Comparative Time-Scale Gene Expression Analysis Highlights the Infection Processes of Two Amoebophrya Strains

Decoding the Arsenal: Protist Effectors and Their Impact on Photosynthetic Hosts

Interactions between various microbial pathogens including viruses, bacteria, fungi, oomycetes, and their plant hosts have traditionally been the focus of phytopathology. In recent years, a significant and growing interest in the study of eukaryotic microorganisms not classified among fungi or oomycetes has emerged. Many of these protists establish complex interactions with photosynthetic hosts, and understanding these interactions is crucial in understanding the dynamics of these parasites within traditional and emerging types of farming, including marine aquaculture. Many phytopathogenic protists are biotrophs with complex polyphasic life cycles, which makes them difficult or impossible to culture, a fact reflected in a wide gap in the availability of comprehensive genomic data when compared to fungal and oomycete plant pathogens. Furthermore, our ability to use available genomic resources for these protists is limited by the broad taxonomic distance that these organisms span, which makes comparisons with other genomic datasets difficult. The current rapid progress in genomics and computational tools for the prediction of protein functions and interactions is revolutionizing the landscape in plant pathology. This is also opening novel possibilities, specifically for a deeper understanding of protist effectors. Tools like AlphaFold2 enable structure-based function prediction of effector candidates with divergent protein sequences. In turn, this allows us to ask better biological questions and, coupled with innovative experimental strategies, will lead into a new era of effector research, especially for protists, to expand our knowledge on these elusive pathogens and their interactions with photosynthetic hosts. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Intracellular development and impact of a marine eukaryotic parasite on its zombified microalgal host

Parasites are widespread and diverse in oceanic plankton and many of them infect single-celled algae for survival. How these parasites develop and scavenge energy within the host and how the cellular organization and metabolism of the host is altered remain open questions. Combining quantitative structural and chemical imaging with time-resolved transcriptomics, we unveil dramatic morphological and metabolic changes of the marine parasite Amoebophrya (Syndiniales) during intracellular infection, particularly following engulfment and digestion of nutrient-rich host chromosomes. Changes include a sequential acristate and cristate mitochondrion with a 200-fold increase in volume, a 13-fold increase in nucleus volume, development of Golgi apparatus and a metabolic switch from glycolysis (within the host) to TCA (free-living dinospore). Similar changes are seen in apicomplexan parasites, thus underlining convergent traits driven by metabolic constraints and the infection cycle. In the algal host, energy-producing organelles (plastid, mitochondria) remain relatively intact during most of the infection. We also observed that sugar reserves diminish while lipid droplets increase. Rapid infection of the host nucleus could be a “zombifying” strategy, allowing the parasite to digest nutrient-rich chromosomes and escape cytoplasmic defense, whilst benefiting from maintained carbon-energy production of the host cell.

Emerging Parasitic Protists: The Case of Perkinsea

The last century has witnessed an increasing rate of new disease emergence across the world leading to permanent loss of biodiversity. Perkinsea is a microeukaryotic parasitic phylum composed of four main lineages of parasitic protists with broad host ranges. Some of them represent major ecological and economical threats because of their geographically invasive ability and pathogenicity (leading to mortality events). In marine environments, three lineages are currently described, the Parviluciferaceae, the Perkinsidae, and the Xcellidae, infecting, respectively, dinoflagellates, mollusks, and fish. In contrast, only one lineage is officially described in freshwater environments: the severe Perkinsea infectious agent infecting frog tadpoles. The advent of high-throughput sequencing methods, mainly based on 18S rRNA assays, showed that Perkinsea is far more diverse than the previously four described lineages especially in freshwater environments. Indeed, some lineages could be parasites of green microalgae, but a formal nature of the interaction needs to be explored. Hence, to date, most of the newly described aquatic clusters are only defined by their environmental sequences and are still not (yet) associated with any host. The unveiling of this microbial black box presents a multitude of research challenges to understand their ecological roles and ultimately to prevent their most negative impacts. This review summarizes the biological and ecological traits of Perkinsea-their diversity, life cycle, host preferences, pathogenicity, and highlights their diversity and ubiquity in association with a wide range of hosts.

