Development and morphology of the glochidium larva of Anodonta cygnea (Mollusca: Bivalvia)

The development of the parasitic glochidium larva of Anodonta cygnea L. is outlined, with reference to earlier studies on unionid larvae, and a comparison is made between the mature glochidia of the three genera, Anodonta, Unio , and Margaritifera.
The glochidium of Anodonta cygnea is anatomically and morphologically specialized in connection with its parasitic existence. The specialized structures are described, and their significance discussed.     

The interrelations of the causative agents of transmissible diseases in ixodid ticks (Ixodidae) with a mixed infection

The data on interactions between pathogens of human diseases with natural focality in ixodid ticks (Ixodidae) with mixed infection are reviewed. Different variants of mixed infection revealed in nature and accompanied by predominantly extracellular, or both extra- and intracellular location of the agents in tick organism are considered. Natural mixed infection of ticks by different pathogens is a normal and commonly occurring phenomenon; moreover, there is no experimental evidence for the existence of any antagonistic relationships between spirochetes, rickettsiae, and piroplasms in such ticks. The absence of antagonism between different agents in vectors largely provides a relative autonomy of corresponding parasitic systems and creates conditions for coexistence of different parasitic systems in mixed natural foci.     

Is there sex-biased resistance and tolerance in Mediterranean wood mouse (Apodemus sylvaticus) populations facing multiple helminth infections?

Sex-biased parasitism is rarely investigated in relation to host tolerance and resistance, which are two defense strategies hosts can adopt when challenged by parasites. Health or fitness deteriorations in less tolerant individuals with increasing parasite burden would be faster than those in more tolerant ones. Hence, the body condition and reproductive potential of an infected individual host can be considered proxies for tolerance to parasitism. We studied Mediterranean populations of the wood mouse (Apodemus sylvaticus) and its helminth parasites. We assessed their resistance using the phytohemagglutinin test and spleen size, and their tolerance using body condition in males and females and testes mass in males. In order to avoid spurious correlations, we took into account the phylogeographic structure of the Mediterranean wood mouse populations. We used a mixed model adapted from the animal model used in quantitative genetics. While helminth infection did not differ between the two sexes, females and males differed in their measured defenses. Females seem to invest more in immune defense with increasing risk of parasite diversity, but also appear to be potentially more tolerant of parasitic diversity. These results suggest the existence of sexual differences in resistance and tolerance, and that measurements of parasitic loads alone could be insufficient to detect any underlying sexual differences in the two strategies that have evolved in response to multiple parasitic attacks.     

Photosynthetic evolution in parasitic plants: insight from the chloroplast genome

Despite the enormous diversity in plant form, structure and growth environment across the seed-bearing plants (angiosperms and gymnosperms), the chloroplast genome has, with few exceptions, remained remarkably conserved. This conservation suggests the existence of universal evolutionary selection pressures associated with photosynthesis-the primary function of chloroplasts. The stark exceptions to this conservation occur in parasitic angiosperms, which have escaped the dominant model by evolving the capacity to obtain some or all of their carbon (and nutrients) from their plant hosts. The consequence of this evolution to parasitism is a relaxation of the evolutionary constraints associated with the need to maintain photosynthetic function, the very function that drove early stages of the ancient symbiotic relationship that produced the contemporary chloroplast. Extreme examples of reductionism among parasitic angiosperms reveals major alterations in chloroplast function with the loss of photosynthetic capacity and, with that, massive alterations in chloroplast genome content. This review highlights emerging patterns in reported gene loss and gene retention in the chloroplast genomes of parasitic plants. Some gene losses appear to occur in the early stages of parasitic evolution, even before the loss of photosynthetic capacity, like the chlororespiratory (ndh) genes. This contrasts with unexpected gene retentions, like that of the rbcL gene responsible for photosynthetic carbon dioxide fixation, and belies current understanding of gene function. The review relates gene retention to current knowledge of protein function and gene processing that has implications to broader aspects of genome conservation in organelles.     

