The coexistence of hybrid and parentalDaphnia: the role of parasites

Abstract

Parasite driven time-lagged negative frequency-dependent selection of hosts has been studied in natural populations by following changes in host genotype frequencies over time. However, such dynamics have not been considered at higher taxonomic levels, for example, between parental species and their hybrids. In a field study on a Daphnia hybrid system, we observed that one Daphnia taxon first was relatively underinfected, but became over-infected after a strong increase in frequency. This finding is consistent with the idea of parasite evolution towards the most frequent host taxon. In two experiments, we investigated whether the assumptions made by a model of negative frequency-dependent selection apply to our host taxa system. First, we showed that the parasite can change the outcome of taxa competition and secondly, we confirmed that the over-infection of one host taxon observed in the field has a genetic basis. Our results indicate that the incorporation of host-parasite interactions at the species level may allow us to gain a more complete picture of forces driving dynamic taxa coexistence in Daphnia hybrid systems. More generally, we suggest that if hybrids coexist in sympatry with parental taxa, the infection patterns as observed under natural conditions may be rather temporal and unstable.

ADaphniaParasite (Caullerya mesnili) Constitutes a New Member of the Ichthyosporea, a Group of Protists near the Animal–Fungi Divergence

“…Parasites can play a regulatory function in host populations when the virulence of the parasite decreases the net growth rate of its host (Anderson and May 1981). Previous work established that C. mesnili decreases fecundity, growth, and life-span of its hosts Wolinska et al 2006), which we confirmed in the present study. Transmission likely occurs when naive hosts ingest spores during grazing.…”

“…The parasite strain used for laboratory studies was collected in 2006 from a natural lake in Switzerland (Greifensee). The parasite was maintained in a D. galeata genotype collected from the same lake by adding newborns from uninfected stock cultures at approximately 2-week intervals (as described in Wolinska et al 2006). The daphniids were reared with an unlimited food supply of the unicellular green algae Scenedesmus obliquus in artificial medium (for details see Jeschke and Tollrian 2000).…”

“…The infection is spread horizontally by a waterborne stage directly from Daphnia to Daphnia, with new infections becoming visible from 8 to 12 days post-exposure . Both lab and field studies have shown C. mesnili to be a virulent parasite: in the Daphnia longispina hybrid complex, egg numbers were reduced by about 95% in infected hosts (Wolinska et al 2006;2007a). In addition, Daphnia galeata infected in the lab with C. mesnili reach a smaller adult size and die earlier than healthy individuals (Lass and Bittner 2002;Wolinska et al 2006).…”

Temperature effects on parasite prevalence in a natural hybrid complex

“…The specificity of host-parasite interactions was also shown to operate between groups of genotypes belonging to hybrid and parental taxa [12,13]. About 10 per cent of animal species are involved in hybridization and potential introgression [14].…”

Biochemical food quality effects on a Daphnia hybrid complex

“…There are several environmental factors influencing the fitness of Daphnia. Temperature (Weider and Wolf 1991), food level (Weider 1993), the predation regime (Spaak and Hoekstra 1997), and infections by parasites (Woliń ska et al 2006) have all been shown to alter the relative fitness of hybrids and parental species in the D. longispina complex. These factors vary in absolute strength and relative significance during the course of the year, ensuring a selective advantage of the hybrids under some conditions and of the parentals under other conditions.…”