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

Abstract

The influence of host and parasite genetic background on infection outcome is a topic of great interest because of its pertinence to theoretical issues in evolutionary biology. In the present study, we use a classical genetics approach to examine the mode of inheritance of infection outcome in the crustacean Daphnia magna when exposed to the bacterial parasite Pasteuria ramosa. In contrast to previous studies in this system, we use a clone of P. ramosa, not field isolates, which allows for a more definitive interpretation of results. We test parental, F1, F2, backcross and selfed parental clones (total 284 genotypes) for susceptibility against a clone of P. ramosa using two different methods, infection trials and the recently developed attachment test. We find that D. magna clones reliably exhibit either complete resistance or complete susceptibility to P. ramosa clone C1 and that resistance is dominant, and inherited in a pattern consistent with Mendelian segregation of a single-locus with two alleles. The finding of a single host locus controlling susceptibility to P. ramosa suggests that the previously observed genotype-genotype interactions in this system have a simple genetic basis. This has important implications for the outcome of host-parasite coevolution. Our results add to the growing body of evidence that resistance to parasites in invertebrates is mostly coded by one or few loci with dominance.

Genetic architecture of resistance in Daphnia hosts against two species of host-specific parasites

“…Furthermore, the data show that variation for resistance is mostly determined by clone identity, as replicates within clones are highly consistent with each other, suggesting little non-genetic variation (Figure 1a). These results were not surprising: a previous study using genetic crosses had shown that D. magna's resistance against P. ramosa is likely based on a locus with Mendelian inheritance, without evidence for non-genetic effects (Luijckx et al, 2012). However, this previous study, using material from Finnish populations exclusively, revealed a second locus strongly linked to the first, coding for resistance against a different P. ramosa genotype (Luijckx et al, 2013).…”

“…Assuming that resistance is dominant and coded by a single locus with two alleles, the proportion of susceptible clones would be expected to be 25%, which is not significantly different from 20.6% (χ 2 = 1.986, P = 0.16, n = 195). This single locus Mendelian segregation was also seen for Pasteuria resistance in a cross between two Finnish D. magna clones (Luijckx et al, 2012). The QTL analysis of these binary (0/1) infection data (clones being treated as susceptible or resistant) revealed a single major QTL that explains 50.04% of the total variation ( Figure 1c and Table 1).…”

“…It sterilizes its host, killing it after about 50 days. Studies of sediment cores have revealed fast evolutionary dynamics between host and parasite suggestive of a Red Queen-like coevolution (Decaestecker et al, 2007) and genetic crosses revealed strong host-parasite genetic interactions based on few loci and strong epistasis (Little et al, 2006;Luijckx et al, 2012;Luijckx et al, 2013), making it a strong candidate for coevolution by NFDS (Ebert, 2008). Nonetheless, although we understand relatively well Daphnia resistance to Pasteuria, we have lacked knowledge about this system's underlying genomic architecture.…”

Host-parasite coevolution in populations of constant and variable size

“…Numerous field studies identified the presence of comprehensive heritable variation in resistance-infectivity patterns for plant and animal populations and their respective pathogens, suggesting that coevolution acts to maintain genetic diversity [3,11,[52][53][54][55]. However, already the first studies, which attempted to explain such variation by cycling dynamics, encountered the problem of stability.…”

Water fleas require microbiota for survival, growth and reproduction

“…As resting eggs are very tolerant of chemical treatment (Vizoso et al, 2005;Luijckx et al, 2012), we used household bleach (sodium hypochlorite) instead of antibiotics to remove the bacteria from the egg surfaces. We also used autoclaved algae instead of axenic live algae as alternative food to the Daphnia.…”