Parasites are more likely to be haploid, while host organisms are more likely to be diploid, according to results of a study in PNAS this week that seeks to explain the variety of ploidy found among eukaryotes.

Scott L. Nuismer and Sarah P. Otto, at the Universities of Idaho and British Columbia, respectively, integrated host–parasite coevolution into existing models of lifecycle evolution in order to generate testable predictions about whether a particular eukaryote should be haploid or diploid. Preexisting models focused only on the effects of mutation on an organism, but could otherwise make no predictions on levels of ploidy, according to the authors.

"It makes perfect sense in hindsight," Otto told The Scientist. Host–parasite interaction is selecting for a host that can recognize as many pathogens as possible, making it better able to clear itself of infection. On the other hand, in many types of interaction, a...

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