Local adaptation of a parasite to solar radiation impacts disease transmission potential,spore yield,and host fecundity*

Environmentally transmitted parasites spend time in the abiotic environment, where they are subjected to a variety of stressors. Learning how they face this challenge is essential if we are to understand how host–parasite interactions may vary across environmental gradients. We used a zooplankton–bacteria host–parasite system where availability of sunlight (solar radiation) influences disease dynamics to look for evidence of parasite local adaptation to sunlight exposure. We also examined how variation in sunlight tolerance among parasite strains impacted host reproduction. Parasite strains collected from clearer lakes (with greater sunlight penetration) were most tolerant of the negative impacts of sunlight exposure, suggesting local adaptation to sunlight conditions. This adaptation came with both a cost and a benefit for parasites: parasite strains from clearer lakes produced relatively fewer transmission stages (spores) but these strains were more infective. After experimental sunlight exposure, the most sunlight-tolerant parasite strains reduced host fecundity just as much as spores that were never exposed to sunlight. Sunlight availability varies greatly among lakes around the world. Our results suggest that the selective pressure sunlight exposure exerts on parasites may impact both parasite and host fitness, potentially driving variation in disease epidemics and host population dynamics across sunlight availability gradients.     

System optimization for attachment of Pasteuria penetrans to Meloidogyne incognita

For optimal mass production of Pasteuria penetrans in vivo, it is important to develop a system that can ensure 100% nematode attachment of the bacteria and high bacterial infection after inoculation. In this study, effects of endospore concentration and centrifugation parameters on attachment were investigated, followed by evaluation of impacts of centrifugation on endospore dislodgement, Meloidogyne incognita juvenile (J2) mortality, J2 infectivity, and bacterial infectivity. Endospore concentration and percentage of attachment fit well to mass-action and logit models, with the former being superior. Centrifugation had no impact on J2 mortality but had a great impact on endospore dislodgement in sand and water, nematode infectivity and bacterial infectivity. At nematode concentration of 2×10³ J2/mL, the optimal system for endospore attachment was developed which consisted of bacteria at 2×10⁴ endospores/mL, and centrifugation at 9000×g for 3 min three times. This system generated 100% attachment with approximately seven endospores/J2. After inoculation of treated nematodes to tomato plants, the inoculum yielded 47% bacterial infection, superior to 17% infection observed in centrifugation at 6000×g. Endospore dislodgement occurred after placing the centrifuged inoculated nematodes in sand or water for 24 and 48 h, which was more severe in centrifugation at 6000 than at 9000×g. Results also indicated that centrifugation led to lower nematode infectivity, regardless of endospore presence and centrifugation at 9000 or 6000×g, compared with the no centrifugation control.     

A Method for Isolation of Pasteuria penetrans Endospores for Bioassay and Genomic Studies

A rapid method for collection of Pasteuria penetrans endospores was developed. Roots containing P. penetrans-infected root-knot nematode females were softened by pectinase digestion, mechanically processed, and filtered to collect large numbers of viable endospores. This method obviates laborious handpicking of Pasteuria-infected females and yields endospores competent to attach to and infect nematodes. Endospores are suitable for morphology studies and DNA preparations.     

Exploitation of immunofluorescence for the quantification and characterization of small numbers of Pasteuria endospores

The Pasteuria group of endospore-forming bacteria has been studied as a biocontrol agent of plant-parasitic nematodes. Techniques have been developed for its detection and quantification in soil samples, and these mainly focus on observations of endospore attachment to nematodes. Characterization of Pasteuria populations has recently been performed with DNA-based techniques, which usually require the extraction of large numbers of spores. We describe a simple immunological method for the quantification and characterization of Pasteuria populations. Bayesian statistics were used to determine an extraction efficiency of 43% and a threshold of detection of 210 endospores g(-1) sand. This provided a robust means of estimating numbers of endospores in small-volume samples from a natural system. Based on visual assessment of endospore fluorescence, a quantitative method was developed to characterize endospore populations, which were shown to vary according to their host.     

Temperature-dependent costs of parasitism and maintenance of polymorphism under genotype-by-environment interactions

The maintenance of genetic variation for infection-related traits is often attributed to coevolution between hosts and parasites, but it can also be maintained by environmental variation if the relative fitness of different genotypes changes with environmental variation. To gain insight into how infection-related traits are sensitive to environmental variation, we exposed a single host genotype of the freshwater crustacean Daphnia magna to four parasite isolates (which we assume to represent different genotypes) of its naturally co-occurring parasite Pasteuria ramosa at 15, 20 and 25 degrees C. We found that the cost to the host of becoming infected varied with temperature, but the magnitude of this cost did not depend on the parasite isolate. Temperature influenced parasite fitness traits; we found parasite genotype-by-environment (G x E) interactions for parasite transmission stage production, suggesting the potential for temperature variation to maintain genetic variation in this trait. Finally, we tested for temperature-dependent relationships between host and parasite fitness traits that form a key component of models of virulence evolution, and we found them to be stable across temperatures.     

