Whitney Laboratory, University of Florida
Correspondence: E-mail: matz{at}whitney.ufl.edu.
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Abstract |
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Key Words: green fluorescent protein anthozoa color evolution
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Introduction |
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The current data set of sequenced and spectroscopically characterized GFP-like proteins of the class Anthozoa includes four color types, possessing different chromophore structures: green, yellow, orange-red, and nonfluorescent purple-blue (Labas et al. 2002). Quite unexpectedly, phylogenetic analysis of the data set showed that proteins of different colors do not constitute separate clades, but are intermixed within the terminal branches of the phylogenetic tree. This fact suggested that the color diversity, instead of being a product of prolonged evolution, originated recently and independently within several lineages (Labas et al. 2002). Deciphering the evolutionary scenario that resulted in such a phylogenetic pattern is a challenging task, which at the present moment is additionally complicated by insufficient understanding of the adaptive role (or roles) of different colors in Anthozoa.
One of the basic issues regarding the evolution of anthozoan colors that can be addressed at the present state of knowledge is the basis of intraspecific color variation. Many coral species exhibit these variations, ranging from fluorescent blue to fluorescent red and nonfluorescent purple (Mazel 1995, 1997; Veron 2000). Two alternative mechanisms of generating diversity of coloration are possible, which are usually called polymorphism and polyphenism. Polymorphism is the situation when differences in color appearance can be traced to differences in the genome, in which case the color appearance of an organism is largely determined at the moment of zygote formation and there is little possibility for it to change afterwards, except for the intensity. This explanation of color diversity in corals seems particularly attractive, considering the relative ease of certain types of color conversion by random mutations (Lukyanov et al. 2000; Gurskaya et al. 2001b). In contrast, polyphenism is recognized when the same genome gives rise to different phenotypes. In the case of corals, color polyphenism would mean the existence of the same collection of genes coding for GFP-like proteins in all color morphs, the differences in color appearance being due to the changes in relative levels of expression of these genes. In this case, the color can be a much more flexible character during the organism's lifetime than it is allowed under the polymorphism model. The color diversity may also be a result of combination of these two models, in which case both allelic polymorphism and expression level variations would play significant roles.
In the present study, we used the great star coral Montastraea cavernosa to gain insight into this problem. This species is one of the most common reef-building corals in the Caribbean, exhibiting several color variants (Mazel 1995, 1997; Veron 2000). Our goals were to evaluate the number of genes coding for GFP-like proteins that are expressed in an individual colony and to determine whether the color differences between morphs are due to variations in expression levels of the color-coding genes or mutations within their coding sequences.
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Methods |
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Cloning of cDNAs for GFP-like Proteins
The complete cloning procedure included three stages: (1) amplification of initial fragments with 11 different combinations of degenerate primers; (2) obtaining 5' and 3' flanks for all detected sequence variants; and (3) amplification and cloning into bacterial expression vectors of all open reading frames (ORFs) using primers designed to match all determined ORF termini. See online Supplementary Material for oligonucleotides' sequences and protocol details. Once the full-scale cloning was done on one colony of M. cavernosa, the last stage (full ORF cloning for bacterial expression) was repeated on two other colonies using the same set of primers designed for ORF termini. The bacterial colonies were screened for fluorescence using a stereomicroscope (Leica MZ FL III), and at least 96 fluorescent clones per cDNA sample were replated as streaks onto new plates, the same clones being also grown as overnight cultures for plasmid isolation and sequencing. The emission spectra of the streaks were measured using the Ocean Optics setup described above.
Contig Assembly
The sequences of the expression constructs were aligned and subjected to phylogenetic analysis using unweighted parsimony within PAUP* package (Swofford 1993) and maximum-likelihood/quartet puzzling approach implemented in TreePuzzle software (Strimmer and von Haeseler 1996). This analysis reliably separated the sequences into groups corresponding to different genes and/or alleles, avoiding possible confusion due to sequencing errors and occasional point mutations introduced during PCRs. The coding sequences were then separated into contigs as suggested by phylogenetic analysis. Each contig was required to include at least two sequences, which must share at least two nucleotide substitutions that were not observed in other sequences (i.e., represent true synapomorphs). Moreover, each contig was required to contain sequences that encoded spectroscopically identical products. For the colony on which full-scale cloning was done, after subdivision of ORFs into separate contigs, the corresponding 5' and 3' cDNA termini were selected manually among the sequences obtained during cDNA flanks' amplification, requiring an exact match within the overlap of the flank sequence and the inferred coding sequence.
