Microgeographic Population Genetic Structure of Baylisascaris procyonis (Nematoda: Ascaroidae) in Western Michigan Indicates the Grand River Is a Barrier to Gene Flow.

Baylisascaris procyonis , the raccoon roundworm, is increasingly being recognized for its zoonotic and public health importance. Fine-scale analyses of the population genetics of this species have been hampered due to a lack of appropriate genetic markers. To this end, we developed 8 novel polymorphic microsatellites for B. procyonis and used these markers to elucidate microgeographic structuring of this parasite in a 500-km(2) study area in western Michigan. Our analyses revealed significant levels of genetic differentiation amongst the 74 worms collected from 10 different raccoons. Critically, Bayesian clustering indicated 2 genetically distinct groups, one on either side of the Grand River which bisects our study area. Estimates of F(ST), and results from AMOVA and isolation by distance, further corroborated a scenario whereby the river is acting as a barrier to gene flow, a rather unexpected finding given the high vagility of raccoons and microgeographic scale of the analysis. It is possible that the Grand River is a major dispersal barrier for B. procyonis because raccoons are most likely to disperse across the river when it is frozen, and worm burden in raccoons approaches zero during the winter.

Reinvestigation of an endogenous meiotic drive system in the mosquito, Aedes aegypti (Diptera: Culicidae).

We have initiated efforts to determine the molecular basis for the M(D) meiotic drive system in the mosquito, Aedes aegypti. The effect of the M(D) gene is a highly male-biased sex ratio, but varies depending on the frequency and sensitivity of a susceptible responder m(s) allele. The M(D) system has potential as a mechanism for driving trangenes for pathogen resistance into natural Ae. aegypti populations. Because all previously existing laboratory strains carrying the M(D) gene have been lost, we have selected for a new strain, T37, that carries a strong driver. Matings between T37 males and drive-susceptible m(s) females result in progeny with highly biased sex ratios, wherein only approximately 14.7% females are produced. We discuss the potential for identifying M(D) candidate genes based on comparisons with the well-described Drosophila melanogaster segregation distorter (SD) meiotic drive system and considerations for release of transgenic Ae. aegypti into natural populations where M(D) and insensitive m3 alleles are likely segregating.

Variation in the structure of Toxoplasma gondii and the roles of selfing, drift, and epistatic selection in maintaining linkage disequilibria.

Previous studies of Toxoplasma gondii, based on samples dominated by clinical isolates, have concluded that its population structure is clonal, despite the sexual reproduction that occurs in cats. To determine whether this applies to non-clinical isolates, we compared patterns of linkage disequilibrium (LD) among seven loci in samples of T. gondii from Brazil and the US. LD was detected in both locations, but it was substantially lower in Brazil. The lower LD in Brazil can be explained by a higher rate of sexual reproduction between different genotypes (outcrossing) because of a higher rate of transmission. The extent of LD between pairs of physically unlinked loci varied significantly in each location. Moreover, the magnitude of LD between corresponding locus pairs in Brazil and the US was correlated, despite minimal gene exchange between the continents (mean FST = 0.19). The heterogeneity among locus pairs and the correlation in LD between physically unlinked locus pairs from different continents suggests that locus-specific factors, such as epistatic selection are involved in maintaining LD in T. gondii. Possibly, the unique life cycle of T. gondii with its unpredictable transmission among diverse host species and distinct ecological habitats requires specific combinations of alleles from multiple loci. The usefulness of typing isolates based on physically unlinked loci is questioned not only by the geographic variation in the reproductive population structure, but mainly by the low overall predictability of the genotype of one locus based on the genotype in another (unlinked) locus. This predictability ranged between 23 and 45%, but was close to nil for a considerable fraction of locus pairs.

Quantitative trait loci conditioning transovarial transmission of La Crosse virus in the eastern treehole mosquito, Ochlerotatus triseriatus.

Quantitative trait loci (QTL) affecting the ability of the Eastern treehole mosquito, Ochlerotatus triseriatus, to transovarially transmit (TOT) La Crosse virus (LAC) were mapped in an F1 intercross. The Holmen strain of O. triseriatus, previously selected for TOT refractoriness, was crossed to the AIDL strain that had been selected for TOT permissiveness. In P1 and F1 parents and 49 F2 individuals, regions of 10 cDNA loci were analysed with single strand conformation polymorphism (SSCP) analysis to identify and orientate linkage groups. Genotypes were also scored at fifty-six random amplified polymorphic DNA (RAPD)-SSCP loci. Twenty-eight F2 offspring were individually analysed for TOT. Three QTL for TOT were detected with standard interval mapping on chromosomes II and III. Alleles at the three loci contributed additively towards determining the overall TOT rate and cumulatively accounted for approximately 53% of the phenotypic variance in TOT.

Transmission dynamics of Toxoplasma gondii on a pig farm.

Transmission of Toxoplasma gondii infection on a pig farm in New England was investigated using genetic and ecological methods to (i) determine if infection of pigs was a result of a single source, such as in an epizootic situation (e.g. outbreak) or of multiple sources, such as in an enzootic situation, (ii) identify the main source species of infection to pigs and (iii) evaluate the role of the environment surrounding the farm as the source of infection on the farm. Genetic characterization of 25 T. gondii isolates from market pigs revealed three distinct genotypes with no evidence of recombinants. These data imply that at least three distinct exposure events occurred during the 7-month lifespan of these pigs. This genotype diversity is consistent with enzootic transmission of T. gondii on the farm. Cats were suspected as the main source of pig infection based on the high seroprevalence (>95%) in pigs. The presence of the two most common T. gondii genotypes in eight isolates from free ranging chickens on this farm corroborated the role of cats because chickens were probably infected through ingestion of oocysts in the soil. The seroprevalence of toxoplasmosis in 163 wild mammals and birds captured around the pig sties (overall 13.1%) increased with proximity to the pig sties. Thus, transmission of T. gondii was higher near the pig sties than in the surrounding environment probably because of increased density of oocysts there. We propose that the farm does not simply reflect its surroundings in terms of strain composition and risk of infection, but that it acts as a reservoir of strains from which the outflow of new infections into its surrounding environment is higher than the inflow.