Genotypic variation in an ecologically important parasite is associated with host species, lake, and spore size

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.