Recombinant Protein Purification and Spectroscopy
Representative clones for each of the detected genes/allelomorphs were grown in a 200 ml volume of LB medium overnight, and the recombinant protein was isolated from them using metalloaffinity chromatography, taking advantage of the 6xHis tags attached at the step of coding sequence amplification, using standard protocol (http://www.qiagen.com/literature/xpreslit.asp#expressionist). Emission spectra of the products were determined using a USB-2000 spectrometer (Ocean Optics); the excitation spectra were measured using an LS50-B spectrofluorometer (Perkin Elmer).
Data Analysis
For contigs' tree reconstruction, the alignment of cDNA sequences was made by ClustalX (Thompson et al. 1997) and corrected manually according to triplet coding and considerations from protein structure (Matz et al. 1999). The tree of contigs was reconstructed using Tree-Puzzle software (Strimmer and von Haeseler 1996), with the following parameters: HKY model of DNA evolution (Hasegawa, Kishino, and Yano 1985) was used for maximum-likelihood distance calculation (parameters estimated from the dataset), gamma distribution of sites variability was assumed (alpha parameter estimated from the data set), and 10,000 puzzling steps were done to obtain support values. For gene conversion detection, GENECONV software (version 1.81) was used (Sawyer 1989; Hartl and Sawyer 1991). The tree of "nonconverted" contigs with addition of the rest of the known scleractinian proteins was constructed as a part of a global phylogenetic tree of all known GFP-like proteins (see online Supplementary Material) using MrBayes software (Huelsenbeck and Ronquist 2001). This program performs a Markov chain Monte Carlo simulation to search for the areas of maximum likelihood on a landscape of all possible trees and then builds a consensus from the trees found in these areas, at the same time calculating the posterior (Bayesian) probability for occurrence of each node. The alignment of coding sequences of all known GFP-like proteins was analyzed (see online Supplementary Material), with the exception of the sequences that were identified as possible products of gene conversion by GENCONV. Due to this criterion, in addition to the three sequences described in this paper (mc6, g6, and r7), amajGFP (GenBank accession number AF168421), dis3GFP (GenBank accession number AF420593), and scubGFP2 (GenBank accession number AY037771) were also excluded. The Markov chain was run with the following settings: mutation rate classes were assigned according to the position within a codon (first, second and third); substitution model was general time reversible (Tavare 1986); number of parallel chains run was four; number of iterations was 200,000; sample frequency was 10; number of initial trees discarded during consensus building ("burn-in" value) was 8,000.
Assessment of Gene Expression Levels in Color Morphs
Pools of fragments of all detected target cDNAs were amplified from each of the morphs using nondegenerate primers that matched any of the cloned cDNA types equally well. The pools were cloned and sequenced, to calculate the number of occurrences of each cDNA type in the mixed populations. Since the primers had the same match quality to all the analyzed genes, and the lengths of the amplified fragments were practically the same, the efficiency of amplification for all the genes should be similar enough to expect that in the resulting product, the relative amounts of different types of GFP-like protein-coding sequences would reflect their original proportion in the mRNA sample. This is the same principle that was utilized in coamplification methods of quantitative PCR, where a control template of known amount was amplified along with an assayed template to provide reference at the amplification endpoint (see Raeymaekers 2000 for review). The sequencing sample size (77 for each morph) guaranteed with 95% confidence that any target mRNA comprising 4% or more of the original fragment pool would be detected.
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Results |
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The emission spectra of the obtained collection can be roughly subdivided into three color bands: cyan, green, and red. It must be noted that in this case "cyan" does not correspond to a separate color class of GFP-like proteins, but rather is a subtype of a green class, as defined by Labas et al. (2002). Cyan and red bands were represented each by a single contig (mc5 and mc1 [fig. 1C and A], respectively), and the rest of the clones were emitting in the green range. Of these, representatives of the contig mc6 were clearly distinguishable from the rest of the green clones by positions of excitation and emission maxima (fig. 1D). The contigs mc2, mc3, and mc4 contained clones with very similar excitation and emission curves (fig. 1B).
