Two species of feminizing microsporidian parasite coexist in populations of Gammarus duebeni

The amphipod crustacean Gammarus duebeni hosts two species of vertically transmitted microsporidian parasites, Nosema granulosis and Microsporidium sp. A. Here it is demonstrated that these co-occurring parasite species both cause infected females to produce female-biased broods. A survey of European G. duebeni populations demonstrates that these two parasites co-occur in six of 10 populations. These findings contrast with the theoretical prediction that two vertically transmitted feminizing parasites should not coexist in a panmictic population of susceptible hosts at equilibrium. Possible explanations for the co-occurrence of the two feminizing microsporidia in G. duebeni include the recent invasion of a new parasite, horizontal transmission of one or both parasites and the spread of alleles for resistance to the dominant parasite in host populations.     

Impact of a Novel, Feminising Microsporidium on its Crustacean Host

We describe the transmission and pathogenic effects of a novel, feminising microsporidium, probably a Nasema species, on its crustacean host Gammarus duebeni. The parasite prevalence in the field was high (46% of females were infected) and the parasite was transovarially transmitted to 91% of embryos of infected females. The impact of the parasite on the host was assessed by means of a host breeding experiment. The parasite feminised 66% of infected host young and was transovarially transmitted by these individuals to the next host generation. The parasite differed from other feminising microsporidia in G. duebeni in that early embryos had a high parasite burden (288 parasites per embryo) and the infection was pathogenic, causing a reduction in both the growth rate of young hosts and in adult size. This study suggests that feminising microsporidia are a diverse group in which a variety of host/pathogen relationships have evolved.     

Three microsporidian pathogens infecting Lymantria dispar larvae do not differ in their success in horizontal transmission

We quantified horizontal transmission of three microsporidian pathogens, Endoreticulatus schubergi, Nosema lymantriae and Vairimorpha disparis that infect Lymantria dispar larvae in an experiment using caged, potted oak plants. Despite marked differences in the modes of spore release from infectious hosts, no significant differences in the transmission success to uninfected, susceptible test hosts were ascertained between the tested microsporidian species. The density of initially inoculated larvae and the exposure period, on the other hand, did influence the number of infected test larvae. Depending on the density of inoculated larvae (10%, 30% or 50%), between 0% and 26% of the test larvae became infected with one of the three tested microsporidian pathogens after an exposure period of 6 days. When the exposure period was 12 days, between 11% and 76% of the test larvae became infected.     

Targeting of host cell lineages by vertically transmitted, feminising microsporidia

Feminising microsporidian parasites are transmitted vertically from generation to generation of their crustacean hosts. Little is known about the mechanisms underpinning vertical transmission, in particular, parasite transmission to the host gonad during host development. Here, we investigate the burden and distribution of two species of vertically transmitted, feminising microsporidia (Dictyocoela duebenum and Nosema granulosis) during early embryogenesis (zygote to eight-cells) of the Gammarus duebeni host. Parasite burden differs between the two parasites with N. granulosis being higher by a factor of 10. Whilst D. duebenum replicates during the first few host cell divisions, there is no increase in N. granulosis burden. Only merogonic parasite stages were observed in the host embryo. Distribution of both parasites was non-random from the two-cell embryo stage, indicating biased parasite segregation at host cell division. Dictyocoela duebenum burden was low in the germline and somatic gonad progenitor cells but was highest in the ectoderm precursors, leading us to propose that the parasite targets these cells and then secondarily infects the gonad later in host development. Targeting by N. granulosis was less specific although there was a persistent bias in parasite distribution throughout host cell divisions. Parasite burden was highest in the ectoderm precursors as well as the germline progenitors leading us to suggest that, in addition to using the ectodermal route, N. granulosis may also target germline directly. Biased segregation will be adaptive for these parasites as it is likely to lead to efficient transmission and feminisation whilst minimising virulence in the host.     

Relationship between the host cell mitochondria and the parasitophorous vacuole in cells infected with Encephalitozoon microsporidia

Encephalitozoon microsporidia proliferate and differentiate within a parasitophorous vacuole. Using the fluorescent probe, calcein, and the mitochondrial probe, MitoTracker-CMXRos, a vital method was developed that confirmed ultrastructural reports that the host cell mitochondria frequently lie in immediate proximity to the parasitophorous vacuole. Morphometry failed to demonstrate any infection-induced increase in host cell mitochondria as there was no correlation between the mitochondrial volume and the extent of infection as judged by the parasitophorous vacuole volume. The total ATP concentration of infected cells did not differ from that of uninfected cells in spite of the increased metabolic demands of the infection. Treatment with 10(-6) M albendazole, more than ten times the antiparasitic IC50 dose, and demecolcine had no subjective effect on the proximity of mitochondria to the parasitophorous vacuole membrane when studied by either transmission electron microscopy or by confocal microscopy even though these drug concentrations affected microtubule structure. Thus, once the association between mitochondria and the parasitophorous vacuole has been established, host cell microtubule integrity is probably not required for its maintenance. It is unlikely that the antimicrosporidial action of albendazole involves physically uncoupling developing parasite stages from host cell organelle metabolic support.     

