Phosphorus use and excretion varies with ploidy level in Daphnia

Abstract

Ploidy affects the amount of DNA per cell. Because phosphorus (P) is required for building DNA, polyploids should have higher P content and be more sensitive to dietary P limitation. Such differences could be due to some combination of differential incorporation and excretion of P. We tested these predictions using diploids and polyploids of the zooplankter Daphnia pulex. After manipulating the P content of natural seston, we fed it to diploids and polyploids and quantified growth and %P. Although neither growth nor %P differed significantly with ploidy, polyploids tended to grow faster on P-spiked seston. Laboratory experiments on four genotypes using Scenedesmus algae cultured under low (LoP) and high (HiP) phosphorus supplies revealed significant P treatment by ploidy interactions in both growth and %P, with polyploids growing faster in HiP, and containing more P in LoP. Radiotracer assays revealed that polyploids incorporated significantly more 33P and excreted significantly less 33P compared with diploids. Ploidy-specific P use could interact with environmental P supply to affect the fitness of genotypes differing in ploidy level. Further, these results indicate unique ecological functions (i.e. P recycling) of lineages differing in ploidy level that should be generally applicable to taxa that have undergone whole-genome duplications.