Fleshy fruits play a vital role in the lifecycles of many plant species by attracting animal dispersers who carry the seeds away from the parent plant. Fruit color is particularly important as a signal to animals that the fruits are ripe and ready to be dispersed. The primary explanation for fruit color diversity—the disperser syndrome hypothesis—posits that fruits should fall into a "bird syndrome" (small fruits with bright colors such as red, black, blue) and a "mammal syndrome" (large fruits with dull colors such as green, brown, or yellow). Despite the importance of fleshy fruits, our knowledge of how fruit colors vary across space and how they evolve is limited. This dissertation, comprised of four chapters, explores global spatial patterning in fruit colors and investigates how fruit colors have evolved in a single clade, Viburnum.
Chapter 1 takes a global perspective to understanding spatial variation in fruit color and size. Previous work had suggested that fruit colors are spatially heterogeneous and that color and size are correlated, but whether there are coherent global patterns has remained unclear. I find that tropical fruiting assemblages are highly diverse in both color and size, and high latitude assemblages are dominated by red-fruited species of generally small to intermediate size. These results are broadly congruent with the disperser syndrome hypothesis, although they raise questions about additional factors that might underlie variation within the bird or mammal syndrome.
Chapter 2 explores the factors that underlie the differences between bird and mammal colors by testing alternative, abiotic hypotheses against the disperser syndrome hypothesis. I find that while dispersers are important correlates of fruit color, the interaction between dispersers and temperature is critical in accounting for global patterns. Mammal colors are only common both when mammalian frugivory is important and when temperatures are relatively warm; otherwise, bird colors dominate. These results suggest that additional variables are needed to explain fruit color variation aside from dispersers alone, and future work should more directly consider these alternatives.
Chapter 3 examines the anatomical underpinnings of blue fruit color in Viburnum. Although blue-fruited Viburnum species do have anthocyanin pigments, they also use a physical mechanism — structural color — to produce their peculiar blue hue. I identify two origins of blue fruits in Viburnum and find that in both cases small lipid droplets are embedded in their cell walls, and these interfere with light to produce color. In one origin, these lipid droplets are arranged into a multilayer reflector that produces an intense blue color. In the second origin, the lipid droplets are only quasi-ordered and as a consequence the color is only weakly blue. Outside of Viburnum, only four origins of structurally colored fruits are known to date, and none are known to embed lipid droplets in the cell wall.
Chapter 4 considers the broader context of fruit color evolution in Viburnum. The blue-fruited Viburnum species also have high nutritional lipid content, and this correlation suggests the possibility of honest signaling and that there may be similar correlations between nutritional content and other fruit colors in Viburnum. I found that different fruit colors in Viburnum do indeed correspond with several additional traits. Blue fruits have high lipid content, low moisture content, a small amount of pulp, and a large, round endocarp. These fruits exhibit a high value (energy dense lipids), high cost (large endocarp to be carried by dispersers) strategy, signaled by the blue fruit color. In stark contrast, red fruits have very watery fruits with low lipid content and a smaller, flattened endocarp. These fruits exhibit a low value, low cost strategy. Overall, Viburnum fruits exhibit syndromes of traits involving color, nutritional content, and morphology. These syndromes differ from the classic bird and mammal syndromes and they occur entirely within a clade of bird-dispersed plants.
Together, these studies suggest that fruit color varies not only along the axis of bird and mammal syndromes, but that a wide variety of alternative factors also influence fruit color evolution. Historically, much of the focus in understanding fruit color has been on the disperser syndrome hypothesis and the degree to which it can adequately explain fruit color variation. The results presented here suggest that greater attention should be given to alternative hypotheses, and that an understanding of how fruit colors evolve within individual clades will ultimately be necessary to explain patterns at community and global scales. Although structural color in fruits is likely to be rare, fruit syndromes such as those in Viburnum may be far more widespread than previously appreciated.
|Advisor:||Donoghue, Michael J|
|Commitee:||Jetz, Walter, Prum, Richard, Jordano, Pedro|
|Department:||Ecology and Evolutionary Biology|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 81/10(E), Dissertation Abstracts International|
|Subjects:||Evolution and Development, Ecology|
|Keywords:||Fruit color, Fruits, Latitudinal gradients, Seed dispersal, Structural color, Viburnum|
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