Figure 7.

Schematic Illustration of Gene Expression Patterns, Comparison of Organ Identity, and Podostemaceae Shoot Evolution.

(A) and (B) Expression patterns of STM (yellow), WUS (blue), and ARP (red) genes in the shoot apex of ordinary angiosperms (A) and in the leaf/bract of H. japonicum (B).

(A) The STM gene is expressed in the SAM but not in leaf primordia (L). The WUS gene is expressed in the organization center (OC). The ARP gene is expressed in leaf primordia (L), complementary to the STM gene.

(B) At an early stage of leaf/bract development (I), Hj STM is expressed in the entire leaf/bract primordium and Hj WUS is expressed in the center. At a later stage (II) (not at maturity), Hj STM expression is restricted in the basal part of the leaf/bract primordium, and Hj PHAN is expressed in the distal part.

(C) The same colors indicate comparable organs that have the same genetic identity. The leaf/bract of Podostemoideae (H. japonicum and C. doianus) is comparable to the shoot of Tristichoideae (T. minor) and typical angiosperms. The proportion of green and brown areas is not to scale.

(D) The sympodium model (modified from Koi and Kato, 2007). Illustrations below the phylogenetic tree (Kita and Kato, 2001) show branching patterns in which brown and green bars indicate shoot and leaf, respectively. Arrowheads indicate the presence of SAMs. A chain of leaves in Podostemoideae is derived from the sympodial shoots of Tristichoideae and Weddellinoideae. On the extreme right side is the development of a SAM-less shoot complex. A new SSM arises from the base of an older SSM and differentiates into an apical leaf.