Plant evolution: a new model of plant architecture based on primitive land plants
A project undertaken at the School of Botany, The University of Melbourne and supervised by Andrew Drinnan
Plants are modular organisms. Most studies of plant modularity have centred on flowering plant morphology and the production of repeated units from the apical meristem. While this is an adequate basis for conceptual plant morphology, it has little explanative power as to how plants became modular in an evolutionary sense; i.e., what is the basic module, and how did plants evolve from a unitary to a modular condition. The answer to this question lies with the structure of lower plants such as bryophytes, fern allies and ferns, for it is this level of land plant evolution at which occurred the transition from a single-celled diploid generation (zygote) in the charaphyte green algae to the complex branched sporophytes of vascular plants. Surprisingly, the embryology and early sporophyte development at this level of plant evolution is poorly known and then only from old literature. Bierhorst (1971), referring to this older literature, stated that “ Not a single study … is sufficiently documented”; little progress has been made since. This information is critical to understanding the evolution of plant modularity, and hence to a comprehensive model of plant construction and architecture.
Recent molecular phylogenetic analyses have resolved extant vascular plants as three major lineages: lycophytes, ferns and their allies, and seed plants. Lycopsids have, for some time, been accepted as the closest relatives (sister group) to the rest of vascular plants, and the monophyly of seed plants is not controversial. However, the pairing of the leafless and rootless Psilotales with Ophioglossales and their inclusion with horsetails and ferns to form the third lineage (moniliforms) was unexpected. The difficulty in moving forward from this phylogeny stems from the lack of a cohesive model of sporophyte construction for lower land plants that allows comparison of the different taxa; e.g., we can’t easliy compare the dichotomous aerial shoots of Psilotum with the “fronds” of Ophioglossum, or these taxa with other ferns. This is a fundamental problem not only for these two taxa, but for all lower land plants from bryophytes through to ferns. We do not understand the structural equivalences between ferns and lycopsids, nor how spropophyte complexity and branching proceeded from bryophytes to tracheophytes. Friedman et a.l (2004), in a recent review on the burgeoning field of evolutionary developmental biology (evo-devo) and its application in land plants, highlights the need for advances in three critical areas, namely molecular developmental genetics, phylogeny, and comparative morphology. Three of the four pressing issues that they highlight as needing urgent address are the development of branching, the homology of roots, and the evolution of leaves, and the plant groups they use to illustrate the present state of uncertainty are lycophytes, psilophytes and ferns. In specifying these three characters and these plant groups they are essentially conceding that there is no satisfactory model of plant morphology and architecture at the base of land plants, and that this deficiency, together with phylogenetic uncertainly, is retarding progress in evolutionary plant biology.
The ultimate objective of this proposal is a general model of sporophyte structure and evolution that is applicable at the lower land plant level, and which permits a comparison of structure and architecture between groups. This will allow a more meaningful comparison of the seemingly bizarre morphologies exhibited by these plants, and provide a solid framework for comparative developmental genetic studies. It will also generate new and properly documented information on the embryology and early sporophyte development in a range of key lower land plant taxa in a consistent and comparable manner, using a new set of microscopic techniques. This will fill an important gap in current knowledge of land plant development and morphology, and describe this process for the first time in a number of Australasian plant species.
I propose to focus in the first instance on lydophytes and ferns, for two reasons. Firstly, they are relatively easy to grow from spores though the gametophyte to the sporophyte stage, so getting a good series of early developmental stages is straighforwad (some cultivation is already underway). Secondly, of the lower plant groups they are the most studied, and there are some intriguing observations of mature structure for which the developmental basis is unknown but crucial. For example, ferns with large apices (e.g. treeferns) have a very different pattern of leaf initiation (and vasculature and root initiation) than those with smaller apices, and the two have been considered fundamentally different (Hebant-Mauri 1993). We need to know the underlying developmental basis.
The main methods will be careful dissection of specimens, and observationby scanning electron Microscopy, confocal laser microscopy, and plasic embedding and microtome sectioning for light microscopy.