Polygenic traits and parasite local adaptation

The extent to which parasites are locally adapted to their hosts has important implications for human health and agriculture. A recently developed conceptual framework--the geographic mosaic theory of coevolution--predicts that local maladaptation should be common and largely determined by the interplay between gene flow and spatially variable reciprocal selection. Previous investigation of this theory has predominately focused on genetic systems of infection and resistance characterized by few genes of major effect and particular forms of epistasis. Here we extend existing theory by analyzing mathematical models of host-parasite interactions in which host resistance to parasites is mediated by quantitative traits with an additive polygenic basis. In contrast to previous theoretical studies predicated upon major gene mechanisms, we find that parasite local maladaptation is quite uncommon and restricted to one specific functional form of host resistance. Furthermore, our results show that local maladaptation should be rare or absent in studies that measure local adaptation using reciprocal transplant designs conducted in natural environments. Our results thus narrow the scope over which the predictions of the geographic mosaic theory are likely to hold and provide novel and readily testable predictions about when and where local maladaptation is expected.     

Parasite recruitment and oceanographic regime: evidence suggesting a relationship on a global scale

We here investigate the relationship between oceanographic processes and variability in parasite recruitment to host populations using existing data from host-parasite systems encountering differing hydrographic conditions. Combined epidemiological data obtained from both exploited fish and cephalopod populations indicate that variability in recruitment of parasite infracommunities tends to be associated with major current systems of the World's oceans. It appears that instability in water masses caused by physical perturbations (e.g. water mass convergence and turbulent mixing in upwelling systems) is associated with instability of trophic interactions over time, which in turn leads to a paucity of parasite communities in that area. The likely relationship between parasite recruitment and oceanographic regime should be extremely useful to the fishing industry and also as an indicator of ecosystem health.     

Comparative Heterochromatin Profiling Reveals Conserved and Unique Epigenome Signatures Linked to Adaptation and Development of Malaria Parasites

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Ditylenchus dipsaci Infestation of Trifolium repens. II. Dynamics of Infestation Development

Trifolium repens (white clover) stolons were inoculated with Ditylenchus dipsaci (stem nematode), and the development of resulting infestations was monitored. Nematodes initially remained confined to superficial locations, concentrating in petiole axils near inoculation points. They were able to migrate slowly from the inidal inoculation points and infest adjacent axils, especially in regions near the stolon tip. As time progressed, in some axils, nematodes migrated through the stolon epidermis and colonized slowly expanding subepidermal pockets of host tissue (ca. 0.2-mm length of stolon/day). In these loci nematodes established exponentially increasing populations, but the rates of locus expansion remained constant, indicating that locus expansion was limited by unidentified host-dependent factors. As a result of increasing population pressure within subepidermal loci, J4 entered a "diapause" state and the rate of egg production by adults declined, thereby reducing rate of population growth to more sustainable levels. Typically, these populations peaked at ca. 10,000 individuals in ca. 160 days occupying 3-cm lengths of stolon. Thereafter, heavily infested regions of stolons started to die, leading to the formation of longitudinal splits in their epidermis. In other axils, nematodes did not migrate into the stolons but remained confined to axils. Some of these populations increased a hundred-fold in 95 days, with population growth ending when petioles started to die. Host plant stolon morphology was affected only when subepidermal stolon populations developed high population levels (>100 nematodes) within close proximity (<2 cm) to active terminal meristems. This occurred either when axillary buds became active on previously infested nodes or when nematodes established endoparasitic populations at locations near the stolon tip during winter and spring, when the rate of stolon extension was limited by low light intensity. Affected stolon tips could "escape" from the influence of such infestations when light intensity and temperature increased. Nematode activity was limited by low temperature rather than light intensity. Global warming is likely to lead to greater damage to infested plants during the winter and early spring because the predicted milder winter temperatures will enhance nematode activity but not necessarily promote stolon growth.     

