Linking Molecular Mechanisms and Evolutionary Consequences of Resource Polyphenism

Abstract

Resource polyphenism-the occurrence of environmentally induced, discrete, and intraspecific morphs showing differential niche use-is taxonomically widespread and fundamental to the evolution of ecological function where it has arisen. Despite longstanding appreciation for the ecological and evolutionary significance of resource polyphenism, only recently have its proximate mechanisms begun to be uncovered. Polyphenism switches, especially those influencing and influenced by trophic interactions, offer a route to integrating proximate and ultimate causation in studies of plasticity, and its potential influence on evolution more generally. Here, we use the major events in generalized polyphenic development as a scaffold for linking the molecular mechanisms of polyphenic switching with potential evolutionary outcomes of polyphenism and for discussing challenges and opportunities at each step in this process. Not only does the study of resource polyphenism uncover interesting details of discrete plasticity, it also illuminates and informs general principles at the intersection of development, ecology, and evolution.

Keywords: competition; developmental switch; flexible stem; genetic assimilation; phenotypic plasticity; polyphenism.

Figure 1

Examples of resource polyphenism. (A) Preserved saccate (a), cruciform (b), and campanulate (c) females of the rotifer Asplanchna sieboldii. These forms differ in their ability to capture and consume large prey items. (B) Omnivore (left) and carnivore (right) spadefoot toad (Spea multiplicata) tadpoles. In addition to the obvious size and jaw musculature differences, carnivores possess a shorter gut, more highly keratinzed mouthparts, and overall activity. Omnivore tadpoles primarily feed on detritus, but carnivores specialize on fairy shrimp and other tadpoles. (C) The stenostomatous (St morph) and eurystomatous (Eu morph) mouth forms of the nematode Pristionchus pacificus. False coloring indicates how the morphs differ in the shape of the dorsal tooth (blue) and the presence of an additional, opposing tooth (yellow). Whereas St worms are microbivores, Eu worms are omnivores capable of consuming both microbes (e.g., yeast, bacteria) and other nematodes. Image in (A) is courtesy of John J. Gilbert and reproduced from Gilbert (2017) with permission from John Wiley and Sons (© 2016 Cambridge Philosophical Society). Image in (B) is courtesy of David W. Pfennig and modified from Levis et al. (2020). Images in (C) are by Erik J. Ragsdale and from Bui et al. (2018).

Figure 2

Highlighted areas where knowledge of resource polyphenism’s proximate mechanisms can inform ultimate questions about ecology and evolution.