Cloning of the unculturable parasite Pasteuria ramosa and its Daphnia host reveals extreme genotype–genotype interactions

Abstract

The degree of specificity in host-parasite interactions has important implications for ecology and evolution. Unfortunately, specificity can be difficult to determine when parasites cannot be cultured. In such cases, studies often use isolates of unknown genetic composition, which may lead to an underestimation of specificity. We obtained the first clones of the unculturable bacterium Pasteuria ramosa, a parasite of Daphnia magna. Clonal genotypes of the parasite exhibited much more specific interactions with host genotypes than previous studies using isolates. Clones of P. ramosa infected fewer D. magna genotypes than isolates and host clones were either fully susceptible or fully resistant to the parasite. Our finding enhances our understanding of the evolution of virulence and coevolutionary dynamics in this system. We recommend caution when using P. ramosa isolates as the presence of multiple genotypes may influence the outcome and interpretation of some experiments.

Expression of parasite genetic variation changes over the course of infection: implications of within-host dynamics for the evolution of virulence

“…These parasite lines have been derived from single spore infections (see Luijckx et al [22]) and therefore represent true single-genotypes. The entire experiment was conducted using the host clone D. magna HO 2 , originally collected in Hungary and being chosen because it is susceptible to all P. ramosa genotypes included in our experiments.…”

“…Unravelling the potential contributions of these evolutionary mechanisms, however, will require characterizing the course of infection for parasite genotypes from populations with differences in Daphnia survival [30]. Although all the parasite clones used in this study were derived from natural isolates, we know little about their natural history (but see [22]). Given the diverse geographical distribution of the parasite origins, each population would have likely varied in the permanence of the population (seasonal or permanent) and the type of predators (fish, invertebrate or none)-all of which would shape the duration of an infection period.…”

“…Hall & Ebert [18] suggested that variability in castration might cause disease dynamics to change with infection age, as they found differences in genetic variation for parasite fitness at early and late stages of the infection process. Indeed, in this system, host and parasite traits like virulence (parasite-induced mortality and fecundity reduction) and resistance show strong G Â G and G Â E interactions (G, genetic; E, environmental) [3,[19][20][21][22], which suggest a genetic basis for variation in pathology caused by P. ramosa. Unravelling how variability in castration dynamics is linked to overall within-host dynamics will help identify how conflict between host and parasites over resources can be resolved.…”

Resistance to a bacterial parasite in the crustacean Daphnia magna shows Mendelian segregation with dominance

“…We used spores from P. ramosa clone C1 (Russia, Moscow) for our infection trials, which, as other P. ramosa clones, shows highly specific infectivity (for more details see Luijckx et al, 2011). This parasite clone is both phenotypically and genetically similar to a naturally occurring parasite in Finland McElroy et al, 2011).…”

“…First, we employ a clone of P. ramosa (single genotype), not field isolates. Field isolates may contain more than one parasite clone (Jensen et al, 2006;Mouton et al, 2007;Ben-Ami et al, 2008;Luijckx et al, 2011). The use of P. ramosa clones negates the complicating factors intrinsic to mixed infections and allows for a more definitive interpretation of experimental results .…”