Leaf form varies widely across angiosperms and may be broadly classified as simple or complex. Phylogenetic analysis of this variation reveals that 1) The ancestral angiosperm had simple leaves, supporting palaeobotanical evidence; 2) There were multiple evolutionary ‘gains’ of complex leaves: not all complex leave are homologous; and 3) There were multiple evolutionary ‘losses’ of simple leaves: not all simple leaves are homologous. A comparison of transition rates suggests that ‘losses’ tend to occur more than ‘gains.’ This preferential origin of simple over complex leaves indicates the prevalence of either evolutionary pressures or developmental constraints, or both. Variation in expression of Knox1 genes was used to examine the developmental basis of this evolutionary variation in morphology. Knox1 (Knotted-like homeobox) genes are expressed in shoot apical meristems and play an important role in maintenance of indeterminate growth. The genes are down regulated in incipient leaf primordia (Po) and stay down regulated in later leaf stages of simple-leaved model species; they are transiently unregulated in later leaf stages of the complex-leaved tomato. The latter mode of KNOX1 protein expression characterizes independently evolved complex leaves across angiosperms. On the other hand, at least two modes of KNOX1 expression characterize developing simple leaves. These results suggest that evolutionary modulation of the Knox1 genetic cascade is involved in multiple evolutionary transitions in leaf form, and that the genetic cascade does not pose a developmental barrier to these morphological changes.

Key words: angiosperm, development, Evolution, Knox expression, leaf