Dissertation Research: Chloroplast Evolution of the Nonphotosynthetic Liverwort Cryptothallus Mirabilis (Aneuraceae)

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AbstractWithin bryophytes (mosses, liverworts, and hornworts), fungal associations are common only in hornworts and liverworts; however, only among liverworts is there a species that relies entirely on its fungal associate, rather than on photosynthesis, for fixed carbon. This dependence on an alternative carbon source is present only in one genus (two species) of liverworts: Cryptothallus (Aneuraceae). Cryptothallus mirabilis is non-photosynthetic, lacks the green pigment chlorophyll, and is therefore whitish yellow in appearance. Carbon is taken up from the fungus, which penetrates the intercellular spaces of the liverwort. The fungus, Tulasnella, satisfies its carbon demand through and ectomycorrhizal association with pine or birch trees. Among flowering plants, the evolution of a non-photosynthetic life strategy is accompanied by a reduction in the chloroplast genome. This reduction results in the loss (physically, or loss of function) of genes that code for enzymes involved in the reactions of photosynthesis. This award will support the dissertation research of a graduate student of Dr. Bernard Goffinet, Norman Wickett, investigating whether the patterns of gene loss observed in non-photosynthetic angiosperms are also present in non-flowering land plants, particularly bryophytes. To accomplish this, the complete chloroplast genome of C. mirabilis will be sequenced, as well as that of a closely related photosynthetic relative. Phylogenetic reconstruction will be used to identify the most closely related photosynthetic liverwort and to test hypotheses of genomic evolution. Comparisons can then be made between the two chloroplast genomes in order to identify changes associated with the loss of photosynthesis and to test hypotheses of genomic evolution. Chloroplast genome sequences will be obtained using a modified shotgun sequencing method developed for non-photosynthetic flowering plants.This study integrates genomics and phylogenetics in order to assess patterns of chloroplast genome evolution in a non-photosynthetic bryophyte. Much of the current understanding in this area arises from studies of flowering plants; this study, therefore, will significantly broaden the taxonomic diversity of studies addressing chloroplast genome evolution accompanying the loss of one of the most fundamental processes of life: photosynthesis. The results of this study will be made available to the public and the scientific community. In the latter case, the results will be particularly informative to current studies addressing the evolution of land plants in general. The unique natural history of this study, and the fundamental nature of its focal questions (genomic evolution when major selection forces are lost), should facilitate the dissemination of findings through local outreach activities and publications in popular journals.