Genetic variation in parasites has important consequences for host-parasite interactions. Prior studies of the ecologically important parasite Metschnikowia bicuspidata have suggested low genetic variation in the species. Here, we collected M. bicuspidata from two host species (Daphnia dentifera and Ceriodaphnia dubia) and two regions (Michigan and Indiana, USA). Within a lake, outbreaks tended to occur in one host species but not the other. Using microsatellite markers, we identified six parasite genotypes grouped within three distinct clades, one of which was rare. Of the two main clades, one was generally associated with D. dentifera, with lakes in both regions containing a single genotype. The other M. bicuspidata clade was mainly associated with C. dubia, with a different genotype dominating in each region. Despite these associations, both D. dentifera- and C. dubia-associated genotypes were found infecting both hosts in lakes. However, in lab experiments, the D. dentifera-associated genotype infected both D. dentifera and C. dubia, but the C. dubia-associated genotype, which had spores that were approximately 30% smaller, did not infect D. dentifera. We hypothesize that variation in spore size might help explain patterns of cross-species transmission. Future studies exploring the causes and consequences of variation in spore size may help explain patterns of infection and the maintenance of genotypic diversity in this ecologically important system.

MichiganField2015.txt and IndianaField2015.txt datasets give the prevalence of Metschnikowia bicuspidata through the season in Daphnia dentifera and Ceriodaphnia dubia host species at each sampling date in 2015 in the Michigan study lakes and the Indiana study lakes respectively.

microsats.data.txt gives the fragment lengths for the microsatellites analyzed for each parasite sample

BenefielInfections.csv and GooseInfections.csv contain the infection data from the cross infection experiments from each lake.

BenefielSporeCounts.csv and GooseSporeCounts.csv give the number of spores per infected animal in the cross infection experiments from each lake.

Spore_Measurements.txt gives the length of spores measured from the cross infection experiment.

FragAnalysisResults.Experiment.csv gives the fragment lengths for the microsatellite loci analyzed in cross infection experiment and genotype assignment given to each genotyped animal.

Use the R code, 'SporeSizeCombinedCode.R', to analyze the data in the way it was analyzed for the manuscript.

See Read me.doc for a description of the variables in each of the datasets.