Abstract
Why is sex ubiquitous when asexual reproduction is much less costly? Sex disrupts coadapted gene complexes; it also causes costs associated with mate finding and the production of males who do not themselves bear offspring. Theory predicts parasites select for host sex, because genetically variable offspring can escape infection from parasites adapted to infect the previous generations. We examine this using a facultative sexual crustacean, Daphnia magna, and its sterilizing bacterial parasite, Pasteuria ramosa. We obtained sexually and asexually produced offspring from wild-caught hosts and exposed them to contemporary parasites or parasites isolated from the same population one year later. We found rapid parasite adaptation to replicate within asexual but not sexual offspring. Moreover, sexually produced offspring were twice as resistant to infection as asexuals when exposed to parasites that had coevolved alongside their parents (i.e. the year two parasite). This fulfils the requirement that the benefits of sex must be both large and rapid for sex to be favoured by selection.
Highlights
One of the great paradoxes of biology is that sex is the dominant mode of reproduction when asexual reproduction is much less costly [1,2,3]
Chief among them is the Red Queen hypothesis, which states that parasite-mediated selection is strongest against common contemporary host genotypes [7,8,9,10]
Studies that directly test the effect of reproductive mode on host resistance to parasitism by comparing the sexual and asexual offspring of a single parent are very rare, and are mainly limited to plant2parasite systems
Summary
One of the great paradoxes of biology is that sex is the dominant mode of reproduction when asexual reproduction is much less costly [1,2,3]. A productive method for testing whether parasitism favours sex over asex involves comparing parasite resistance of obligately asexual and sexual host lineages of a particular host species [16,17,18] Such studies have effectively demonstrated that obligate sexual lineages can outcompete obligate asexual lineages in the face of parasitism. We recorded the two principal infection characteristics: proportion of hosts infected and Pasteuria transmission spore density per infected host By analysing both the overall trends and underlying family-level genetic correlations, we were able to add to previous work examining the relative fitness of sexually and asexually produced offspring [19,23]. Our findings demonstrate that the parasite population evolved rapidly in the field, whereas our laboratory experiments indicate temporal changes in parasite-mediated selection on host genotypes, favouring host sex over asex
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