Evol Ecol Res 2: 1031-1043 (2000)     Full PDF if your library subscribes.

Parasite heterogeneity affects infection success and the occurrence of within-host competition: An experimental study with a cestode

Claus Wedekind and Andreas Rüetschi

Abteilung Verhaltensökologie, Zoologisches Institut, Universität Bern, CH-3032 Hinterkappelen, Switzerland

Address all correspondence to Claus Wedekind, Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK.
e-mail: c.wedekind@ed.ac.uk


It has been suggested that a main benefit of sex is genetic heterogeneity among offspring, so as to be better prepared for unpredictable interactions faced in the next generation. This may be especially important in the co-evolutionary arms race between parasites and their hosts. The aim of this study was to determine whether parasite heterogeneity influences transmission and growth of the cestode Schistocephalus solidus in its copepod host. S. solidus can either reproduce by selfing or by outcrossing, producing different degrees of genetic heterogeneity in the offspring. It is, however, not sufficient to compare selfed with outcrossed parasites, because these two types may also vary, for example in their load of genetic damage caused by inbreeding depression. We therefore used 10 parasite sibships to infect copepods, each with six larvae, that came from either a single clutch (‘pure’ exposure; i.e. low heterogeneity) or a mixture of two clutches (‘mixed’ exposure; i.e. high heterogeneity). We found that infection was more likely in mixed exposures (increase in prevalence >50%). We also found that multiple infections were more common in pure exposures. Parasite transmission rates were therefore only slightly increased in mixed exposures (22%). Since parasite growth was reduced in multiple infections, parasites from mixed exposures were on average more than 50% larger than those from pure exposures at a time when they were infectious to the next intermediate host. One day after exposure, we distributed the copepods between two different housing conditions, and let the parasites grow within the hosts for 2 weeks. These two conditions resulted in different parasite growth rates and different host mortalities. However, the conditions did not have a significant influence on the prevalence and the parasite transmission rates either in the pure or the mixed exposures. Sexual reproduction in this parasite appears to be advantageous because heterogeneity increases the range of hosts that can be infected, while it decreases the occurrence of within-host competition. The parasite–host interaction that determines how many worms reach their infectious stage appears to take place during the very early stages of infection.

Keywords: antagonistic co-evolution, copepod, frequency-dependent selection, lottery model, Schistocephalus solidus.

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