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cDNAs of GFP-like Proteins in Color Morphs
Two color morphs (red and green) were compared with regard to the spectral properties and expression levels of the encoded GFP-like proteins. The red morph exhibited orange-red emission in coenosteum (tissue between polyps) and oral disk, whereas the tentacles were weakly green-fluorescent. The green morph had a cyan-colored coenosteum, green oral disk, and green or nonfluorescent tentacles (see fig. 2C and D for in vivo emission curves).
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The green colony yielded six contigs (fig. 3A), designated g1.1, g1.2, g4, g5.1, g5.2, and g6. Two of them (g5.1 and g5.2) were similar to the previously identified mc5 (cyan) and exhibited identical emission spectra of the protein products. There were two green-coding cDNAs similar to others identified previously: contig g6 was identical to mc6, and contig g4 clustered with mc4. The fifth contig was closer to mc1 (red) than to any other types by sequence similarity and indel pattern. However, it contained 58 nucleotide substitutions (9.3% overall difference) in comparison with mc1, and moreover, the corresponding protein product emitted in the green range with no trace of a red peak. Surprisingly, one of the contigs was nearly identical to the red-coding mc1 (contig g1.1) and indeed encoded a red fluorescent product. This contig was represented by two identical expression clones, which contained at least four shared nucleotide substitutions in comparison with the previously encountered red-coding cDNAs (mc1, r1.1, and r1.2). After the finding of these clones, the red fluorescence was actually detected in several isolated areas of the green morph.
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Discussion |
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We can therefore conclude that there are at least three separate loci coding for GFP-like proteins in M. cavernosa, which correspond to the clades in the contig tree and three major colors: cyan, green, and red (fig. 3A). It must be noted that these three colors represent only two color/structural classes of GFP-like proteins as suggested by Labas et al. (2002), since cyan proteins have the same chromophore as greens (Gurskaya et al. 2001b) and therefore should be considered a subtype of structural green class. Furthermore, the green color, as represented by the clade mc2/3/4, must involve at least two loci, because there are two cases when three different sequences that fell into this clade were obtained from the same specimen (mc2-mc3-mc4 and r2-r3-r4). Most probably, the sequences not included into any of the clades (contigs mc6/g6, r7 and g1.2) also represent separate loci, on the basis of the high nucleotide divergence (difference between each other and mean difference with any of the major clades is 9% to 15%). On the other hand, it is possible that sequences within the major clades represent alleles of the same locus (or two loci in the case of the mc2/3/4 clade), although the within-clade differences are still high (2.7% to 3.7% between sequences from a single specimen). Therefore, the conservative estimate of the number of color-coding loci in M. cavernosa is at least four (two corresponding to the cyan and red clades and two corresponding to the mc2/3/4 clade) and can be as high as seven if contigs mc6, r7, and g1.2 represent separate loci.
Polymorphism Versus Polyphenism
Polymorphism of a trait generally means that in natural populations it comes in several states; the states are inheritable and directly or indirectly correspond to allelic variations at certain loci. Color polymorphism is common in all organisms, from flowers (Subramaniam and Rausher 2000) to bears (Lynch and Ritland 1999), and provides an excellent basis for studies in color evolution and ecology. On the other hand, polyphenism means that the trait is not preset genetically and is determined by the environmental cues. In this case, the color of an organism, combined with the knowledge of factors that affect it, may help to draw conclusions about life history of the organism or its current condition, depending on when the color is determined. Thus, in butterflies, the wing color pattern may be an indicator of the diel cycle conditions (Nijhout 1997) or temperature (Roskam and Brakefield 1999) under which the animal has developed (see Beldade and Brakefield 2002 for review). In grasshoppers and locusts, color polyphenism may be dependent on population density and was suggested to be a basis for evolution of aposemantic ("warning") coloration (Sword 2002). A remarkable case of polyphenism that may include color is seen in tadpoles of several frog species, which assume a different morphology and behavior depending on the presence of predators (dragonfly larvae) in the pond (McCollum and Van Buskirk 1996; Van Buskirk, McCollum, and Werner 1997).
