The evolution of genes encoding for green fluorescent proteins: insights from cephalochordates (amphioxus)

Fluorescent proteins generate a genetic color polymorphism and counteract oxidative stress in intertidal sea anemones

Fluorescent proteins (FPs) are ubiquitous tools in research, yet their endogenous functions in nature are poorly understood. In this work, we describe a combination of functions for FPs in a clade of intertidal sea anemones whose FPs control a genetic color polymorphism together with the ability to combat oxidative stress. Focusing on the underlying genetics of a fluorescent green "Neon" color morph, we show that allelic differences in a single FP gene generate its strong and vibrant color, by increasing both molecular brightness and FP gene expression level. Natural variation in FP sequences also produces differences in antioxidant capacity. We demonstrate that these FPs are strong antioxidants that can protect live cells against oxidative stress. Finally, based on structural modeling of the responsible amino acids, we propose a model for FP antioxidant function that is driven by molecular surface charge. Together, our findings shed light on the multifaceted functions that can co-occur within a single FP and provide a framework for studying the evolution of fluorescence as it balances spectral and physiological functions in nature.

Optical Clearing and Light Sheet Microscopy Imaging of Amphioxus

Cephalochordates (amphioxi or lancelets) are representatives of the most basally divergent group of the chordate phylum. Studies of amphioxus development and anatomy hence provide a key insight into vertebrate evolution. More widespread use of amphioxus in the evo-devo field would be greatly facilitated by expanding the methodological toolbox available in this model system. For example, evo-devo research on amphioxus requires deep understanding of animal anatomy. Although conventional confocal microscopy can visualize transparent amphioxus embryos and early larvae, the imaging of later developmental stages is problematic because of the size and opaqueness of the animal. Here, we show that light sheet microscopy combined with tissue clearing methods enables exploration of large amphioxus specimens while keeping the surface and the internal structures intact. We took advantage of the phenomenon of autofluorescence of amphioxus larva to highlight anatomical details. In order to investigate molecular markers at the single-cell level, we performed antibody-based immunodetection of melanopsin and acetylated-α-tubulin to label rhabdomeric photoreceptors and the neuronal scaffold. Our approach that combines light sheet microscopy with the clearing protocol, autofluorescence properties of amphioxus, and antibody immunodetection allows visualizing anatomical structures and even individual cells in the 3D space of the entire animal body.

Sea as a color palette: the ecology and evolution of fluorescence

Fluorescence and luminescence are widespread optical phenomena exhibited by organisms living in terrestrial and aquatic environments. While many underlying mechanistic features have been identified and characterized at the molecular and cellular levels, much less is known about the ecology and evolution of these forms of bioluminescence. In this review, we summarize recent findings in the evolutionary history and ecological functions of fluorescent proteins (FP) and pigments. Evidence for green fluorescent protein (GFP) orthologs in cephalochordates and non-GFP fluorescent proteins in vertebrates suggests unexplored evolutionary scenarios that favor multiple independent origins of fluorescence across metazoan lineages. Several context-dependent behavioral and physiological roles have been attributed to fluorescent proteins, ranging from communication and predation to UV protection. However, rigorous functional and mechanistic studies are needed to shed light on the ecological functions and control mechanisms of fluorescence.

Nitric Oxide regulates mouth development in amphioxus

The development of the mouth in animals has fascinated researchers for decades, and a recent study proposed the modern view of recurrent evolution of protostomy and deuterostomy. Here we expanded our knowledge about conserved traits of mouth formation in chordates, testing the hypothesis that nitric oxide (NO) is a potential regulator of this process. In the present work we show for the first time that NO is an essential cell signaling molecule for cephalochordate mouth formation, as previously shown for vertebrates, indicating its conserved ancestral role in chordates. The experimental decrease of NO during early amphioxus Branchiostoma lanceolatum development impaired the formation of the mouth and gill slits, demonstrating that it is a prerequisite in pharyngeal morphogenesis. Our results represent the first step in the understanding of NO physiology in non-vertebrate chordates, opening new evolutionary perspectives into the ancestral importance of NO homeostasis and acquisition of novel biological roles during evolution.

Acropora digitifera Encodes the Largest Known Family of Fluorescent Proteins that Has Persisted during the Evolution of Acropora Species

Fluorescent proteins (FPs) are well known and broadly used as bio-imaging markers in molecular biology research. Many FP genes were cloned from anthozoan species and it was suggested that multi-copies of these genes are present in their genomes. However, the full complement of FP genes in any single coral species remained unidentified. In this study, we analyzed the FP genes in two stony coral species. FP cDNA sequences from Acropora digitifera and Acropora tenuis revealed the presence of a multi-gene family with an unexpectedly large number of genes, separated into short-/middle-wavelength emission (S/MWE), middle-/long-wavelength emission (M/LWE), and chromoprotein (CP) clades. FP gene copy numbers in the genomes of four A. digitifera colonies were estimated as 16–22 in the S/MWE, 3–6 in the M/LWE, and 8–12 in the CP clades, and, in total, 35, 31, 33, and 33 FP gene copies per individual shown by quantitative PCR. To the best of our knowledge, these are the largest sets of FP genes per genome. The fluorescent light produced by recombinant protein products encoded by the newly isolated genes explained the fluorescent range of live A. digitifera, suggesting that the high copy multi-FP gene family generates coral fluorescence. The functionally diverse multi-FP gene family must have existed in the ancestor of Acropora species, as suggested by molecular phylogenetic and evolutionary analyses. The persistence of a diverse function and high copy number multi-FP gene family may indicate the biological importance of diverse fluorescence emission and light absorption in Acropora species.