Extracellular Vesicles from Parasitic Helminths Contain Specific Excretory/Secretory Proteins and Are Internalized in Intestinal Host Cells

The study of host-parasite interactions has increased considerably in the last decades, with many studies focusing on the identification of parasite molecules (i.e. surface or excretory/secretory proteins (ESP)) as potential targets for new specific treatments and/or diagnostic tools. In parallel, in the last few years there have been significant advances in the field of extracellular vesicles research. Among these vesicles, exosomes of endocytic origin, with a characteristic size ranging from 30–100 nm, carry several atypical secreted proteins in different organisms, including parasitic protozoa. Here, we present experimental evidence for the existence of exosome-like vesicles in parasitic helminths, specifically the trematodes Echinostoma caproni and Fasciola hepatica. These microvesicles are actively released by the parasites and are taken up by host cells. Trematode extracellular vesicles contain most of the proteins previously identified as components of ESP, as confirmed by proteomic, immunogold labeling and electron microscopy studies. In addition to parasitic proteins, we also identify host proteins in these structures. The existence of extracellular vesicles explains the secretion of atypical proteins in trematodes, and the demonstration of their uptake by host cells suggests an important role for these structures in host-parasite communication, as described for other infectious agents.     

Rethinking convergence in plant parasitism through the lens of molecular and population genetic processes

The autotrophic lifestyle of photosynthetic plants has profoundly shaped their body plan, physiology, and gene repertoire. Shifts to parasitism and heterotrophy have evolved at least 12 times in more than 4000 species, and this transition has consequently left major evolutionary footprints among these parasitic lineages. Features that are otherwise rare at the molecular level and beyond have evolved repetitively, including reduced vegetative bodies, carrion-mimicking during reproduction, and the incorporation of alien genetic material. Here, I propose an integrated conceptual model, referred to as the funnel model, to define the general evolutionary trajectory of parasitic plants and provide a mechanistic explanation for their convergent evolution. This model connects our empirical understanding of gene regulatory networks in flowering plants with classical theories of molecular and population genetics. It emphasizes that the cascading effects brought about by the loss of photosynthesis may be a major force constraining the physiological capacity of parasitic plants and shaping their genomic landscapes. Here I review recent studies on the anatomy, physiology, and genetics of parasitic plants that lend support to this photosynthesis-centered funnel model. Focusing on nonphotosynthetic holoparasites, I elucidate how they may inevitably reach an evolutionary terminal status (i.e., extinction) and highlight the utility of a general, explicitly described and falsifiable model for future studies of parasitic plants.     

Why do all host species not show defense against avian brood parasitism: evolutionary lag or equilibrium?

Avian brood parasitism reduces the reproductive success of hosts and is therefore expected to select for host defenses against parasitism, such as an ability to reject parasitic eggs. Field studies have shown that some hosts recognize and reject parasitism, whereas others do not, and the degree of the defense varies from population to population. One long-standing debate concentrates on the differences in the distribution of host defenses observed in hosts parasitized by the brown-headed cowbird and the common cuckoo. The cowbird's hosts show either few or nearly perfect defenses, whereas the cuckoo's hosts have defenses varying from none to complete, with most falling in between the two extremes. To explore the mechanisms underlying this pattern, I constructed a mathematical model in which host defense is assumed to be genetically determined and analyzed how the host defense is established under parasitic pressure. The model shows that differences in the defense-level distribution can be attributed to the difference in the parasite's breeding strategy, generalized or specialized: hosts parasitized by generalists show perfect, none, or intermediate levels of the defense depending on the host abundances, whereas hosts parasitized by specialists always exhibit either none or intermediate levels of the defense if the parasite lacks counter defenses such as egg mimicry. This result provides a testable explanation for the existence of accepter species of the brown-headed cowbird, which might reconcile the previously conflicting hypotheses.     

