Microsporidia are a large group of microbial eukaryotes composed exclusively of obligate intracellular parasites of other eukaryotes. Almost 150 years of microsporidian research has led to a basic understanding of many aspects of microsporidian biology, especially their unique and highly specialized mode of infection, where the parasite enters its host through a projectile tube that is expelled at high velocity. Molecular biology and genomic studies on microsporidia have also drawn attention to many other unusual features, including a unique core carbon metabolism and genomes in the size range of bacteria. These seemingly simple parasites were once thought to be the most primitive eukaryotes; however, we now know from molecular phylogeny that they are highly specialized fungi. The fungal nature of microsporidia indicates that microsporidia have undergone severe selective reduction permeating every level of their biology: From cell structures to metabolism, and from genomics to gene structure, microsporidia are reduced.

Microsporidia: Biology and Evolution of Highly Reduced Intracellular Parasites

Parasitism, aptly defined as one of the 'living-together' strategies (Trager, 1986), presents a dynamic system in which the parasite and its host are under evolutionary pressure to evolve new and specific adaptations, thus enabling the coexistence of the two closely interacting partners Microsporidia are very frequently encountered obligatory intracellular protistan parasites that can infect both animals and some protists and are a consummate example of various aspects of the 'living-together' strategy Microsporidia, relatives of fungi in the superkingdom Opisthokonta, belong to the relatively small group of parasites for which the host cell cytoplasm is the site of both reproduction and maturation The structural and physiological reduction of their vegetative stage, together with the manipulation of host cell physiology, enables microsporidia to live in the cytosolic environment for most of their life cycle in a way resembling endocytobionts The ability to form structurally complex spores and the invention and assembly of a unique injection mechanism enable microsporidia to disperse within host tissues and between host organisms, resulting in long-lasting infections Microsporidia have adapted their genomes to the intracellular way of life, evolved strategies how to obtain nutrients directly from the host and how to manipulate not only the infected cells, but also the hosts themselves The enormous variability of host organisms and their tissues provide microsporidian parasites a virtually limitless terrain for diversification and ecological expansion This review attempts to present a general overview of microsporidia, emphasising some less known and/or more recently discovered facets of their biology

Microsporidia and ‘The Art of Living Together’

There is a significant and increasing interest in using fungi as biocontrol agents for insect pests in agricultural systems, and also a growing interest in the basic biology of insect-fungal associations from the perspective of parasitism, symbiosis, and infection. Vega and Blackwell are well-regarded workers in this field, and they have assembled an impressive short book of ten chapters to cover the most important topics in this field, incorporating new molecular techniques wherever possible. The book should appeal to worker in ecology, entomology, mycology, plant pathology, and biological control and pest management.

Insect-Fungal Associations: Ecology and Evolution

This chapter reviews the morphological features and development of the microsporidia. The general life cycle pattern of the microsporidia can be divided into three phases: the infective or environmental phase, the proliferative phase, and the sporogony or spore forming phase. The chapter discusses the host-parasite interface during parasite development. Microsporidia with host-produced interfacial envelopes are those that are separated from the hyaloplasm by host-produced membranes during proliferation through spore formation. Microsporidia having indirect contact between host- and parasite-produced interfacial envelopes are separated from the hyaloplasm by membranes and/or secretions produced by both the host and the parasite. The proliferative phase has been referred to as schizogonic and merogonic by some authors, however, different authors have assigned different types of nuclear activity to the terms merogony and schizogony with respect to the microsporidia. Recognizing the diversity of polaroplast morphology and organization in various species of microsporidia, Larsson described five types of polaroplast structural arrangements. In addition to environmental spores, some microsporidia produce autoinfective spores which become activated and germinate within the same host in which they were just produced. A few microsporidia of the genus Amblyospora have two host cycles with morphologically different spores in each and different development in the males and females of the same host species.

Developmental Morphology and Life Cycles of the Microsporidia

Migration of macromolecules under flow: the physical origin and engineering implications

Spores of the microsporidium Nosema algerae were stimulated to germinate in vitro while observed with video-enhanced contrast microscopy. Field-by-field playback of tape-recorded sequences yielded the first serial illustrations and kinematic analysis of the explosive discharge of the polar filament and the sporoplasm. The filament emerges from the anterior pole of the spore in a regularly pitched helicoidal course along a nearly straight axis, with a mean maximum instant velocity of 105 μm/s. Just before elongation is completed the filament tip follows a tortuous path that often results in a curved or spiralling terminal configuration. Then elongation stops and, after a lag that may vary from less than 15 to over 500 ms, the sporoplasm pours out at the filament tip forming a globule that quickly grows up to a size larger than its original volume within the spore. Concomitantly, the helical filament becomes straightened and frequently the spore body is pulled forward. Thereafter a relaxed filament, usually 5–10% shorter than when maximally extended, remains connecting the empty spore case and the sporoplasmic droplet. Experiments with hyperosmolar media produced a considerable slowdown of filament extrusion and often precluded sporoplasm discharge. The present results are fully consistent with the hypothesis of a hydrostatic pressure-triggered mechanism of spore germination, and revealed that the process is composed of two discrete phases separated by a variable lag: (1) complete eversion of the polar filament, and (2) passage of the main sporoplasm mass along the tube. The data provide a preliminary basis toward the conception of a quantitative physical model of microsporidian spore germination. © 1992 Wiley-Liss, Inc.

Dynamics of Polar Filament Discharge and Sporoplasm Expulsion by Microsporidian Spores

Flow experiments through capillaries with 0.2% xanthan in aqueous solution and 0.1 N NaCl brine were carried out to study the influence of the molecular conformation on the flow development at relatively low shear rates, from 20s−1 to 400s−1. Capillaries with a wide range of length-to-diameter ratios, L/D = 4.5 to 1015 were used. The apparent viscosity as a function of L/D at a constant shear rate shows a continuous decrement of the viscosity as L/D increases, until an asymptotic value is reached. The decrement in the apparent viscosity is partially explained in terms of slip. It was found that slip is a function of L/D as well as shear stress, i.e., slip develops during flow, thus inducing spatial anisotropy in the fluid until a stable state is reached. However, the substantial difference in apparent viscosity between short capillaries and capillaries longer than 300 D may be attributed to dominant elongational flow due to the contraction in the small capillaries and slip in long capillaries. The flow in a sufficiently long capillary can be divided in four regions rather than three, as is usually assumed. In the first region, which corresponds to the entry, elongational and shear flow coexist and elongational flow dominant. In the second region, end effects and slip development are coupled. In the third region the flow is fully developed and end effects are negligible. However, the fluid shows physical characteristics different from those of the fluid at rest, as a consequence of prior slip development. The fourth zone is the exit region in which the velocity rearranges due to the change of boundary conditions. The length of each region depends on the conformation of the macromolecules and shear rate. In addition, it was found that the stiffness of xanthan increases with the increase of the ionic strength. Finally, a performance of Bagley's analysis in the whole range of L/D studied showed that the use of the Bagley correction is not a reliable way to correct for end effects when the flow is not fully developed and/or in the presence of slip.

José Tejero

Papers

Dynamics of Polar Filament Discharge and Sporoplasm Expulsion by Microsporidian Spores

Flow development of xanthan solutions in capillary rheometers