One key question in understanding the origins of biodiversity is whether novel phenotypes promote ecological diversification by opening new niche dimensions, or whether diversification is driven by existing niche heterogeneity. This has proven difficult to resolve because phenotypic and ecological diversification often interact, and the origin of ancient phenotypes can be murky. Diversification of resource acquisition may be particularly important because it constrains what organisms have available for allocation to competing downstream functions. In consumers, such diversification is common, driven by diversity in the resource base. However, resource acquisition diversity in producers is infrequently considered, perhaps because there is limited variation in the available form of most abiotic resources. We used common-garden experiments and phylogenetic comparative methods to examine light capture in Cryptophytes, a phylum of single-celled eukaryotic algae, to better understand diversification in the acquisition of an abiotic resource. Cryptophytes originated through a secondary endosymbiosis that resulted in distinctive pigment-protein complexes, the cryptophyte phycobiliproteins. These are products of genes from both the ancestral host and endosymbiont lineages, and may have been a phenotypic trigger for diversification of light capture.
Results/Conclusions
Examining a billion years of Cryptophyte evolution, we found that phycobiliprotein characteristics were associated with diversification of light capture, while non-phycobiliprotein pigments and cell size were not. Both the wavelength of maximum absorption and quantity of phycobiliproteins were positively associated with photosynthetically usable radiation, a measure of light capture. The phycobiliprotein quantity relationship disappeared when scaled to cell size, indicating that evolving larger cells does not increase light capture. Furthermore, phycobiliproteins were evolutionarily labile with repeated transitions and reversals, although there was a general trend toward improved light capture. Thus the endosymbiotic origin of cryptophyte phycobiliproteins provided an evolutionary spark that drove diversification of light capture, the abiotic resource that is the foundation of photosynthesis.