The evolution of life-history traits in parasitic and free-living platyhelminthes: a new perspective

Parasite life histories have been assumed to be shaped by their particular mode of existence. To test this hypothesis, we investigate the relationships between life-history traits of free-living and parasitic platyhelminthes. Using phylogenetically independent contrasts we examine patterns of interspecific covariation in adult size, progeny volume, daily fecundity, total reproductive capacity, age at first reproduction and longevity. The correlations obtained indicate a similar causal chain of life history variations for free-living and parasitic platyhelminthes. These results suggest that increased longevity favours delayed reproduction. Furthermore, growth pattern determines adult body size and age at maturity. For platyhelminthes, whether free-living or parasitic, the total reproductive capacity is found to be directly determined by the size of the worm. Within this group the parasitic way of life does not seem to influence the basic patterns of life history evolution. Received: 20 September 1997 / Accepted: 1 March 1998     

Shedding of antibody complexes by Strongyloides ratti (Nematoda) larvae

Antigens on the epicuticular surface of Strongyloides ratti infective third-stage larvae (L3) could be demonstrated by an indirect fluorescent antibody technique under certain conditions. Infective L3 shed anti-antibody complexes at room temperature, but not at 4 C or in the presence of sodium azide or colchicine. Shedding of antibody did not appear to involve epicuticular antigens, and only occurred when anti-rat IgG was complexed to rat anti-larval antibody. However, parasitic L3 removed from rats did not exhibit this shedding reaction, suggesting that an important developmental change in cuticle physiology occurs during the transition from a free-living existence to a parasitic mode. The ability to shed foreign objects from the epicuticle of free-living infective L3 may be a defensive or protective response to soil microorganisms.     

地老虎六索线虫的生物学特性调查

在岳阳地区农田菜地,寄生于小地老虎幼虫Agrotis ypsilon的线虫都是地老虎六索线虫Hexamermis agrotis。田间调查其寄生率高达44%~68%。并对地老虎六索线虫的生活史及温度、湿度、土壤肥料、日照等自然因素对该线虫生存繁殖的影响进行了调查。     

A survival-reproduction trade-off in entomopathogenic nematodes mediated by their bacterial symbionts

In this work, we investigate the investment of entomopathogenic Steinernema nematodes (Rhabditidae) in their symbiotic association with Xenorhabdus bacteria (Enterobacteriaceae). Their life cycle comprises two phases: (1) a free stage in the soil, where infective juveniles (IJs) of the nematode carry bacteria in a digestive vesicle and search for insect hosts, and (2) a parasitic stage into the insect where bacterial multiplication, nematode reproduction, and production of new IJs occur. Previous studies clearly showed benefits to the association for the nematode during the parasitic stage, but preliminary data suggest the existence of costs to the association for the nematode in free stage. IJs deprived from their bacteria indeed survive longer than symbiotic ones. Here we show that those bacteria-linked costs and benefits lead to a trade-off between fitness traits of the symbiotic nematodes. Indeed IJs mortality positively correlates with their parasitic success in the insect host for symbiotic IJs and not for aposymbiotic ones. Moreover mortality and parasitic success both positively correlate with the number of bacteria carried per IJ, indicating that the trade-off is induced by symbiosis. Finally, the trade-off intensity depends on parental effects and, more generally, is greater under restrictive environmental conditions.     

Why is mimicry in cuckoo eggs sometimes so poor?

I propose that the existence of imperfect adaptations (e.g. egg mimicry) in brood parasites and their hosts (e.g. discrimination abilities) could reflect age-dependent territory and nest-site selection patterns of the host. Studies of various passerines indicate that (1) older breeders tend to occupy nest sites of higher quality than do young birds (ideal despotic distribution resulting from interference competition), (2) nest-site selection affects the risk of parasitism in various habitats, (3) egg recognition in passerines has a strong learning component (therefore naive breeders tend to accept whereas older birds tend to reject parasitic eggs). Because young naive birds, who tend to accept parasitic eggs, usually breed in low-quality areas where they are frequently parasitised, while old experienced birds, who tend to reject parasitic eggs, breed in high-quality areas where they are rarely parasitised, the distribution of acceptors and rejecters with respect to the risk of parasitism is non-random, i.e. populations of some host species may consist of heavily parasitised acceptors and weakly parasitised rejecters. Therefore, the selection pressure exerted by the host on the parasite should be weaker than if brood parasitism was randomly distributed among naive and experienced breeders and affect adaptations such as egg mimicry. This could explain the existence of imperfect adaptations in some brood parasite-host systems.     

