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Mechanical and chemical convergence of joints in three lineages of articulated coralline algae

University of British Columbia

Macroalgae living in wave-swept environments experience a high degree of mechanical stress. Flexible algae can mitigate this stress by bending with the waves, taking on smaller, more streamlined shapes that reduce drag. Coralline algae are limited in their flexibility due to having cell walls enriched in calcium carbonate, which cause thalli to be mostly rigid. While crustose species may avoid drag by growing prostrate along the substrate, most upright coralline species have uncalcified articulations (genicula) that allow them to retain flexibility despite their calcified constraint. Articulated corallines have evolved from crustose ancestors at least three separate times, leading to articulated species within the Corallinoideae, Lithophylloideae, and Metagoniolithoideae. The repeated evolution of genicula, and the rarity of upright coralline species without them, suggests that they play a key role in the ecological success of erect corallines. While previous studies have noted structural and developmental differences among genicula in the three evolutionary lineages, I address multiple levels of genicular organization to investigate the depth of convergent evolution of these structures. I found that genicular tissues are stronger and more extensible than other fleshy seaweed tissues, reflecting the fact that genicula must undergo a high degree of stress and strain to compensate for rigidity elsewhere in the algal thallus (Ch. 2). Differences exist between articulated clades; corallinoids are particularly strong, while lithophylloids are often highly extensible (Ch. 2). Articulated clades also differ in the way genicular morphology and tissue properties are adjusted to increase thallus flexibility; corallinoids possess a high number of genicula, metagoniolithoids possess long genicula, and lithophylloids possess particularly pliant genicular tissues (Ch. 3). Both the content and structure of polysaccharides in the genicular cell wall varies depending on subfamily, reflecting differences in genicular development and potentially causing differences in material properties (Ch. 4). Results from polarized microscopy suggest that the arrangement of polysaccharides within the cell wall also plays a role in how genicular tissue responds to mechanical stress (Ch. 5). In summary, while genicula may serve similar functions in corallinoids, lithophylloids, and metagoniolithoids, I show that there is more than one way to build an articulated coralline. [An errata to this thesis/dissertation was made available on 2022-01-11.]

Text

2018-08-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/66732

Botany

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/