Models of Blue Green Adn Green Algea Competion Modle

doi: 10.7717/peerj.10654. eCollection 2021.

Freshwater sponge hosts and their green algae symbionts: a tractable model to understand intracellular symbiosis

Affiliations

• PMID: 33614268
• PMCID: PMC7882143
• DOI: 10.7717/peerj.10654

Free PMC article

Freshwater sponge hosts and their green algae symbionts: a tractable model to understand intracellular symbiosis

Chelsea Hall  et al. PeerJ. 2021 .

Free PMC article

Abstract

In many freshwater habitats, green algae form intracellular symbioses with a variety of heterotrophic host taxa including several species of freshwater sponge. These sponges perform important ecological roles in their habitats, and the poriferan:green algae partnerships offers unique opportunities to study the evolutionary origins and ecological persistence of endosymbioses. We examined the association between Ephydatia muelleri and its chlorophyte partner to identify features of host cellular and genetic responses to the presence of intracellular algal partners. Chlorella-like green algal symbionts were isolated from field-collected adult E. muelleri tissue harboring algae. The sponge-derived algae were successfully cultured and subsequently used to reinfect aposymbiotic E. muelleri tissue. We used confocal microscopy to follow the fate of the sponge-derived algae after inoculating algae-free E. muelleri grown from gemmules to show temporal patterns of symbiont location within host tissue. We also infected aposymbiotic E. muelleri with sponge-derived algae, and performed RNASeq to study differential expression patterns in the host relative to symbiotic states. We compare and contrast our findings with work in other systems (e.g., endosymbiotic Hydra) to explore possible conserved evolutionary pathways that may lead to stable mutualistic endosymbioses. Our work demonstrates that freshwater sponges offer many tractable qualities to study features of intracellular occupancy and thus meet criteria desired for a model system.

Keywords: Algae; Freshwater sponges; Model system; RNA seq; Symbiosis.

©2021 Hall et al.

Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1. Freshwater sponges in natural habitat growing at the outflow of the dam.

Several sponge species are present - some harbor green algae, some do not. (Inset) Example of sponge harboring green algae. The sponge was growing on the underside of a rock, which has been turned over. The portion of the sponge that would have been exposed to sunlight (bottom portion of the sponge) is green due to the presence of algal. Tissue protected from sunlight is devoid of algae (top portion of sponge colony).

Figure 2. Infection of aposymbiotic E. muelleri sponges.

(A) Schematic of infection process. Inset shows electron micrograph of algal engulfment by sponge cell. (B) E. muelleri without algae and 24 h post-infection with algal symbionts. O (osculum), C (canal).

Figure 3. Confocal time series of E. muelleri choanoderm region after infection with Chlorella-like symbionts.

(A) Aposymbiotic E. muelleri (B) E. muelleri 4 h post-infection. (C) E. muelleri 24 h post-infection. (D) E. muelleri 6 days post-infection. Note cells with multiple algae. Images show DNA in blue, F-actin in green, and autofluorescence of algal cells in red. Scale bars 30 µm.

Figure 4. Transmission electron microscopy of intracellular algal symbionts after E. muelleri infections.

(A) E. muelleri 4 h post-infection. (B) Multiple infected cells 24 h post-infection. C. Once cell with multiple algal symbionts 24 h post-infection. Scale bars 2 µm.

Figure 5. Confocal image at 24 h post-infection showing multiple intracellular algal symbionts in one sponge cell.

Images show DNA in blue, F-actin in green, and autofluorescence of algal cells in red. Scale bars 20 µm.

Figure 6. Heatmap of differentially expressed genes in RNASeq analysis.

Relative expression of differentially expressed sponge genes where red hues represent comparatively upregulated genes and blue hues represent comparatively downregulated genes (scale at right). Data shown compares triplicate samples for aposymbiotic and 24 h post-infected sponges. Gene IDs are provided at the right of each expression profile.

Figure 7. Enrichment categories for Gene Ontology and KEGG.

(A). Statistics of gene ontology (GO) enrichment across the most represented GO categories for expressed sponge genes comparing aposymbiotic and 24 h post-infection E muelleri. (B) Statistics of KEGG pathway enrichment across the differential sponge gene expression for aposymbiotic and 24 h post-infection E muelleri. Size of dots correspond to gene number while colors correspond to p values. Scales are given on the right.

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