Endopolyploidy (increased cell ploidy) occurs during normal development in many eukaryotes. In higher plants, endopolyploidy is usually the result of endoreduplication – endonuclear DNA replication that produces chromosomes with multivalent chromatids. According to the ‘karyoplasmic ratio’ theory, a cell's cytoplasmic volume is proportional to its nuclear DNA content. On p. 3817, Christian Chevalier and co-workers test this theory by analysing the structure of endoreduplicated nuclei in tomato fruit, which reach very high ploidy levels during their development. The researchers show that endopolyploidy in tomato pericarp (the fleshy part of the fruit) leads to the formation of polytene chromosomes. Pericarp nuclei, they report, have a complex structure in which numerous deep grooves are filled with mitochondria and in which there is a fairly constant ratio between nuclear surface area and the nuclear volume. Finally, they provide the first direct evidence that endoreduplication triggers enhanced transcription. Together, these results support the karyoplasmic theory and suggest that endoreduplication is associated with the complex cellular organisation that is required for tomato fruit development.