Effect of microclimate on the development of parasitic arthropods

The paper contains a survey of methods used and experience gained during the 25-year research of the influence of microclimate on the existence and development of parasitic arthropods at the Institute of Parasitology, Czechoslovak Academy of Sciences. Research methods by means of apparatuses, organization of long-term experiments and procedures in the assessment of results are described. An emphasis is put on the inter-disciplinary character of such working trend and on the advantage of creating complex working teams, in which the biologist must be the initiator and coordinator of the studies.     

Testing for ecological limitation of diversification: a case study using parasitic plants

Abstract Imbalances in phylogenetic diversity could be the result of variable diversification rates, differing limits on diversity, or a combination of the two. We propose an approach to distinguish between rates and limits as the primary cause of phylogenetic imbalance, using parasitic plants as a model. With sister-taxon comparisons, we show that parasitic plant lineages are typically much less diverse than their autotrophic sisters. We then use age estimates for taxa used in the sister-taxon comparisons to test for correlations between clade age and clade diversity. We find that parasitic plant diversity is not significantly correlated with the age of the lineage, whereas there is a strong positive correlation between the age and diversity of nonparasitic sister lineages. The Ericaceae sister pair Monotropoideae (parasitic) and Arbutoideae (autotrophic) is sufficiently well sampled at the species level to allow more parametric comparisons of diversification patterns. Model fitting for this group supports ecological limitation in Monotropoideae and unconstrained diversification in Arbutoideae. Thus, differences in diversity between parasitic plants and their autotrophic sisters might be caused by a combination of ecological limitation and exponential diversification. A combination of sister-taxon comparisons of diversity and age, coupled with model fitting of well-sampled phylogenies of focal taxa, provides a powerful test of likely causes of asymmetry in the diversity of lineages.     

Co-evolutionary arms race between brood parasites and their hosts at the nestling stage

Traditionally it was thought that the arms race between brood parasites and their hosts was confined to the egg stage of the breeding cycle because many host species are able to reject mimetic parasitic eggs but they are unable to reject strongly different parasitic chicks. However, recently, new cases of chick rejection, discrimination, or mimicry, have been published confirming the possibility that an equivalent arms race to that found at the egg stage could ever be played out at the chick stage. Here, I review the evidence for the existence of a co-evolutionary arms race at the nestling stage. Recent findings include new deceiving strategies used by brood parasitic chicks, defensive strategies used by foster parents and adaptive strategies of host nestlings. This review shows that both chick discrimination and relationships between brood parasites and their hosts at the nestling stage are much more complicated than previously believed. At least in some brood parasite-host systems, an arms race at the nestling stage is working.     

Great Spotted Cuckoo Nestlings but not Magpie Nestlings Starve in Experimental Age‐Matched Broods

Nestlings of non‐evicting avian brood‐parasites have to compete for food with foster parents' own nestlings. The outcome of these competitive contests is determined mainly by body size differences between parasitic and host nestlings. As part of the coevolutionary arms race between brood parasites and their hosts at the nestling stage, it has been reported that some host foster parents discriminate against parasitic chicks and are reluctant to feed them. Here, by experimentally creating size‐matched broods of different composition (only magpie Pica pica chicks, only great spotted cuckoo Clamator glandarius chicks or mixed broods), we show that great spotted cuckoo chicks starved in 20.2 per cent (17 of 84) of the parasitized magpie nests even in absence of size asymmetries, while in none (0 of 72) of the nests a magpie chick starved. As far as we know, this is the first record of non‐evictor brood parasitic nestlings starving without being smaller than their host nestmates in a frequently used host species. Nest composition had no effect on chick starvation. The cuckoo nestling starved even in two of the nests occupied by only one cuckoo chick. Our results could be explained by (1) magpies being reluctant to feed cuckoo chicks; (2) parasitic chicks receiving lower‐quality food items or cuckoo nestlings being sensitive to some particular component of the diet (e.g. cereal grains); and (3) the existence of cuckoo chick discrimination ability by magpie foster parents.     