COMPARATIVE POPULATION GENETIC STRUCTURE OF A PARASITE (FASCIOLOIDES MAGNA) AND ITS DEFINITIVE HOST

The population genetic structure of the American liver fluke, Fascioloides magna, and its definitive host the white-tailed deer, Odocoileus virginianus, was examined in South Carolina. Flukes were significantly more common in deer from river-swamp habitat than upland areas and prevalence increased with host age. The distribution of flukes among deer occurred as a negative binomial with the mean dispersion parameter, k, equal to 0.17 and the range from 0.10 to 1.11 within local areas. Significant spatial genetic differentiation was observed for flukes and deer. Patterns of genetic distance in flukes were not concordant with those of the definitive host nor were they related to geographic distance between sample locations. Spatial genetic differentiation among flukes reflected the tendency for individual hosts to harbor multiple individuals from a limited number of parasite clones. The large population size of the parasite and movements of the definitive host tend to counteract factors that lead to spatial differentiation.     

Partial Characterization of Cytosolic Superoxide Dismutase Activity in the Interaction of Meloidogyne incognita with Two Cultivars of Glycine max

The closely related soybean (Glycine max) cultivars Centennial and Pickett 71 were confirmed to be resistant and susceptible, respectively, to the root-knot nematode Meloidogryne incognita. Increases in superoxide dismutase (SOD) activity were detected in roots of both soybean cultivars 48 hours following inoculation. Superoxide dismutase activity increased in roots of the susceptible cultivar overall, but declined after 96 hours in roots of the resistant cultivar. The isoelectric points of SOD isolated from preparasitic and parasitic developmental stages of the nematode appeared to differ. The SOD activity increased dramatically as nematodes matured and enlarged. Plant and nematode SOD were present as ca. 40-kDa cuprozinc dimers. Initial increases in SOD activity in infected tissue appeared to involve nematode regulation of plant gene expression. However, as the nematode enlarged, SOD activity could be detected within the female body only.     

Influence of Pratylenchus penetrans on Plant Growth and Water Relations in Potato

Plants of potato (Solanum tuberosum) cultivars Katahdin and Superior were inoculated with 0, 1,500, or 15,000 Pratylenchus penetrans. Transpiration, measured in the greenhouse with a porometer after 56 days of growth, was not significantly different among nematode inoculum levels or between cultivars. The rate of xylem exudation from decapitated root systems of Katahdin plants inoculated with 1,500 or 15,000 P. penetrans and Superior plants inoculated with 15,000 P. penetrans was lower than from noninoculated plants. Root weight of Katahdin and Superior was not affected by P. penetrans inoculum level. Transpiration of plants inoculated with 0, 500, 5,000 or 50,000 P. penetrans was recorded weekly from 14 to 56 days after planting. No consistent effects of nematode inoculum density on transpiration rate were observed. Root hydraulic conductivity was lower in Katahdin plants inoculated with 266 P. penetrans per plant and in Chippewa with 5,081 per plant than in noninoculated plants. Nematodes reduced leaf area of Superior, Chippewa, and Katahdin and root dry weight of Chippewa but had no effect on growth of Hudson, Onaway, or Russet Burbank plants. Assessing nematode effects on root hydraulic conductivity may provide a measure of the tolerance of potato cultivars to nematodes.     

Clutch size in parasitoids: the egg production rate as a constraint

Summary In many species of insect parasitoids, adult females mature eggs as they search their environment for hosts. In such species, the number of mature eggs, at the point of finding a host, is a function of the interhost time and the rate of egg maturation. Assuming that interhost search times are variable, we use a version of the marginal value theorem to derive a decision rule for optimizing the time spent exploiting individual hosts; this indirectly determines clutch size. We find that a threshold search time exists above which a female should simply lay her currently mature eggs and depart from the host. However, when the search time has been less than the threshold, a female should oviposit, but then remain on the host to mature and lay additional eggs, until the threshold time is reached.     

Diagnosis of Malaria Parasites Plasmodium spp. in Endemic Areas: Current Strategies for an Ancient Disease

Fast and effective detection of the causative agent of malaria in humans, protozoan Plasmodium parasites, is of crucial importance for increasing the effectiveness of treatment and to control a devastating disease that affects millions of people living in endemic areas. The microscopic examination of Giemsa-stained blood films still remains the gold-standard in Plasmodium detection today. However, there is a high demand for alternative diagnostic methods that are simple, fast, highly sensitive, ideally do not rely on blood-drawing and can potentially be conducted by the patients themselves. Here, the history of Plasmodium detection is discussed, and advantages and disadvantages of diagnostic methods that are currently being applied are assessed.