Polyploidy, the Nucleotype, and Novelty: The Impact of Genome Doubling on the Biology of the Cell

Polyploidy is a key mechanism of genome evolution and speciation, particularly in plants. Many aspects of polyploidy have been elucidated with the tools that have become available during the molecular genetics and genomics revolution. Nevertheless, significant questions remain about how genome doubling per se, in the absence of hybridization, is capable of generating evolutionary novelty. This is particularly true at the cellular level, where since the discovery of polyploidy it has been assumed that increased cell size plays a key role in physiological and developmental changes associated with genome doubling, usually through changing the surface-to-volume ratio. Cell size, nuclear volume, and cell cycle duration have been hypothesized to be among a suite of nucleotypic characters, defined as phenotypic traits influenced by bulk DNA amount, irrespective of genic content. We update this old but still relevant concept, focusing on what current knowledge from cell biology can elucidate about how quantitative differences between diploids and isogenic autopolyploids could lead to phenotypic differences. Much remains to be learned before causality and correlation can be distinguished in the complex network of interactions among the cytoplasm, the nucleus and other organelles, the cell cycle, metabolism, and intra- and intercellular transport. It is clear that many effects of polyploidy are likely to be cell type specific and are conditioned by the different genomic architectures of species in which genome doubling occurs. The rapidly developing ability to study key processes such as transcription and the movement of molecules in single cells will enable experiments capable of addressing fundamental questions about potentially nucleotypic aspects of genome doubling and how these interact with genotypes to produce phenotypic novelty.