Fluorescent Protein Evolution - Gene Duplication and Loss in Acropora

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Animal color patterns involving fluorescent proteins (FPs) are critical to providing colors to corals and are known to have expanded in these animals, and FPs are perhaps nowhere more exploited than by corals. Coral colors are due primarily to green (GFP), cyan (CFP), and red (RFP) FP emission, as well as in combination with purple or blue non-fluorescent chromoproteins (ChrPs).

FPs serve as photoprotective host pigments for corals and their symbionts. FPs become excited when exposed to high-energy ultraviolet radiation and subsequently emit lower-energy visible light. This phenomenon may help reduce oxidative stress both in corals and their algal symbionts. Additionally, FPs are likely involved in attracting symbionts to coral hosts and optimizing photosynthetic activity within coral polyps. 

Acropora species lost a gene for cystathionine beta-synthase, an enzyme essential for cysteine biosynthesis (Shinzato et al., 2011), they depend upon dinoflagellates of the family Symbiodiniaceae and/or other symbiotic organisms to supply this amino acid. Therefore, Acropora species are more sensitive to stress such as bleaching. Previous reports have suggested that bleaching events trigger colorization in reef-building corals.

Genome-wide survey identified 219 FP genes in Acropora species. Molecular phylogeny revealed that the 15 Acropora species each have 9–18 FP genes, whereas the other acroporids examined have only two (below left figure), suggesting a pronounced expansion of the FP genes in the genus Acropora. 

Different evolutionary histories of lineage-specific duplication and loss were discovered among GFP/CFPs, RFPs, and ChrPs. Synteny analysis revealed core GFP/CFP, RFP, and ChrP gene clusters, in which a tandem duplication of the FP genes was evident. The expansion and diversification of Acropora FPs may have contributed to the present-day richness of this genus. Therefore, exploring the number and function of FP candidate genes in corals is important for understanding their survival strategies.

Kashimoto et al., 2021 Genes