Academician E. N. Pavlovskiĭ's parasitology school in the Zoological institute RAS

The first parasitological division in the Zoological Museum was created in 1924 by the initiative of E. N. Pavlovsky and A. A. Schtakelberg and originally had a named "The permanent commission on the study of malaria mosquitoes". In the process of reorganisation of the Zoological Museum into Zoological Institute in 1930, it was modified into the Department of Parasitology with the E. N. Pavlovsky as a head. In 1934-1935, two laboratories were formed within this department: the Laboratory arachno-entomology and Laboratory of parasitic worms. In subsequent history of ZIN, these parasitological laboratories existed at first as subdivisions of the Department of Parasitology and finally the they were reorganised into independent administrative divisions. The study of parasitic and blood-sucking arthropodes is concentrated in the Laboratory of Parasitology (the head Yu. S. Balashov). A creation of the most important concepts of ecological parasitology was taking place in the Zoological Institute in the middle of 30th. E. N. Pavlovsky for the first time had formulated the principle of an organism as an environment for parasites, the concept of communities of parasitic organisms (concept of parasitocoenosis), and the theory of natural focuses of transmissive diseases. In the process of development of these scientific generalisations, a scientific direction named "Academician E. N. Pavlovsky's school of thought in parasitology" was formed in the USSR in 40-50th. In the frame of this school of thought, the main tusks of the Laboratory of Parasitology ZIN are to work out fundamental problems in ecology, systematics and morphology of parasitic and blood-sucking ticks, mites and insects. Within the ecological parasitology, different aspects of host-parasite relationships are studied at organism and population levels. The main basis of systematics studies of parasitic arthropodes is a scientific collection including over 250,000 samples. Based on this material, 40 key books and monographs on the USSR's fauna were created. Over 20 doctors of science and 50 candidates of science have been prepared within the laboratory or under the promotion of its stuff during 70 years of the existence of the Laboratory of Parasitology.     

The Population Structure of Species in Agamic Protozoans Using Leishmania As an Example

The problem of intraspecific variability in agamic protozoans is analyzed using Leishmania as an example on the basis of data obtained within the last decade using modern methods of molecular biology and biochemistry. We demonstrate local populations that are a result of coevolutionary interaction with other members of the parasitic system: mosquitoes (Diptera, Phlebotominae) and mammals. The existence of a population structure substantiates the notion of species for Leishmania genus; their properties fit within the generally accepted frames of these taxon criterions.     

Epigenetics: A key regulator of platyhelminth developmental biology?

The Platyhelminthes (flukes/flatworms) are a large group of derived metazoans beautifully adapted for existence in diversely challenging ecosystems. As tractable examples of development and self-regeneration or as causative agents of aquacultural, veterinary and biomedically-relevant parasitic diseases, the platyhelminths are subject to intensive inter-disciplinary research. Given the complex lifestyles exhibited by individuals within this phylum, we postulate that epigenetic processes feature in many aspects of platyhelminth lifecycle diversity, development and environmentally-driven adaptations.     

Ultraviolet and green parts of the colour spectrum affect egg rejection in the song thrush (Turdus philomelos)

Much attention has been devoted to understanding the evolution of egg mimicry in avian brood parasites. The majority of studies have been based on human perception when scoring the mimicry of the parasitic egg. Surprisingly, there has been no detailed study on the recognition and sensitivity towards differently coloured parasitic eggs. We investigated effect of different colours of the experimental eggs measured by ultraviolet (UV)-visible reflectance spectrophotometry on rejection behaviour in the song thrush ( Turdus philomelos ). We carried out a set of experiments with four blue model eggs representing mimetic eggs, whereas six other colours represented nonmimetic eggs. Our results revealed that two colours originally designed as a mimetic were rejected at a high rate, whereas one group of the nonmimetic was accepted. A multiple regression model of absolute differences between song thrush and experimental eggs on rejection rate showed that the level of mimicry in the UV and green parts of the colour spectrum significantly influenced egg rejection in the song thrush. To our knowledge, this is the first detailed study showing that different colour perception by the birds can affect their responses towards the parasitic egg. These findings suggest that the combination of UV and visible ranges of the spectra plays a major role in the evolution of discrimination processes, as well as in the evolution of the mimicry of the parasitic egg.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 269–276.