Genetic basis for leaf form diversity
High-throughput sequencing technologies have revolutionized biology but the identification of causal genetic changes underpinning the diversity of biological forms remains a significant challenge. This is particularly relevant where classical approaches based on genetic crosses are difficult to use to link genotypic and phenotypic variation. Leaves of seed plants offer attractive prospects to understand the molecular basis of morphological change because they exhibit substantial heritable variation. Comparisons between the reference plant Arabidopsis thaliana, which bears simple leaves and its complex-leaved relative Cardamine hirsuta have led to considerable insight on how leaf form develops and diversifies. To gain further insight into the mechanisms that gave rise to different leaf shapes in these two species, we compared their gene expression profiles in developing leaves. We identified transcription factors expressed at higher levels in Cardamine than in Arabidopsis, and expressed them in Arabidopsis to test whether they are sufficient to generate leaf complexity in simple leaves. We also reduced their expression in Cardamine to assess whether they are necessary for compound leaf development. Our results reveal a skewed distribution of effect sizes, with a handful of differentially expressed transcription factors showing a disproportionate ability to alter leaf shape in the interspecies gene transfer assays. Our findings suggest that change in leaf morphology between species occurs via a limited number of evolutionary paths, influenced by a defined set of major-effect molecular players. We extend this approach at the family level to study how the balance of conservation and divergence of the pathways regulating growth and form generated the tremendous morphological diversity in nature.