buaya et al 2021 ) using a compound light microscope (Imager2, Carl Zeiss Göttingen, Germany) with DIC, and photographs were taken using a Zeiss Axiocam MRc5 (Carl Zeiss, Göttingen, Germany). In addition, cellulose presence in the thallus wall of the diatom parasitoid was tested using zinc iodine chloride solution (Carl Roth GmbH, Germany).…”

“…Some PCR amplicons were cloned as previously described (Buaya et al 2020b ). The resulting sequences were edited and assembled using Geneious (version 5.6, Biomatters, Auckland, New Zealand), and the assembled consensus sequences were added to the dataset of Buaya et al ( 2020b ), supplemented with additional sequences from Buaya et al ( 2021 ). Subsequently, alignments were done using mafft version 7 as implemented on the mafft webserver (Katoh et al 2019 ), with the Q-INS-i algorithm and a gap opening penalty of 2.4 to account for the high conservation of the nrSSU.…”

“…Cyclotella, Pleurosira), with most of the known species isolated from temperate regions (Buaya and Thines 2019 c; Scherffel 1925 ; Sparrow 1933 ; Zopf 1884 ). In addition to the recently-described species Miracula moenusica (Buaya and Thines 2019b ), only three other freshwater species ( Aphanomycopsis bacillariacearum, Diatomophthora gillii, Ectrogella bacillariacearum ) have been investigated for their molecular phylogeny (Buaya et al 2019a , 2021 ; Buaya and Thines 2020a ), leaving the majority of known species unplaced. Freshwater diatom parasitoids were classified in the genera Aphanomycopsis (Scherffel 1925 ), Ectrogella (Zopf 1884 ), Lagenidium (Zopf 1878 ), Miracula (Buaya et al 2017 ), and Olpidiopsis (Cornu 1872 ).…”