Phylogenetic trees
We constructed the plant phylogenetic tree using an updated version of the mega-phylogeny published by (Zanne et al. 2014) as the backbone. For those genera and species in our dataset that were absent from the meta-phylogeny, we used S.PhyloMaker (available at https://github.com/jinyizju/) to add them to their respective families (in the case of genera) and genera (in the case of species) in the mega-phylogeny under Scenario 3 (Qian & Jin 2016). The polytomies were resolved by the multi2di function in the ape package in R. Finally, our phylogenetic tree had 161 species (following APG IV, Fig. S1). We estimated phylogenetic signals for genome size by employing Blomberg’s K and Pagel’s λ tests, with the significance being estimated with randomization and likelihood ratio tests in the package phylosignal in R (Keck et al. 2016) (Fig. S1).