Kirsten Fisher
Dissertation Summary:
Heterochronic changes in relative timing and/or rates of development in a descendant relative to its ancestor serve as a theoretical background for explaining morphological diversification. Although the seminal works on heterochrony were written with animal systems in mind, and research in the last decades has mainly emphasized animal examples, the same principles can be applied to the study of plant development. The effects of heterochrony are expressed differently in plant evolution due to fundamental differences in plant and animal development, and while some patterns of development may be comparable in plants and animals, the modularity and heirarchical organization of plants add another level of complexity to their development that prohibits simple extrapolation from animal examples. Comparative developmental studies on plants are needed to improve our understanding of how heterochronic changes at the different levels of plant ontogeny affect their morphological diversification.
My thesis research will focus on a group of Paleotropical mosses, the Syrrhopodon involutus complex. Members of this complex are united by an unusual leaf morphology, in which more than half of the leaf area is occupied by dead, empty cells. The general leaf morphology which unites the taxa of this complex appears early in the ontogeny of many mosses, suggesting that the S. involutus complex may be an excellent system for understanding the relationship between heterochronic changes and morphological diversification.
The overall goal of my dissertation is to combine phylogenetic, biogeographic, and ontogenetic data in order to understand how changes in ontogeny relate to patterns of diversification. Comparative studies will take place at two basic phylogenetic levels:
1. a higher-level phylogeny which establishes the relationship of the S. involutus complex to its sister taxon within the genus Syrrhopodon, based on morphological characters and DNA sequence data from the chloroplast gene rps4. This phylogeny will provide the basis upon which to investigate the hypothesis that the unusual leaf morphology which characterizes the S. involutus complex is the product of a heterochronic change in the ontogeny of the group.
2. a lower-level phylogeny of the S.involutus complex with taxa sampled from throughout the geographic range of the group, based on morphological characters and sequence data from the chloroplast (rps4) and nuclear (GPD) genomes. This phylogeographic information can then provide a context for a comparative study of the heteroblastic series within the S. involutus complex, to determine if there is a trend towards increased paedomorphosis in taxa which occur farther into the eastern end of the group’s range (e.g. across South Pacific islands).
Other Projects:
Plants provide excellent opportunities for ontogenetic studies, as they are iterative, modular organisms. On any given plant shoot, an apical meristematic region moves through space as it divides, leaving behind progressively older derivative tissues. In this way, the indeterminate primary meristem produces a series of determinate modules (e.g. leaves, bracts, petals) that are iterated along the main axis. If the final morphology of later modules is different than that of earlier ones, the series is termed heteroblastic.
Mosses are particularly suited for developmental studies because their organization is inherently hierarchical, owing to the derivation of all their organs and tissues from a single initial cell (as opposed to a meristematic region of cells in seed plants). In acrocarpous mosses, a new branch module is initiated sympodially following the production of archegonia, and the heteroblastic series is reiterated along each new branch, providing an investigator with numerous replicates. Earlier studies have indicated that this heteroblastic ontogenetic sequence is highly conserved in the mosses . These studies have determined that this high level of conservation allows for the direct comparison of ontogeny between taxa, even those that are phylogenetically disparate.
REFERENCES CITED:
Allsop, A. 1967. Heteroblastic development in vascular plants. Advances in Morphogenesis 6: 127-171.
Goebel, K. 1898. Organography of Plants, Especially of the Archegoniatae and Spermophyta. Oxford University Press, UK.
Mishler, B. D., and E. De Luna. 1991. The Use of Ontogenetic Data in Phylogenetic Analyses of Mosses. Pages 121-167 in N. G. Miller, ed. Advances in Bryology, Vol. 4. Bryophyte Systematics. J. Cramer in Der Gebrueder Borntraeger Verlagsbuchhandlung, Stuttgart, Germany;