Is the Dinoflagellate Amoebophrya Really Missing an mtDNA?

Mitochondrial DNA (mtDNA) is a universal hallmark of aerobic eukaryotes. That is why the recent suggestion by John et al. (2019. An aerobic eukaryotic parasite with functional mitochondria that likely lacks a mitochondrial genome. Sci Adv. 5(4):eaav1110.) that the aerobic dinoflagellate Amoebophrya sp. strain AT5 (Syndiniales) lacks mtDNA was so remarkable. Here, by reanalyzing recently published genomic and transcriptomic data from three Amoebophrya strains, we provide evidence of a cryptic, highly reduced mtDNA in this clade. More work is needed before one can definitively say if Amoebophrya has or does not have an mtDNA, but for now, the data are pointing toward the existence of one. Ultimately, we urge caution when basing supposedly absent genomic features on single line evidences.

Rapid protein evolution, organellar reductions, and invasive intronic elements in the marine aerobic parasite dinoflagellate Amoebophrya spp

BACKGROUND

Dinoflagellates are aquatic protists particularly widespread in the oceans worldwide. Some are responsible for toxic blooms while others live in symbiotic relationships, either as mutualistic symbionts in corals or as parasites infecting other protists and animals. Dinoflagellates harbor atypically large genomes (~ 3 to 250 Gb), with gene organization and gene expression patterns very different from closely related apicomplexan parasites. Here we sequenced and analyzed the genomes of two early-diverging and co-occurring parasitic dinoflagellate Amoebophrya strains, to shed light on the emergence of such atypical genomic features, dinoflagellate evolution, and host specialization.

RESULTS

We sequenced, assembled, and annotated high-quality genomes for two Amoebophrya strains (A25 and A120), using a combination of Illumina paired-end short-read and Oxford Nanopore Technology (ONT) MinION long-read sequencing approaches. We found a small number of transposable elements, along with short introns and intergenic regions, and a limited number of gene families, together contribute to the compactness of the Amoebophrya genomes, a feature potentially linked with parasitism. While the majority of Amoebophrya proteins (63.7% of A25 and 59.3% of A120) had no functional assignment, we found many orthologs shared with Dinophyceae. Our analyses revealed a strong tendency for genes encoded by unidirectional clusters and high levels of synteny conservation between the two genomes despite low interspecific protein sequence similarity, suggesting rapid protein evolution. Most strikingly, we identified a large portion of non-canonical introns, including repeated introns, displaying a broad variability of associated splicing motifs never observed among eukaryotes. Those introner elements appear to have the capacity to spread over their respective genomes in a manner similar to transposable elements. Finally, we confirmed the reduction of organelles observed in Amoebophrya spp., i.e., loss of the plastid, potential loss of a mitochondrial genome and functions.

CONCLUSION

These results expand the range of atypical genome features found in basal dinoflagellates and raise questions regarding speciation and the evolutionary mechanisms at play while parastitism was selected for in this particular unicellular lineage.

Dinoflagellate Host Chloroplasts and Mitochondria Remain Functional During Amoebophrya Infection

