Development of the insect pathogenic alga Helicosporidium

Helicosporidium sp. infection in a California kingsnake (Lampropeltis californiae): Spillover of a pathogen of invertebrates to a vertebrate host

Helicosporidium is a genus of nonphotosynthetic, green algae in the family Chlorellaceae, closely related to Prototheca. It is a known pathogen of invertebrates, and its occurrence in vertebrates has not been documented. A captive, 10-month-old, male, albino California kingsnake (Lampropeltis californiae) was submitted for necropsy. Gross examination revealed hemorrhagic laryngitis and a red mottled liver. Histologically, intravascular, intramonocytic/macrophagic and extracellular, eukaryotic organisms were observed in all tissues. These organisms stained positive with Grocott-Gomori methenamine silver and periodic acid-Schiff and were variably acid-fast and gram-positive. Ultrastructural analysis revealed approximately 4 µm vegetative multiplication forms and cysts with 3 parallel ovoid cells and a helically coiled filamentous cell. A polymerase chain reaction with primers targeting Prototheca, amplicon sequencing, and Bayesian phylogenetic analysis confirmed it clustered within Helicosporidium sp. with 100% posterior probability. The genus Helicosporidium was found to nest within the genus Prototheca, forming a clade with Prototheca wickerhamii with 80% posterior probability.

Protists in the Insect Rearing Industry: Benign Passengers or Potential Risk?

As the insects for food and feed industry grows, a new understanding of the industrially reared insect microbiome is needed to better comprehend the role that it plays in both maintaining insect health and generating disease. While many microbiome projects focus on bacteria, fungi or viruses, protists (including microsporidia) can also make up an important part of these assemblages. Past experiences with intensive invertebrate rearing indicate that these parasites, whilst often benign, can rapidly sweep through populations, causing extensive damage. Here, we review the diversity of microsporidia and protist species that are found in reared insect hosts and describe the current understanding of their host spectra, life cycles and the nature of their interactions with hosts. Major entomopathogenic parasite groups with the potential to infect insects currently being reared for food and feed include the Amoebozoa, Apicomplexa, Ciliates, Chlorophyta, Euglenozoa, Ichtyosporea and Microsporidia. However, key gaps exist in the understanding of how many of these entomopathogens affect host biology. In addition, for many of them, there are very limited or even no molecular data, preventing the implementation of molecular detection methods. There is now a pressing need to develop and use novel molecular tools, coupled with standard molecular diagnostic methods, to help unlock their biology and predict the effects of these poorly studied protist parasites in intensive insect rearing systems.

Methods for Manipulating Cryptococcus Spores

Spores are essential for the long-term survival of many diverse organisms, due to their roles in reproduction and stress resistance. In the environmental human fungal pathogen, Cryptococcus, basidiospores are robust cells with the ability to cause disease in animal models of infection. Here we describe methods for producing and purifying Cryptococcus basidiospores in quantities sufficient for large-scale analyses. The production of high numbers of pure spores has facilitated the development of new assays, including quantitative germination assays, and enabled transcriptomic, proteomic, and virulence studies, leading to discoveries of behaviors and properties unique to spores and spore-mediated disease.

A lack of parasitic reduction in the obligate parasitic green alga Helicosporidium

The evolution of an obligate parasitic lifestyle is often associated with genomic reduction, in particular with the loss of functions associated with increasing host-dependence. This is evident in many parasites, but perhaps the most extreme transitions are from free-living autotrophic algae to obligate parasites. The best-known examples of this are the apicomplexans such as Plasmodium, which evolved from algae with red secondary plastids. However, an analogous transition also took place independently in the Helicosporidia, where an obligate parasite of animals with an intracellular infection mechanism evolved from algae with green primary plastids. We characterised the nuclear genome of Helicosporidium to compare its transition to parasitism with that of apicomplexans. The Helicosporidium genome is small and compact, even by comparison with the relatively small genomes of the closely related green algae Chlorella and Coccomyxa, but at the functional level we find almost no evidence for reduction. Nearly all ancestral metabolic functions are retained, with the single major exception of photosynthesis, and even here reduction is not complete. The great majority of genes for light-harvesting complexes, photosystems, and pigment biosynthesis have been lost, but those for other photosynthesis-related functions, such as Calvin cycle, are retained. Rather than loss of whole function categories, the predominant reductive force in the Helicosporidium genome is a contraction of gene family complexity, but even here most losses affect families associated with genome maintenance and expression, not functions associated with host-dependence. Other gene families appear to have expanded in response to parasitism, in particular chitinases, including those predicted to digest the chitinous barriers of the insect host or remodel the cell wall of Helicosporidium. Overall, the Helicosporidium genome presents a fascinating picture of the early stages of a transition from free-living autotroph to parasitic heterotroph where host-independence has been unexpectedly preserved.

Helicosporidium is a highly-adapted obligate parasite of animals. Its evolutionary origins were unclear for almost a century, but molecular analysis ultimately and surprisingly showed that it is a green alga, which means it has undergone an evolutionary transition from autotrophy to parasitism comparable to that of the malaria parasite Plasmodium and its relatives. Such transitions are often associated with the loss of biological functions that are no longer necessary in their novel environment and with the development of molecular mechanisms, sometimes quite sophisticated, to invade and take advantage of their hosts. Yet, very little is actually known about the early stages of the transition of a free-living organism to an obligate intracellular parasite. Here we sequenced the genome and transcriptome of Helicosporidium, and use it to show that the outcome of this transition is quite different from that of Plasmodium.

