Characterizing the Xenoma of Vairimorpha necatrix Provides Insights Into the Most Efficient Mode of Microsporidian Proliferation

Abstract

Microsporidia are a group of obligated intracellular parasites that can infect nearly all vertebrates and invertebrates, including humans and economic animals. Microsporidian Vairimorpha necatrix is a natural pathogen of multiple insects and can massively proliferate by making tumor-like xenoma in host tissue. However, little is known about the subcellular structures of this xenoma and the proliferation features of the pathogens inside. Here, we characterized the V. necatrix xenoma produced in muscle cells of silkworm midgut. In result, the whitish xenoma was initially observed on the 12th day post infection on the outer surface of the midgut and later became larger and numerous. The observation by scanning electronic microscopy showed that the xenoma is mostly elliptical and spindle with dense pathogen-containing protrusions and spores on the surface, which were likely shedding off the xenoma through exocytosis and could be an infection source of other tissues. Demonstrated with transmission electron microscopy and fluorescent staining, the xenoma was enveloped by a monolayer membrane, and full of vesicle structures, mitochondria, and endoplasmic reticulum around parasites in development, suggesting that high level of energy and nutrients were produced to support the massive proliferation of the parasites. Multiple hypertrophic nuclei were found in one single xenoma, indicating that the cyst was probably formed by fusion of multiple muscle cells. Observed by fluorescence in situ hybridization, pathogens in the xenoma were in merongony, sporogony, and octosporogony, and mature stages. And mature spores were pushed to the center while vegetative pathogens were in the surface layer of the xenoma. The V. necatrix meront usually contained two to three nuclei, and sporont contained two nuclei and was wrapped by a thick membrane with high electron density. The V. necatrix sporogony produces two types of spores, the ordinary dikaryotic spore and unicellular octospores, the latter of which were smaller in size and packed in a sporophorous vesicle. In summary, V. necatrix xenoma is a specialized cyst likely formed by fusion of multiple muscle cells and provides high concentration of energy and nutrients with increased number of mitochondria and endoplasmic reticulum for the massive proliferation of pathogens inside.

An Ectosymbiosis-Based Mechanism of Eukaryogenesis

“…However, Microsporidia developed hexokinases bearing secretion signal sequences, and an elaborate extrusion device consisting of a coiled polar tube with an anchoring disc for their spores; and there are 150 genera and more than 1,200 species of Microsporidia capable of infecting virtually all animal phyla accompanied by rapid proliferation within the varied hosts 59 . They can even direct host biology to the formation of cystlike xenoma that provided high concentrations of energy and nutrients to support massive growth of the microsporidian parasites 60 . Such vitality suggests that the divergence and rapid evolution of Microsporidia could be due to their adaptations to widely different hosts more than intrinsic genomic instability.…”

The gut commensal bacterium Enterococcus faecalis LX10 contributes to defending against Nosema bombycis infection in Bombyx mori

“…Many studies have found negative impacts of microsporidian infections on foraging efficiency, wing deformity, lifespan, colony fitness, fecundity and survival in honeybees and bumblebees. 45,46 Other studies have also explored whether microsporidian infection reduced the weight of the silk gland and growth rate due to the reduced intake of food and its utilization compared to those of healthy silkworms. 47 Many factors, such as initial spore dose, silkworm species, physiological resistance, nutrition status and immune response, may influence the virulence of N. bombycis to silkworms, which may contribute to the detoxification of B. mori.…”

“…Moreover, the growth of silkworms and food consumption were significantly inhibited, leading to substantial loss in cocoon yield after infection with N. bombycis . Many studies have found negative impacts of microsporidian infections on foraging efficiency, wing deformity, lifespan, colony fitness, fecundity and survival in honeybees and bumblebees 45,46 . Other studies have also explored whether microsporidian infection reduced the weight of the silk gland and growth rate due to the reduced intake of food and its utilization compared to those of healthy silkworms 47 .…”

An ectosymbiosis-based mechanism of eukaryogenesis

“…However, Microsporidia developed hexokinases bearing secretion signal sequences, and an elaborate extrusion device consisting of a coiled polar tube with an anchoring disc for their spores; and there are 150 genera and more than 1,200 species of Microsporidia capable of infecting virtually all animal phyla accompanied by rapid proliferation within the varied hosts 59 . They can even direct host biology to the formation of cyst-like xenoma that provided high concentrations of energy and nutrients to support massive growth of the microsporidian parasites 60 . Such vitality suggests that the divergence and rapid evolution of Microsporidia could be due to their adaptations to widely different hosts more than intrinsic genomic instability.…”