In the present study, two different color morphs of Montastraea cavernosa, green and red, were found to contain, and express, the same functional suite of color-coding genes. We also demonstrated that the differences in color appearances of these morphs were because of differences in expression levels (specifically, mRNA abundance) of these genes relative to one another, and not because some of the coding sequences in one or another morph were altered by mutations. These data certainly rule out at least the most straightforward polymorphism scenario, whereby the color differences are due to variations in protein sequences encoded by alleles at color-coding loci. However, the relative expression levels may also be genetically predetermined. Thus, for Pocillopora damicornis, Takabayashi and Hoegh-Guldberg (1995) demonstrated that the intensity of pink coloration, which is most probably due to a GFP-like protein from a nonfluorescent class (Lukyanov et al. 2000), can be a dynamic trait since it is light-inducible. However, only originally pink colonies showed noticeable induction, which was interpreted as evidence of genetic control. It must be noted, however, that Yu et al. (2000) recently argued that pink and brown morphs of P. damicornis show enough ecological and physiological differences to suggest that they may represent different species. In any case, our data for M. cavernosa can only be interpreted as polyphenism if factors other than genetic background are found to determine the color at some point of the coral's life cycle, or changes in the hue of coloration are observed during the lifetime of the coral. Todd et al. (2002a, 2002b) documented environment-induced color change in a Faviid coral Favia speciosa (same family as M. cavernosa): fragments of the green colony changed their color to red after 4 to 7 months after relocation to greater depth. It is tempting to note that our superficial observations from field sites where the samples were collected are in accord with these results. Although both green and red M. cavernosa colonies can be found in shallow (3 to 5 m) and deep (12 to 14 m) waters, the red colonies seem to be more abundant at greater depths. Another case of color change was observed in a specimen of mushroom coral Fungia fungites, which went from brown to intense purple and then to bright fluorescent green over the period of several months, apparently in response to varying lighting conditions (R. Rowan, personal communication). So, in Favia speciosa and Fungia fungites, color definitely is a polyphenetic trait. It appears highly probable that the same is also true for Montastraea cavernosa, especially taking in account its multilocus principle of color coding that is well suited to produce phenotypic plasticity. This issue can be settled in further experiments to confirm whether or not the color in M. cavernosa is inheritable, and if not (as the polyphenism scenario predicts), which factors determine the color and when they act during the life cycle. At the moment, we do not have enough data to discuss possible functions of the three different colors of M. cavernosa without this information at hand, not to mention the color ecology and physiology components that are missing for M. cavernosa and only beginning to be developed for other coral species.
Evolution of Intraspecific Sequence/Color Diversity
Our data suggest that the color of Montastraea cavernosa is the product of three mechanisms that give rise to sequence diversity of the color-coding genesgene duplication, gene conversion, and point mutations. Gene duplications played a major role, leading to separation of genes for three main colors, cyan, green, and red. Some of the duplication events happened even before separation of the families Faviidae, Mussidae, and Trachphyllidae, as suggested by the placement of non-Montastraea sequences within the phylogenetic tree. All these three families belong to the suborder Faviina. According to the fossil record, Mussidae may have existed as a separate lineage since the mid-Jurassic (about 170 to 190 MYA), whereas separation of Faviidae and Trachyphyllidae happened much later, in the Eocene (about 50 MYA) (Veron 1995). The non-Faviidae sequences (scubGFP1 representing family Mussidae, and Kaede, or tgeoRFP, representing Trachyphyllidae) do not form sister groups to the cluster of M. cavernosa proteins, as it would be expected if the families' separation preceded the genes' separation. Instead, Kaede protein falls within the mc1 clade (in accord with its red emission color), whereas scubGFP1 is basal to the group of mc1-mc2/3/4 clades (fig. 3B). This topology indicates that the three color lineages separated before separation of Faviidae and Trachyphyllidae, whereas separation of Mussidae from other represented coral families preceded the divergence of mc1 and mc2/3/4 (red and green) lineages. The split between mc5 clade (cyan) and the rest of colors is the most ancient, preceding the separation of all three coral families. This finding was unexpected, since cyan GFP-like proteins were previously considered merely a variation within a green class (Labas et al. 2002). Apparently, at least in studies devoted to color evolution and ecology, cyan should be regarded as a separate class of GFP-like proteins, most probably having specific function in corals.