The Non-Photosynthetic Algae Helicosporidium spp.: Emergence of a Novel Group of Insect Pathogens

Since the original description of Helicosporidium parasiticum in 1921, members of the genus Helicosporidium have been reported to infect a wide variety of invertebrates, but their characterization has remained dependent on occasional reports of infection. Recently, several new Helicosporidium isolates have been successfully maintained in axenic cultures. The ability to produce large quantity of biological material has led to very significant advances in the understanding of Helicosporidium biology and its interactions with insect hosts. In particular, the unique infectious process has been well documented; the highly characteristic cyst and its included filamentous cell have been shown to play a central role during host infection and have been the focus of detailed morphological and developmental studies. In addition, phylogenetic analyses inferred from a multitude of molecular sequences have demonstrated that Helicosporidium are highly specialized non-photosynthetic algae (Chlorophyta: Trebouxiophyceae), and represent the first described entomopathogenic algae. This review provides an overview of (i) the morphology of Helicosporidium cell types, (ii) the Helicosporidium life cycle, including the entire infectious sequence and its impact on insect hosts, (iii) the phylogenetic analyses that have prompted the taxonomic classification of Helicosporidium as green algae, and (iv) the documented host range for this novel group of entomopathogens.

The mitochondrial genome of the entomoparasitic green alga helicosporidium

BACKGROUND

Helicosporidia are achlorophyllous, non-photosynthetic protists that are obligate parasites of invertebrates. Highly specialized, these pathogens feature an unusual cyst stage that dehisces inside the infected organism and releases a filamentous cell displaying surface projections, which will penetrate the host gut wall and eventually reproduce in the hemolymph. Long classified as incertae sedis or as relatives of other parasites such as Apicomplexa or Microsporidia, the Helicosporidia were surprisingly identified through molecular phylogeny as belonging to the Chlorophyta, a phylum of green algae. Most phylogenetic analyses involving Helicosporidia have placed them within the subgroup Trebouxiophyceae and further suggested a close affiliation between the Helicosporidia and the genus Prototheca. Prototheca species are also achlorophyllous and pathogenic, but they infect vertebrate hosts, inducing protothecosis in humans. The complete plastid genome of an Helicosporidium species was recently described and is a model of compaction and reduction. Here we describe the complete mitochondrial genome sequence of the same strain, Helicosporidium sp. ATCC 50920 isolated from the black fly Simulium jonesi.

METHODOLOGY/PRINCIPAL FINDINGS

The circular mapping 49343 bp mitochondrial genome of Helicosporidium closely resembles that of the vertebrate parasite Prototheca wickerhamii. The two genomes share an almost identical gene complement and display a level of synteny that is higher than any other sequenced chlorophyte mitochondrial DNAs. Interestingly, the Helicosporidium mtDNA feature a trans-spliced group I intron, and a second group I intron that contains two open reading frames that appear to be degenerate maturase/endonuclease genes, both rare characteristics for this type of intron.

CONCLUSIONS/SIGNIFICANCE

The architecture, genome content, and phylogeny of the Helicosporidium mitochondrial genome are all congruent with its close relationship to Prototheca within the Trebouxiophyceae. The Helicosporidium mitochondrial genome does, however, contain a number of novel features, particularly relating to its introns.

First record of the insect pathogenic alga Helicosporidium sp. (Chlorophyta: Trebouxiophyceae) infection in larvae and pupae of Rhizophagusgrandis Gyll. (Coleoptera, Rhizophaginae) from Turkey

The predator beetle Rhizophagus grandis Gyll. (Coleoptera, Rhizophaginae) is one of the most important biological control agents, mass-bred and used to suppress populations of an important pest: the great spruce bark beetle, Dendroctonus micans. The achlorophyllous alga Helicosporidium sp. was first discovered in the pest. Later it was also found in the predator, but only in the adults. In this study, the pathogenic alga Helicosporidium sp. was discovered in larvae and early pupae of R. grandis for the first time. The morphological characteristics of the pathogenic alga were revealed by light and electron microscopy. Infection rates of Helicosporidium sp. in the larvae and pupae of R. grandis were 23.5% and 6.25%, respectively.

Host age and pathogen dosage impact cyst morphogenesis in the invertebrate pathogenic alga Helicosporidium sp. (Chlorophyta: Trebouxiophyceae)

Helicosporidium sp. is a pathogenic alga that replicates in the hemolymph of various invertebrate hosts. Morphogenesis of the infectious life stage, the cyst, occurs in the infected host, but to date cannot be induced in vitro. Using larvae of the heterologous host Helicoverpa zea, we examined potential factors influencing pathogenicity and in vivo cyst production of the alga and the impact of infection on host survival. Factors tested were cyst dosage administered per os (ranging from 10(2) to 10(5) cysts per larva) and host age at exposure (third, fourth, and fifth larval instar). Cyst production occurred between 7 and 13days after treatment, regardless of host age at treatment. Increasing dosage increased both percent infection and mortality, but cyst production did not track the total infection response. Increasing host age at exposure mitigated dosage effects on infection and mortality and also elevated cyst production in later-treated larvae. Only the highest dosage produced a significant decrease in the overall time to death. Moderate cyst dosages and later host ages were most effective at regenerating Helicosporidium cysts.