Developmental transitions during the evolution of plant form

jane a. langdale and c. jill harrison the invasion of land

Land plants evolved from aquatic algal ancestors. The algae are a poly-phyletic group from which the transition to land, and acquisition of developmental features associated with land plants, have occurred many times (Lewis and McCourt 2004). Recent phylogenetic evidence points to the charophyte algal lineage as the sister group to the land plants (Figure 16.1). Developmental features shared by charophytes and land plants are cell cleavage by phragmoplasts, plasmodesmatal connections between cells, and a placental link between haploid and diploid phases of growth (Marchant and Pickett-Heaps 1973). These and other features of derived charophytes, in particular growth from an apical cell in the gametophyte (Graham 1996, Graham et al. 2000), suggest that many of the cellular characteristics required for the development of land plants may have evolved within their common stem group.

from haploid to diploid

The major character that distinguishes land plants from charophyte algae is the development of a diploid embryo. In charophytes, the majority of the life cycle is represented by the haploid gametophyte and only the unicellular zygote is diploid, undergoing meiosis immediately after formation. Embryo development represents a major growth transition in that meiotic division of the zygote is delayed and diploid sporophytic cells divide by mitosis, giving rise to a multicellular body. Although in the earliest land plants the gametophyte generation

Evolving Pathways: Key Themes in Evolutionary Developmental Biology, ed. Alessandro Minelli and Giuseppe Fusco. Published by Cambridge University Press. # Cambridge University Press 2008.

Figure 16.1 Phylogenetic relationships between extant land plants. The diagram has been compiled from phylogenies aimed at resolving particular nodes of the plant tree (Pryer et al. 2001, Lewis and McCourt 2004), although there is still conflict between topologies retrieved by different researchers. Angiosperm species mentioned in the text are Arabidopsis thaliana, Populus tremuloides, Zea mays, Antirrhinum majus, Nicotiana sylvestris and Pisum sativum.

Figure 16.1 Phylogenetic relationships between extant land plants. The diagram has been compiled from phylogenies aimed at resolving particular nodes of the plant tree (Pryer et al. 2001, Lewis and McCourt 2004), although there is still conflict between topologies retrieved by different researchers. Angiosperm species mentioned in the text are Arabidopsis thaliana, Populus tremuloides, Zea mays, Antirrhinum majus, Nicotiana sylvestris and Pisum sativum.

remained dominant, this transition in growth pattern led to a dramatic change in life history such that the sporophyte generation gradually became the dominant form (Graham 1985, Kenrick 1994, Graham and Wilcox 2000). Consequently, the dominant part of the life cycle became diploid and thus was better protected against deleterious mutations.

The transition from growth in water to growth on land required innovations that aided reproductive success in a new, drier environment (see Figure 16.2 for a schematic sequence of character acquisitions). Most land plants have waxy cuticles enclosing vegetative tissue and sporopollenin-coated spores, both of which protect against desiccation. Cuticle formation subsequently necessitated the development of stomata to permit gaseous exchange with the environment. Cuticle formation, and the re-localisation of sporopollenin deposition from the zygote in charophytes to the spores in land plants, are shared ancestral character states (plesiomorphies) of land plants. Possession of stomata may also be a land-plant plesiomorphy but because liverworts do not possess true stomata, this scenario would imply a loss in this group. The alternative explanation is that stomatal formation is homoplastic.

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