Evolutionary Developmental Biology (Evo-Devo)
Graham, Linda , Cardona-Correa, Christopher .
Experimental evidence that the direction of illumination controls gametophytic body orientation in modern bryophytes and that this process was inherited from ancestral algae.
The environmental influences most important to early land plants and their modern bryophyte descendants in maintaining normal dorsiventral or axial gametophytic body orientation are unknown. Nor is it known whether such controls originated on land or were inherited from aquatic algal ancestors. To answer these questions, we devised an experimental system that allowed us to concurrently (thereby reducing between-experiment experimental error) evaluate growth responses to: 1) direction of illumination by cool white fluorescent lamps at a consistent irradiance level, 2) gravity, and 3) surface touch. We used this system to serially examine physical effects on body orientation in several modern early-divergent, cultured streptophytes standing as proxies for ancestral aquatic algae and early bryophyte-like plants having dorsiventral (prostrate) or axial gametophytic body orientation. The green alga Coleochaete orbicularis and green gametophytes of the liverworts Blasia pusilla and Marchantia polymorpha, the moss Sphagnum compactum, and the fern Ceratopteris richardii were studied. We assessed responses by making replicate observations of gametophytic body orientation in replicate cultures of each species after a period of exposure to control (illuminated from above) and experimental conditions (illuminated from below). In all cases we observed that light direction played a more important role in gametophytic body orientation than gravity or surface touch. Grown in aqueous media, C. orbicularis displayed normal, attached, monostromatic, dorsiventral morphology when lit from above; when illuminated from below this alga occurred as suspended irregular clumps lacking distinctive hairs. When lit from below, new growth of dorsiventral plant gametophytes on agarized media in petri plates twisted so that ventral surfaces bearing rhizoids faced the overlying air space. Axial gametophytes of Sphagnum moss exposed to light only from below burrowed into an agar substrate rather than growing into an overlying air space. Although negative phototropism of streptophyte rhizoids (which we also observed) has previously been demonstrated, ours is the first analysis of physical factors most likely to impact whole gametophytic body orientation in a range of early-diverging streptophytes. Based on these results, we conjecture that: 1) early land plants inherited light-directed gametophytic body orientation from ancestral algae, 2) directional light control of land plant gametophytic body orientation originated prior to gravity-induced effects, 3) directional light allowed early gametophyte-dominant land plants growing on unstable substrata to reorient themselves after physical disturbance, and 4) directional light is key to the process by which unicellular spores generate multicellular, dorsiventral or axial gametophytic bodies of modern bryophytes.
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1 - University Of Wisconsin, Botany Dept, 211 Birge Hall, 430 Lincoln Drive, Madison, WI, 53706, USA
2 - University of Wisconsin, Botany, 430 Lincoln Drive, Madison, WI, 53706, USA
bryophyte gametophyte orientation.
Presentation Type: Poster:Posters for Topics
Location: Battelle South/Convention Center
Date: Monday, July 9th, 2012
Time: 5:30 PM