The nematode Caenorhabditis elegans is a powerful model for the study of host-pathogen interactions and their co-evolution. The microsporidian Nematocida parisii was the first characterized intracellular pathogen of C. elegans (Troemel et al. 2008). N. parisii proliferates in C. elegans intestinal cells and is transmitted horizontally through spores. By sampling rhabditid nematodes worldwide, we found ten new microsporidia species in 9 rhabditid nematode species. We tested the host range of these microsporidia species and measured intraspecific variation in sensitivity of the host. We further plan to detect genetic loci underlying this observed intraspecific variation in host sensitivity, using Quantitative Trait Locus (QTL) mapping.
Specificities of infection of seven nematode-infecting microsporidia were tested on four nematode species (C. elegans, C. briggsae, Oscheius tipulae and O. sp. 3). N. parisii, N. sp. 2 only infect Caenorhabditis, but not O. tipulae or O. sp. 3. To the infection of N. sp. 1, Caenorhabditis are more sensitive than Oscheius. N. sp. 3 infects both Caenorhabditis and Oscheius species. Microsporidia JUm408 and JUm1505 infect Oscheius spp., but not Caenorhabditis. Microsporidia JUm2551 infects both Oscheius species, JUm408 only infects Oscheius sp. 3, JUm1505 only infects O. tipulae.
Variation in sensitivity of ten wild C. elegans strains to N. sp. 1 infection was revealed by food consumption tests using GFP-labeled E. coli in the presence or absence of microsporidia, complemented by progeny production and longevity assays on a subset of these strains. For genetic studies, we then chose JU2825 as the sensitive strain and JU1249 as the resistant strain. We started to map the genetic loci underlying their difference in sensitivity to N. sp. 1. We plan to generate bulk segregating populations after crossing them (Test group); in parallel, we have a Control group where we mixed JU2825 and JU1249 without crossing them (they reproduce by selfing). We will treat both groups with or without N. sp. 1. By following allelic proportions in the Control (uncrossed) group using pyro-sequencing of SNPs between the two strains, we can monitor the increase in frequency of JU1249 in the presence of microsporidia and determine when it approaches 100% in the population. Then the infected population in the Test (crossed) group should have recovered large numbers or progeny with beneficial alleles linked to relevant loci. We will pool-sequence each of the test populations and analyze the data for significant shifts in allele proportions along the genome of the infected versus uninfected populations, thus potentially revealing quantitative trait loci affecting sensitivity to N. sp. 1.