A Question of Ploidy

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Science  06 Aug 2004:
Vol. 305, Issue 5685, pp. 754-755
DOI: 10.1126/science.305.5685.754e

Eukaryotes are classically considered to be diploid, apart from a brief flirtation with haploidy during reproduction. In fact, variety in ploidy is more likely to be the rule among eukaryotes. Polyploidy is especially common in plants, where it can have spectacular morphological results; some cancerous states are linked with changes in somatic cell ploidy; and some species of sturgeon can even exhibit octoploidy.

Nuismer and Otto contend that selection favors diploidy in host species and haploidy in parasitic species when there is a single ploidy locus. The reason is that parasites that express a narrow array of antigens or elicitors are more successful at evading the host immune system, whereas hosts with a wide variety of recognition molecules are more apt to catch invading parasites. In this scenario, alleles that increase host resistance tend to be dominant, and those that enhance parasite virulence tend to be recessive. A survey of many thousands of heterotrophic protests showed that those with parasitic lifestyles were three to four times more likely to be haploid than diploid. A striking example is the diploid parasite Trypanosoma brucei, which sequesters its large family of variant surface glycoprotein genes within haploid regions of its genome. — CA

Proc. Natl. Acad. Sci. U.S.A. 101, 11036 (2004).

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