Parasitism drives host genome evolution: Insights from the Pasteuria ramosa–Daphnia magna system

Because parasitism is thought to play a major role in shaping host genomes, it has been predicted that genomic regions associated with resistance to parasites should stand out in genome scans, revealing signals of selection above the genomic background. To test whether parasitism is indeed such a major factor in host evolution and to better understand host–parasite interaction at the molecular level, we studied genome‐wide polymorphisms in 97 genotypes of the planktonic crustacean Daphnia magna originating from three localities across Europe. Daphnia magna is known to coevolve with the bacterial pathogen Pasteuria ramosa for which host genotypes (clonal lines) are either resistant or susceptible. Using association mapping, we identified two genomic regions involved in resistance to P. ramosa, one of which was already known from a previous QTL analysis. We then performed a naïve genome scan to test for signatures of positive selection and found that the two regions identified with the association mapping further stood out as outliers. Several other regions with evidence for selection were also found, but no link between these regions and phenotypic variation could be established. Our results are consistent with the hypothesis that parasitism is driving host genome evolution.     

Characterization of a major Mycoplasma penetrans lipoprotein and of its gene

Abstract A novel mycoplasmal species designated as Mycoplasma penetrans has been isolated recently from patients infected with human immunodeficiency virus. p35, a major antigen extracted from the membrane of this mycoplasma using Triton X-114 has been found to be a lipoprotein. After proteolytic treatment of p35, the sequence of one of the resulting peptides was determined and a corresponding oligonucleotide was deduced. Using this oligonucleotide as a probe the p35 gene was cloned and sequenced. Sequence analysis revealed an amino-terminal signal peptide with a potential acylation site which would result in a 35.3 kDa mature product. In addition, the p35 gene was followed by an open reading frame with a corresponding polypeptide partially homologous to p35, in particular to the N-terminus region.     

Suppression of Meloidogyne incognita and M. javanica by Pasteuria penetrans in Field Soil

The role of Pasteuria penetrans in suppressing numbers of root-knot nematodes was investigated in a 7-year monocuhure of tobacco in a field naturally infested with a mixed population of Meloidogyne incognita race 1 and M. javanica. The suppressiveness of the soil was tested using four treatments: autoclaving (AC), microwaving (MW), air drying (DR), and untreated. The treated soil bioassays consisted of tobacco cv. Northrup King 326 (resistant to M. incognita but susceptible to M. javanica) and cv. Coker 371 Gold (susceptible to M. incognita and M. javanica) in pots inoculated with 0 or 2,000 second-stage juveniles of M. incognita race 1. Endospores of P. penetrans were killed by AC but were only slightly affected by MW, whereas most fungal propagules were destroyed or inhibited in both treatments. Root galls, egg masses, and numbers of eggs were fewer on Coker 371 Gold in MW, DR, and untreated soil than in AC-treated soil. There were fewer egg masses than root galls on both tobacco cultivars in MW, DR, and untreated soil than in the AC treatment. Because both Meloidogyne spp. were suppressed in MW soil (with few fungi present) as well as in DR and untreated soil, the reduction in root galling, as well as numbers of egg masses and eggs appeared to have resulted from infection of both nematode species by P. penetrans.     

The animal reservoir of Tunga penetrans in severely affected communities of north-east Brazil

Tungiasis is a zoonotic ectoparasitosis caused by the sand flea Tunga penetrans L. (Siphonaptera: Tungidae). This disease is hyperendemic in poor communities of north-east Brazil, causing considerable morbidity in affected human populations, but the animal reservoirs have not been investigated previously in Brazil. To assess the prevalence and intensity of T. penetrans infection in domestic and peri-domestic animals, as well as in the human population, we surveyed two typical communities of north-east Brazil: an urban slum and a traditional fishing village. In the slum we examined 849 humans, 121 cats, 82 dogs, 2 pigs, 2 rabbits, 1 monkey and 56 rodents, comprising 34 rats (Rattus rattus L.) and 22 mice (Mus domesticus L). In the fishing village we examined 505 humans, 68 dogs, 37 cats, 7 donkeys, 4 cattle, 3 pigs and 1 monkey. Tungiasis was common among dogs and cats of both communities, with respective prevalence rates of 67.1% (95% CI: 55.8-77.1) and 30.9% (95% CI: 20.2-43.3) in dogs, 49.6% (95% CI: 40.4-58.8) and 32.4% (95% CI: 18.0-49.8) in cats. Slum rats were 41.2% (95% CI: 24.6-59.3) infested, but the other animals were not. Human prevalence rates were 54.4% (95% CI: 51.0-57.8) in the slum and 52.1% (95% CI: 47.6-56.5) in the fishing village. High prevalence rates (range 31-67%) of tungiasis in humans, pets and rats (but apparently not other animals) indicate the need for an eco-epidemiological approach to control of this anthropo-zoonotic problem.