is a major photosynthetic prokaryote in nutrient-limited, open ocean environments and an important participant in the global carbon cycle. This phototroph is distinct from other members of the cyanobacterial lineage to which it belongs because it utilizes a chlorophyll / light-harvesting complex as its major antenna, instead of phycobilisomes. Recently, genes encoding the phycobiliprotein phycoerythrin were identified in several isolates, thus making it the only extant photosynthetic prokaryote to possess a chlorophyll antenna as well as phycobiliprotein genes. In order to understand the evolution of phycobiliproteins in this genus, the authors have sequenced the phycoerythrin genes of two isolates that are the most deeply branching in the lineage and share the highest degree of 16S rDNA sequence similarity to phycobilisome-containing marine . Sequence analyses suggest that within the lineage, the selective forces shaping the evolution of the phycoerythrin gene set have not been uniform. Although strains that are most closely related to marine possess genes (, ) encoding both subunits of phycoerythrin, a more recently evolved strain is shown to lack and to possess a degenerate form of . Differences in phycoerythrin gene sequences between and appear to be consistent with a model of elevated mutation rates rather than relaxed selection. This suggests that although phycoerythrin is not a major constituent of the light-harvesting apparatus in , as it is in , the and genes are still under selection, albeit a different type of selection than in . The evolution of the light-harvesting antenna complex provides an important system for understanding the origins and scope of phylogenetic diversity in ocean ecosystems.


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