Dinoflagellates are major components of phytoplankton that play critical roles in many microbial food webs, many of them being hosts of countless intracellular parasites. The phototrophic dinoflagellate Scrippsiella acuminata (Dinophyceae) can be infected by the microeukaryotic parasitoids Amoebophrya spp. (Syndiniales), some of which primarily target and digest the host nucleus. Early digestion of the nucleus at the beginning of the infection is expected to greatly impact the host metabolism, inducing the knockout of the organellar machineries that highly depend upon nuclear gene expression, such as the mitochondrial OXPHOS pathway and the plastid photosynthetic carbon fixation. However, previous studies have reported that chloroplasts remain functional in swimming host cells infected by Amoebophrya. We report here a multi-approach monitoring study of S. acuminata organelles over a complete infection cycle by nucleus-targeting Amoebophrya sp. strain A120. Our results show sustained and efficient photosystem II activity as a hallmark of functional chloroplast throughout the infection period despite the complete digestion of the host nucleus. We also report the importance played by light on parasite production, i.e., the amount of host biomass converted to parasite infective propagules. Using a differential gene expression analysis, we observed an apparent increase of all 3 mitochondrial and 9 out of the 11 plastidial genes involved in the electron transport chains (ETC) of the respiration pathways during the first stages of the infection. The longer resilience of organellar genes compared to those encoded by the nucleus suggests that both mitochondria and chloroplasts remain functional throughout most of the infection. This extended organelle functionality, along with higher parasite production under light conditions, suggests that host bioenergetic organelles likely benefit the parasite Amoebophrya sp. A120 and improve its fitness during the intracellular infective stage.

Distribution and genetic diversity of the parasitic dinoflagellate Amoebophrya in coastal waters of China

Amoebophrya is an obligate endoparasite infecting wide ranges of marine organisms in coastal and oceanic waters. The parasitoid has received growing attention, due to its enormous genetic diversity in seawaters and suppressive effects on the growth of host dinoflagellates. Harmful algal blooms (HABs) caused by planktonic dinoflagellates have significantly impacted the coastal environment and mariculture in China. Series of studies have been conducted to reveal the occurrence mechanism and negative impacts of HABs in past decades, while the factors contributing to the recession of HABs have rarely been studied. Thus, the host range, prevalence and diversity of Amoebophrya along the coastline of China were systemically investigated to facilitate future studies on the ecological roles of the parasitoid. Overall, 10 dinoflagellate taxa were found to be infected by Amoebophrya spp., and the prevalence ranged from 0.03% to 2.50%. Sequencing of environmental genomic DNA revealed substantial diversity and significant regional heterogeneity of Amoebophryidae sequences derived from 12 coastal bays, while no significant correlation was observed among geographical locations. Phylogenetic analyses of 18S rDNA sequences derived from individual Amoebophrya-infected cells indicated the host divergence of the parasitoid and lend credence to the multiple species assumption. The results further revealed the broad host range, wide distribution and substantial diversity of Amoebophrya in the coastal waters of China, that should not be neglected in future studies on the succession of HABs, as well as the ecological significance of this parasitoid in marine microbial food webs.

A precedented nuclear genetic code with all three termination codons reassigned as sense codons in the syndinean Amoebophrya sp. ex Karlodinium veneficum

Amoebophrya is part of an enigmatic, diverse, and ubiquitous marine alveolate lineage known almost entirely from anonymous environmental sequencing. Two cultured Amoebophrya strains grown on core dinoflagellate hosts were used for transcriptome sequencing. BLASTx using different genetic codes suggests that Amoebophyra sp. ex Karlodinium veneficum uses the three typical stop codons (UAA, UAG, and UGA) to encode amino acids. When UAA and UAG are translated as glutamine about half of the alignments have better BLASTx scores, and when UGA is translated as tryptophan one fifth have better scores. However, the sole stop codon appears to be UGA based on conserved genes, suggesting contingent translation of UGA. Neither host sequences, nor sequences from the second strain, Amoebophrya sp. ex Akashiwo sanguinea had similar results in BLASTx searches. A genome survey of Amoebophyra sp. ex K. veneficum showed no evidence for transcript editing aside from mitochondrial transcripts. The dynein heavy chain (DHC) gene family was surveyed and of 14 transcripts only two did not use UAA, UAG, or UGA in a coding context. Overall the transcriptome displayed strong bias for A or U in third codon positions, while the tRNA genome survey showed bias against codons ending in U, particularly for amino acids with two codons ending in either C or U. Together these clues suggest contingent translation mechanisms in Amoebophyra sp. ex K. veneficum and a phylogenetically distinct instance of genetic code modification.