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Reversal of fortunes: Comparative transcriptomics of a key evolutionary shift in the life history of land plants.

Project Contact: Sean Graham, University of British Columbia

All land plants have a form of sexual reproduction known as sporic meiosis, in which there are alternating multicellular haploid and diploid phases (generations) with distinct plant bodies. Haploid-phase plants or gametophytes produce sperm and eggs, whereas the end products of diploid-phase plants (sporophytes) are the meiotic spores that germinate into gametophytes. The angiosperm gametophyte has become very reduced and is nutritionally dependent on (and effectively housed inside) the “parental” diploid plant. In mosses and relatives, in contrast, the gametophyte is the dominant generation, and it bears a nutritionally dependent sporophyte. Other land plants are somewhat intermediate, in that their gametophytes can be reduced but still free-living/photosynthetic. Viewed this way, flowering plants (sporophytes) are the culmination of a long evolutionary reduction series in the haploid generation.

Dominant sporophytes helped pave the way for the ecological dominance of vascular plants on the land. The switch to a highly reduced haploid phase of vascular plants was a landmark innovation in land-plant evolution, because it required major shifts in body plan, physiology, nutritional status and sexual reproduction. The proposed transcriptome sampling will help uncover the genes involved in this revolutionary life-history shift.

We propose to perform a comparative transcriptome analysis of the switch from dominant gametophytes to dominant sporophytes by sampling both haploid and diploid generations in a selection of plants chosen to closely “phylogenetically bracket” this major shift in life-history. They will represent fairly dense sampling of the deepest splits in vascular plants, in addition to their probable bryophyte sister group (the hornworts). Undergraduate students at UBC (and potentially at University of Alberta) will be involved in collecting material locally and in culturing vascular-plant gametophytes.

Proposed methodology

Taxon sampling The plants in which this key shift took place are all extinct. Fortunately we can sample living vascular plants and bryophytes that bracket the shift. Our taxon sampling for bryophytes will focus on the probable sister group of vascular plants (hornworts). For vascular plants we will include various members of two major clades (lycophytes and monilophytes, the latter is the large clade comprising ferns and relatives) that have retained photosynthetic, independently living gametophytes. This latter criterion is important for sampling of vascular plants because:

(1) Photosynthetic vascular-plant gametophytes are generally easier and faster to grow in culture than those that are heterotrophic (i.e., dependent on fungi or maternal tissue for nutrition);

(2) Photosynthetic vascular plant gametophytes are likely to be closer in form to those that existed just before the shift to dominant sporophytes, as they need to retain all the machinery for independent existence.

Tissue sampling and analysis In all cases we will sample both sporophyte and gametophyte tissue from species that represent diverse lineages. This will permit pairwise comparisons of transcriptome profiles both within species (i.e., between the two ploidy or generational phases) and between major lineages (e.g., of vascular plants to outgroups). A major goal is to identify overlapping or unique sets of genes on either side of the vascular plant/hornwort split, to provide insights into the sets of genes that became reduced or diversify after this. This will allow us to address what is needed for nutritionally independent gametophyte existence in all land plants, and for independent sporophyte existence. Examples of the latter include the lignified vascular tissue (xylem) and branching in sporophytes (diploid plants), found exclusively in vascular plants. The inclusion of diverse lineages from both sides of this split will therefore allow us to focus on the core genes involved in the shift of dominant generations.