The coexistence of hybrid and parental Daphnia: 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 under-infected, 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.

Figure 1

Changes in the frequencies of D. galeata (grey area) in the Daphnia population of Greifensee from January 1998 to May 2005. The black dots show the frequencies of D. galeata in infected subsamples during the 2001, 2003 and 2004 epidemics. The prevalence of C. mesnili in the entire Daphnia population (data available since June 2002) is indicated with a dashed line. In the sub-figure, the annual mean frequencies (± s.e.) of D. galeata in infected (black dots) and non-infected (grey triangles) subsamples are shown. The significant interaction is visible. Data before July 2002 have been published in different form by Keller & Spaak (2004) and Wolinska et al. (2004).

Figure 2

Changes in (a) taxa and (b) clone frequencies in the non-infected (left) and infected (right) treatment of the competition experiment. Mean taxon/clone frequencies (±s.e.) are shown (grey area and solid lines, D. galeata; white area and dotted lines, D. hyalina). Clonal identities are given for those clones with relative frequencies (within-taxon) that differed significantly between treatments (see table 1).

Figure 3

Proportion (±s.e.) of infected individuals, (a) D. galeata, (b) hybrids and (c) D. hyalina, in the infection experiment, in relation to taxon, clone and exposure time to parasite spores. Parasites from the 2003 epidemic were used for this experiment. Clones that had carried an infection at the time of their isolation in 2002 are labelled with asterisks. The arrows indicate clones which were tested for a variation in their fitness reduction (see text).