Evolution of organization of the shoot apical meristem

Based on expected mutation rates in initial cells, it has been assumed that SAMs with plural initial cells probably have an advantage over SAMs with a single apical cell, because the increased number of initial cells can result in increased somatic mutation buffering when mutation rates are high (Klekowski, 1988). Phylogenetically, it is also assumed that SAMs with the single apical cell are primitive and SAMs with plural initial cells evolved from them, because the bryophyte sister to vascular plants commonly bears a single apical cell, though in the gametophyte generation (Mishler and Churchill, 1984; Kato and Imaichi, 1997). Bryophyte sporophytes have an unbranched axial body with no SAMs, but they have either an apical cell or no apical cell when young, depending on the class (Schuster, 1984; Crum, 2001). Although it remains an open question whether the bryophyte apical cell is homologous to that of pteridophytes (Friedman et al., 2004), it is possible that the sporophytes of ancestors of lycophytes and euphyllophytes (ferns and seed plants, Pryer et al., 2004) recruited the bryophyte apical cell (Kato and Akiyama, 2005). If this is the case, SAMs with plural initial cells evolved in lycophytes and seed plants independently in different clades (Figure 3.4).

Fossil evidence is quite limited, but the apex of the Devonian fossil vascular plant, Rhynia gwynn-vaughanii, appears to have several rectangular merophytes in place of a single apical cell (Edwards, 1994). This is not congruent with the general account that the apical cell based SAM is more primitive. Taking into account PD network traits along with the vascular plant phylogeny, an alternative evolutionary scenario seems equally plausible for SAM evolution - the apical cell based SAMs with LPD were derived from IPD SAMs with plural initial cells by reduction in initial cell numbers associated with loss of secondary PD networks (Figure 3.4). Ferns with LPD SAMs have only primary PD networks, which are formed in cell plates during cytokinesis, while seed-plant IPD SAMs can form secondary PD networks in addition to primary PD networks, the former of which are inserted into pre-existing walls when the walls expand during development (van der Schoot and Rinne, 1999). If secondary PD networks were lost, the need for all adjacent cells to communicate via their PD networks would exert a strong selection pressure for maintaining single apical cells in plants that only have primary PD networks (Cooke et al., 1996). It is difficult to distinguish secondary from primary PD networks, but the elegant work by Gunning (1978) demonstrated the absence of secondary PD networks in the fern genus Azolla. We must clarify whether PD networks can develop secondarily in pre-existing

Figure 3.5 Drawings of leaf lamina development of Lygodium japonicum (modified from Mueller, 1982a). (A)-(C) Leaf apical cell (la) and pinna initiation (between arrows), and pinna formation by marginal meristem. (D)-(F) Lobe formation of the first leaf. la, leaf apical cell; t, trichome; mm, marginal meristem. Scale bar 100 |im.

Figure 3.5 Drawings of leaf lamina development of Lygodium japonicum (modified from Mueller, 1982a). (A)-(C) Leaf apical cell (la) and pinna initiation (between arrows), and pinna formation by marginal meristem. (D)-(F) Lobe formation of the first leaf. la, leaf apical cell; t, trichome; mm, marginal meristem. Scale bar 100 |im.

walls in Lycopodiaceae and Isoetaceae with IPD SAMs. Furthermore, comparative analyses of bryophyte PD networks in a phylogenetic framework are crucial to clarify the evolution of various SAM structures.

3.3 Leaf

Megaphylls and microphylls are generally thought to have evolved independently in association with the evolution of stems. Megaphylls are simple or compound with branched vascular bundles and have a small parenchymatous portion (leaf gap) at the divergence point of the leaf trace from the stem bundle. Microphylls are scaly or needle-like with unbranched vascular bundles and no leaf gap. These two leaf types differ markedly in development, suggesting different origins and evolutionary pathways (Gifford and Foster, 1989).

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