Diversity and Hidden Host Specificity of Chytrids Infecting Colonial Volvocacean Algae

Abstract

Chytrids are zoosporic fungi that play an important, but yet understudied, ecological role in aquatic ecosystems. Many chytrid species have been morphologically described as parasites on phytoplankton. However, the majority of them have rarely been isolated and lack DNA sequence data. In this study we isolated and cultivated three parasitic chytrids, infecting a common volvocacean host species, Yamagishiella unicocca. To identify the chytrids, we characterized morphology and life cycle, and analyzed phylogenetic relationships based on 18S and 28S rDNA genes. Host range and specificity of the chytrids was determined by cross-infection assays with host strains, characterized by rbcL and ITS markers. We were able to confirm the identity of two chytrid strains as Endocoenobium eudorinae Ingold and Dangeardia mamillata Schröder and described the third chytrid strain as Algomyces stechlinensis gen. et sp. nov. The three chytrids were assigned to novel and phylogenetically distant clades within the phylum Chytridiomycota, each exhibiting different host specificities. By integrating morphological and molecular data of both the parasitic chytrids and their respective host species, we unveiled cryptic host-parasite associations. This study highlights that a high prevalence of (pseudo)cryptic diversity requires molecular characterization of both phytoplankton host and parasitic chytrid to accurately identify and compare host range and specificity, and to study phytoplankton-chytrid interactions in general.

Characterization of the mycobiome of the seagrass,Zostera marina, reveals putative associations with marine chytrids

“…This analysis revealed that the SV8 complex (and thus also all ASVs within this complex) groups within the order Lobulomycetales in the phylum Chytridiomycota (Figure 5). Specifically, the SV8 complex is nested within the recently defined Novel Clade SW-I (Van den Wyngaert et al, 2018) and is sister to several culture independent sequences obtained from the marine ecosystem (Hassett et al, 2017).…”

“…Although many of the ASVs observed here associated with Z. marina are still unidentified, we were able to phylogenetically place the SV8 complex in Novel Clade SW-I in the Lobulomycetales. Known members of Novel Clade SW-I are parasitic chytrids of freshwater algae and diatoms (Frenken et al, 2017; Rad-Menéndez et al, 2018; Van den Wyngaert et al, 2018). It is possible that the SV8 complex’s high relative abundance both on and in Z. marina leaves may indicate that it is directly infecting host plant tissues and suggests that seagrasses may provide a novel niche for marine chytrids.…”

“…This latter interpretation seems to be supported by the observation that the SV8 complex has a higher mean relative abundance in epiphyte washes relative to endophyte washes. However, the closest cultivated relative of the SV8 complex forms sporangia outside of host cells (Van den Wyngaert et al, 2018) and thus, we might expect to find more zoospores in the epiphyte washes. Thus the higher relative abundance of the SV8 complex in the epiphyte washes is consistent with both explanations.…”

“…Closely related sequences to the sequence from the PCR products above were identified using NCBI’s Standard Nucleotide BLAST’s megablast option with default settings. These results were then used to guide a literature search to identify additional sequences for inclusion during phylogenetic reconstruction (Table S4) (Hassett et al, 2017; Karpov et al, 2014; Rad-Menéndez et al, 2018; Simmons et al, 2009; Van den Wyngaert et al, 2018).…”

Comparison of sterol and fatty acid profiles of chytrids and their hosts reveals trophic upgrading of nutritionally inadequate phytoplankton by fungal parasites

“…). As expected, zoospores of both chytrid species were rich in lipids, presenting a large lipid globule, a characteristic morphological feature of chytrids (Powell et al ., ; Van den Wyngaert et al ., ). Chytrids’ high lipid contents could explain the observed increase in total FA contents in Host+Sporangia fraction compared with the uninfected diatom host.…”