Transmission as a predictor of ecological host specificity with a focus on vertical transmission of microsporidia

Consideration of vertical transmission is particularly important for understanding the life cycles of entomopathogens that are naturally occurring in invertebrate populations, are a problem in beneficial insect colonies, or are under consideration as classical biological control agents. Empirical studies generally corroborate the evolutionary hypothesis that virulence should be relatively low for pathogen species that utilize vertical transmission as one mechanism for maintenance in the host population. Nevertheless, many entomopathogens with significant effects on host populations are vertically as well as horizontally transmitted. In addition to gaining a better understanding of pathogen-host interactions and population dynamics, studies of the host range and specificity of putative biological control agents can benefit by using transmission studies to better predict ecological host specificity from physiological data. Horizontal transmission requires a tightly organized host-pathogen relationship to succeed, but still involves, albeit restricted by host behavior and pathogen dosage, the physiological susceptibility of the nontarget host. Vertical transmission studies can provide increased stringency for determining the ecological host specificity of a species and may be one very accurate predictor of the ability of a pathogen to successfully host-switch when introduced into a na?ve population.     

Prevalence,transmission and mortality associated with Nosema fumiferanae infections in field populations of spruce budworm Choristoneura fumiferana

• 1 The prevalence, intensity and transmission of Nosema fumiferanae (Thomson) (Microsporidae) infections and potential impacts on the survival of field populations of spruce budworm Choristoneura fumiferana (Clem.) were examined in three plots in New Brunswick, Canada, from 1983 to 1992.
• 2 The highest prevalence of N. fumiferanae infection in post‐hibernation second‐instar larvae occurred in the plot where prevalence in female pupae was the highest in the previous generation, suggesting higher rates of vertical transmission. There was little change in the prevalence of N. fumiferanae infections between the second and sixth instars in the later generations. In the two other plots, N. fumiferanae prevalence increased by approximately 25% from the second to sixth larval stadia. Coincident with the changes in N. fumiferanae prevalence were substantial declines in the populations of spruce budworms, making it difficult to determine rates of horizontal transfer of the disease.
• 3 In all plots and in all years, there were progressive increases in the intensity of N. fumiferanae infections (spore loads/individual) from the second to sixth instars and pupae.
• 4 Annual spruce budworm mortality associated with N. fumiferanae was ≤15% of all mortality in reared specimens and was positively correlated with but generally less than 30% of annual N. fumiferanae prevalence.

     

Naturally occurring single and double infection with Wolbachia strains in the butterfly Eurema hecabe: transmission efficiencies and population density dynamics of each Wolbachia strain

Wolbachia belonging to Alphaproteobacteria are transovarially transmitted bacteria responsible for reproductive alterations in a wide range of arthropods. In natural populations of the butterfly Eurema hecabe, there are two different types of Wolbachia-infected individuals. Individuals singly infected with Wolbachia strain wHecCI exhibit strong cytoplasmic incompatibility, whereas those doubly infected with wHecCI and wHecFem exhibit feminization. Here, we examined the infection frequencies and population densities of each Wolbachia strain in different host tissues (ovary, testis, fat body, midgut, Malpighian tubule and leg), and the cost of infection in offspring produced by single-infected and double-infected mothers of E. hecabe. The vertical transmission rate of wHecCI was nearly 100%, and that of wHecFem was c. 80%. The wHecCI densities were 10(3)-10(4)-fold higher than the wHecFem densities. In most tissues, the wHecCI densities were significantly higher in offspring of single-infected mothers than in offspring of double-infected mothers. In offspring of double-infected mothers, however, the wHecCI densities were not affected by the presence of wHecFem, suggesting a lack of interaction between the wHecCI and wHecFem densities. The offspring development time was dependent on the infection status of the mothers. These results imply that the maternal infection status affects the Wolbachia densities and fitness of offspring.     

Long‐term selection experiment produces breakdown of horizontal transmissibility in parasite with mixed transmission mode

Evolutionary transitions from parasitism toward beneficial or mutualistic associations may encompass a change from horizontal transmission to (strict) vertical transmission. Parasites with both vertical and horizontal transmission are amendable to study factors driving such transitions. In a long‐term experiment, microcosm populations of the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata were exposed to three growth treatments, manipulating vertical transmission opportunities over ca. 800 host generations. In inoculation tests, horizontal transmission propagules produced by parasites from a “high‐growth” treatment, with elevated host division rates increasing levels of parasite vertical transmission, showed a near‐complete loss of infectivity. A similar reduction was observed for parasites from a treatment alternating between high growth and low growth (i.e., low levels of population turn‐over). Parasites from a low‐growth treatment had the highest infectivity on all host genotypes tested. Our results complement previous findings of reduced investment in horizontal transmission and increased vertical transmissibility of high‐growth parasites. We explain the loss of horizontal transmissibility by epidemiological feedbacks and resistance evolution, reducing the frequency of susceptible hosts in the population and thereby decreasing the selective advantage of horizontal transmission. This illustrates how environmental conditions may push parasites with a mixed transmission mode toward becoming vertically transmitted nonvirulent symbionts.