Parasites are more likely to be haploid, while host organisms
are more likely to be diploid, according to results of a study
in Proceedings of the National Academy of Sciences USA this week
that seeks to explain the variety of ploidy found among
eukaryotes (Proc Natl Acad Sci USA 2004,
DOI:10.1073/pnas.0403151101). 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 us. 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,