Gene conversion is copying of sequence information between two different loci within a genome and can be a prominent evolutionary mechanism for gene families (Graham 1995; Ohta 2000). When happening on the scale of a whole gene, this mechanism homogenizes the sequences within a gene family (Elder and Turner 1995; Graham 1995; Kupriyanova 2000), whereas conversion of gene fragments produces an opposite effect: it generates combinatorial diversity (McCormack, Tjoelker, and Thompson 1991; Knight and Winstead 1997). Among M. cavernosa GFP-like proteins, two sequences (mc6/g6 and r7) appear to be resulting from gene conversion events of the latter (diversifying) type. In both cases, a fragment mapping to the same region within the ORF was copied from a sequence belonging to mc2/3/4 clade (table 1). Diversifying gene conversion seems to be a minor factor in evolution of the GFP-like proteins of M. cavernosa and Anthozoa in general, since it is detected only within a few sequences. In addition to mc6/g6 and r7, within the collection of 30 currently known GFP-like proteins except the ones from Montastraea genus (see online Supplementary Material), GENECONV suggested gene conversion events in only three sequences: amajGFP (GenBank accession number AF168421), dis3GFP (GenBank accession number AF420593), and scubGFP2 (GenBank accession number AY037771) (Labas et al. 2002; Matz, Lukyanov, and Lukyanov 2002), affecting stretches of ORF 35 to 77 bases long (data not shown). It is tempting to speculate that rare occurrence of gene conversion between the gene family members indicates that they are rarely located close to each other within the genome (Graham 1995; Ohta 2000). However, this conclusion requires estimation of the frequency of "homogenizing" gene conversion on a whole-gene scale, which would require extensive analysis of genome sequences and is therefore beyond the scope of this work.
On the basis of molecular organization of GFP-like proteins and in vitro mutagenesis experiments, we earlier suggested that the third mechanism of diversity generation, point mutations, should play a special role in the evolution of colors within the GFP-like protein family: it should drive the change of red-shifted colors into green. Therefore, the very persistence of red-emitting proteins in nature can be viewed as an argument in favor of adaptive significance of the red color, since natural selection for the red must be involved to counterbalance the random mutation drive (Labas et al. 2002). We further speculated that if both red and green colors are adaptively significant, there may be cases when the color-converted reds that became green were not eliminated from the population, but shifted function according to their new color. The data presented here provide indirect evidence in favor of this hypothesis. First, the persistence of the separate lineage of red-emitting proteins since before the coral families diverged is consistent with the idea that red color is adaptively significant and is maintained by natural selection. Second, the finding of an abundantly expressed green-emitting protein belonging to the red clade by sequence similarity as well as by indel pattern (contig g1.2 from the green morph) may indicate that the predicted shift-function scenario has been realized. However, these facts should be treated with caution. In a multigene family with high frequency of gene duplication (and, most probably, gene loss) there is ample room for alternative explanation for the observed phylogenetic pattern. To settle the question about color evolution pathways, detailed statistical analysis of selection forces exerted upon individual sites within the protein family members (Suzuki and Gojobori 1999; Yang and Bielawski 2000) is currently underway, as well as analysis of ancestral sequences at the nodes of phylogenetic tree (Chang, Kazmi, and Sakmar 2002).
Conclusions
We found that in Montastraea cavernosa, the color differences between colonies are explained by varying levels of expression (specifically, transcript abundances) of a set of genes coding for GFP-like proteins. The set includes at least four, and probably up to seven, separate genetic loci and encodes proteins emitting at three general wavebands: cyan (wide emission peak at around 495 nm), green (narrow emission peak at 505 to 520 nm), and red (emission at 580 nm). Proteins grouped by the criterion of emission waveband also compose separate clades in the phylogenetic tree. Gene duplication should be considered a major mechanism that led to the present-day color diversity. Placement of GFP-like proteins from other representatives of stony corals within the phylogenetic tree suggests that these gene duplications started before separation of the coral families, as early as the Jurassic period. Further studies will be directed at understanding details of color evolution history and pinpointing factors that affect and maintain the existing color diversity.
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Supplementary Material Online |
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Phylogenetic tree constructed by MrBayes using the above alignmentphylogenetic_tree.pdf.
Alignment of full-length cDNA sequences and synthetic expression constructs reported here, in FASTA formatMCs-cDNA.fas.
The same alignment as in MCs-cDNA.fas, trimmed for phylogenetic analysis, in FASTA format (was used to construct the unrooted tree in fig. 3A)MCs-cDNA_trim.fas.
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Acknowledgements |
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Footnotes |
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