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 on gene expression, 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 mostly asexually reproducing greater duckweed (Spirodela polyrhiza) perform across a favorable control environment and 5 urban pollutants (iron, salt, manganese, copper, and aluminum). By quantifying the population growth rate of asexually reproducing 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 hypothesis that polyploids are more tolerant of stressful conditions and can maintain fitness better than diploids across heterogeneous stresses. These results may help predict that polyploids may be likely to persist in stressful environments, such as those caused by urbanization and other human activities. Go big or go home: polyploidy increases tolerances to urban pollutants. Many organisms exist in heavily anthropogenically impacted habitats. A major, but common, genetic alteration known as polyploidy, which results from the complete doubling of an organism's genome, may allow these "polyploids" to better handle the stress of these polluted environments than their unaltered "diploid" parents. We provide a rare test of this idea across 5 different urban pollutants using duckweed (Spirodela polyrhiza), a small rapidly clonally reproducing aquatic plant. We show that polyploids grew as fast or faster than their diploid progenitors when exposed to 2 of the pollutants, considering actively growing propagules, and all 5 when considering dormant propagules. Polyploids can also maintain population sizes across the various pollutants, compared to benign conditions, better than the diploids. This supports the hypothesis that polyploids may perform better under polluted conditions, with important implications for their distribution in the face of environmental stressors, including those caused by human urbanization.