Taxonomic revision of the genus Zygorhizidium: Zygorhizidiales and Zygophlyctidales ord. nov. (Chytridiomycetes, Chytridiomycota)

Abstract

During the last decade, the classification system of chytrids has dramatically changed based on zoospore ultrastructure and molecular phylogeny. In contrast to well-studied saprotrophic chytrids, most parasitic chytrids have thus far been only morphologically described by light microscopy, hence they hold great potential for filling some of the existing gaps in the current classification of chytrids. The genus Zygorhizidium is characterized by an operculate zoosporangium and a resting spore formed as a result of sexual reproduction in which a male thallus and female thallus fuse via a conjugation tube. All described species of Zygorhizidium are parasites of algae and their taxonomic positions remain to be resolved. Here, we examined morphology, zoospore ultrastructure, host specificity, and molecular phylogeny of seven cultures of Zygorhizidium spp. Based on thallus morphology and host specificity, one culture was identified as Z. willei parasitic on zygnematophycean green algae, whereas the others were identified as parasites of diatoms, Z. asterionellae on Asterionella, Z. melosirae on Aulacoseira, and Z. planktonicum on Ulnaria (formerly Synedra). According to phylogenetic analysis, Zygorhizidium was separated into two distinct order-level novel lineages; one lineage was composed singly of Z. willei, which is the type species of the genus, and the other included the three species of diatom parasites. Zoospore ultrastructural observation revealed that the two lineages can be distinguished from each other and both possess unique characters among the known orders within the Chytridiomycetes. Based on these results, we accommodate the three diatom parasites, Z. asterionellae, Z. melosirae, and Z. planktonicum in the distinct genus Zygophlyctis, and propose two new orders: Zygorhizidiales and Zygophlyctidales.

Single dominant diatom can host diverse parasitic fungi with different degree of host specificity

“…In another experiment, however, C1 could also infect Au. granulata weakly (< 5% prevalence of infection) if they incubate for more than 10 d (Seto et al 2020). Another strain of Z. melosirae Host-parasite network between two diatoms and fungi in Lake Inba based on the results of single-spore PCR.…”

Trophic position, elemental ratios and nitrogen transfer in a planktonic host–parasite–consumer food chain including a fungal parasite

“…(Williams 2011 ); strain HS-SYN2 isolated from Feldberger Haussee, Germany), its host-specific chytrid parasitic consumer (strain SVdW-SYN-CHY1 isolated from Melzer See, DE) and Keratella cochlearis (isolated from Lumen Pond, The Netherlands). The chytrid has been identified as Zygophlyctis planktonica (Seto et al 2020 ).…”

“…The average cell length of our Synedra population was about 70 µm, which is larger than the reported preferred algal particle size of < 12 µm for the rotifer Keratella (Edmondson 1965 ), making Synedra cells likely too large to be easily ingested by Keratella (Frenken et al 2016 ) . Chytrid infection of the diatom host starts with free swimming zoospores (diameter ca 2–3 µm; Seto et al 2020 ) finding and attaching to a host cell. Subsequently, the zoospores encyst and form epibiontic sporangia while penetrating and digesting the host through a rhizoidal system.…”

“…The zoospore suspension was prepared by gravity filtration of a heavily infected Synedra culture through a 5 µm mesh size plankton net. This mesh size excludes Synedra (average cell length 70 µm) and let zoospores pass through as their average diameter is 2–3 µm (Seto et al 2020 ). At day 8 of treatment 3a, half of the volume of these Synedra -chytrid cultures was harvested, set to 20% infection prevalence by adding uninfected Synedra cells ( 15 N labelled for the N-transfer experiment) and used to start treatment 3b.…”

The Collection of Zoosporic Eufungi at the University of Michigan (CZEUM): introducing a new repository of barcoded Chytridiomyceta and Blastocladiomycota cultures

“…The availability of pure cultures facilitated chytrid systematics via transmission electron microscopic studies of zoospores (Barr 1980). More recently, the availability of DNA sequencing data has led to the description of new orders in the last 15 years (Letcher et al 2006(Letcher et al , 2008Mozley-Standridge et al 2009;Simmons et al 2009;Longcore and Simmons 2012b;Karpov et al 2014Karpov et al , 2016Seto et al 2020). Additional orders are likely to exist as evidenced by environmental DNA studies that show a poor match between cultured diversity and uncultured (Freeman et al 2009;Davis et al 2016).…”