Neopolyploidy increases stress tolerance and reduces fitness plasticity across multiple urban pollutants: support for the 'general purpose' genotype hypothesis.

Whole genome duplication is a common macromutation with extensive impacts from gene expression, to cellular function, and whole organism phenotype. As a result, it has been proposed that polyploids have general purpose genotypes that perform better than their diploid progenitors under stressful conditions. Here we test this hypothesis in the context of stresses presented by anthropogenic pollutants. Specifically, we tested how multiple neotetraploid genetic lineages of the Greater Duckweed (Spirodela polyrhiza) perform across a favorable control environment and five urban pollutants (iron, salt, manganese, copper, and aluminum). By quantifying the population growth rate of duckweed over multiple generations we found that across most pollutants, but not all, polyploidy decreased the growth rate of actively growing propagules but increased that of dormant ones. Yet, when considering total propagule production, polyploidy increased tolerance to most pollutants and polyploids maintained population-level fitness across pollutants better than diploids. Furthermore, broad-sense genetic correlations in growth rate among pollutants were all positive in neopolyploids but not so for diploids. Our results provide a rare test and support for the hypotheses that polyploids are more tolerant of stressful conditions and can maintain fitness better than diploids across heterogenous stresses. These results may help predict the distribution of polyploids across stress gradients such as those caused by urbanization and other human activities.