Coral reefs under rapid climate change and ocean acidification

Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by at least 2 degrees C by 2050 to 2100, values that significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved. Under conditions expected in the 21st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef communities and carbonate reef structures that fail to be maintained. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reefs increasingly toward the tipping point for functional collapse. This review presents future scenarios for coral reefs that predict increasingly serious consequences for reef-associated fisheries, tourism, coastal protection, and people. As the International Year of the Reef 2008 begins, scaled-up management intervention and decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.

Multiplex serum cytokine monitoring as a prognostic tool in rheumatoid arthritis

OBJECTIVE

Early optimized therapy of rheumatoid arthritis (RA) results in improved outcomes. The initiation of optimized therapy is hindered by the difficulty of early diagnosis and the limitations of current disease activity and therapeutic response assessment tools. Identifying patients requiring early combination DMARD/biologic therapy is currently a significant clinical challenge given the lack of definitive prognostic criteria. Since cytokines are soluble intracellular signaling molecules that modulate disease pathology in RA, we tested the recent conjecture that en mass serum cyto-kine measurement and monitoring will provide a useful tool for effective therapeutic management in RA.

METHODS

We assayed the levels of 16 serum cytokines in 18 RA patients treated prospectively with methotrexate and from 18 unaffected controls. Specific mechanistic aspects of inflammatory pathology in the periphery could be discerned on a patient-specific basis from patients' serum cytokine profiles, information that may aid in the design of anti-cytokine biologic therapy. A serum Cytokine Activity Index (CAI) was also created using multi-variant analysis methods.

RESULTS

Distinct cytokines were significantly elevated in RA patients relative to controls, and three distinct clusters with correlations to disease activity were identified. The Cytokine Activity Index correlated well with the therapeutic res-ponse; responders and non-responders in this cohort were distinguishable as early as one month post initiation of methotrexate therapy, well before clinical assessments of response are commonly completed.

CONCLUSION

Clinical assessment tools could be derived from this approach that may provide a means to continually track patients, allowing intervention strategies to be better evaluated on a patient-specific basis and to identify residual cytokine activity that could be used to guide combination therapy.

Circulating cytokines in Norwegian patients with psoriatic arthritis determined by a multiplex cytokine array system

OBJECTIVES

Serum cytokines play an important role in the pathogenesis of psoriatic arthritis (PsA) by initiating and perpetuating various cellular and humoral autoimmune processes. The aim of this study was to describe a broad spectrum of T- and B-cell cytokines, growth factors and chemokines in patients with PsA and healthy individuals.

METHODS

A novel protein array system, denoted as multiplex cytokine assay was utilized to measure simultaneously the levels of 23 circulating cytokines of patients with PsA and healthy individuals. Additionally, correlational clustering and discriminant function analysis (DFA), two multivariate, supervised analysis methods were employed to identify a subset of biomarkers in order to describe potential functional inter-relationships among these pathological cytokines and identify biomarkers with prognostic and diagnostic utility.

RESULTS

Univariate analysis demonstrated that serum levels of a complex set of immune and inflammatory modulating cytokines are significantly up-regulated in patients with PsA relative to unaffected controls including interleukin (IL)-10, IL-13, interferen (IFN)-alpha, epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), fibroblast growth factor [CCL3 macrophage inflammatory protein (MIP)-1alpha], CCL4 (MIP-1beta) and CCL11 (Eotaxin), while granulocyte-colony stimulating factor was significantly reduced in PsA patients. Correlational clustering was able to discriminate among, and hence subclassify, patients with varying levels of disease activity, which may prove useful in guiding therapy in these apparently phenotypically distinct disease subsets. DFA identified EGF, IFN-alpha, VEGF, CCL3 (MIP-1alpha) and IL-12p40 as analytes with the strongest discriminatory power among various PsA patients and controls.

CONCLUSIONS

Our findings suggest that these factors modulate PsA pathology and the articular involvement in a synergistic manner. Identifying factors could be used in the development of clinical diagnostic tests, which are valuable to guide evidence-based diagnosis and disease management of PsA.

RE. Template-driven gene selection procedure

The hierarchical clustering and statistical techniques usually used to analyse microarray data do not inherently represent the underlying biology. Herein, a hybrid approach involving characteristics of both supervised and unsupervised learning is presented. This approach is based on template matching in which the interaction of the variables of inherent malignancy and the ability to express the malignant phenotype are modelled. Immortalised normal urothelial cells and bladder cancer cells of different malignancy were grown in conventional two-dimensional tissue culture and in three dimensions on extracellular matrices (ECMs) that were either permissive or restrictive for expression of the malignant phenotype. The transcriptome represents the effects of two variables--inherent malignancy and the modulatory effect of ECM. By assigning values to each of the biological variables of inherent malignancy and the ability to express the malignant phenotype, a template was constructed, which encapsulated the interaction between them. Gene expression correlating both positively and negatively with the template was observed, but when iterative correlations were carried out, the different models for the template converged on the same actual template. A subset of 21 genes was identified, which correlated with two a priori models or an optimised model above the 95% confidence limits identified in a bootstrap resampling with 5000 permutations of the data set. The correlation coefficients of expression of several genes were > 0.8. Analysis of upstream transcriptional regulatory elements (TREs) confirmed that these genes were not a randomly selected set of genes. Several TREs were identified as significantly over-expressed in the sample of 20 genes for which TREs were identified, and the high correlations of several genes were consistent with transcriptional co-regulation. The authors suggest that the template method can be used to identify a unique set of genes for further investigation.

A genome-scale assessment of peripheral blood B-cell molecular homeostasis in patients with rheumatoid arthritis

OBJECTIVE

While rheumatoid arthritis (RA) is considered a prototypical autoimmune disease, the specific roles of B-cells in RA pathogenesis is not fully delineated.

METHODS

We performed microarray expression profiling of peripheral blood B-cells from RA patients and controls. Data were analysed using differential gene expression analysis and 'gene networking' analysis (characterizing clusters of functionally inter-relelated genes) to identify both regulatory genes and the pathways in which they participate. Results were confirmed by quantitative real-time polymerase chain reaction and by measuring the levels of 10 serum cytokines involved in the pathways identified.

RESULTS

Genes regulating and effecting the cell-cycle, proliferation, apoptosis, autoimmunity, cytokine networks, angiogenesis and neuro-immune regulation were differentially expressed in RA B-cells. Moreover, the serum levels of several soluble factors that modulate these pathways, including IL-1beta, IL-5, IL-6, IL-10, IL-12p40, IL-17 and VEGF were significantly increased in this cohort of RA patients.

CONCLUSIONS

These results outline aspects of the multifaceted role B-cells play in RA pathogenesis in which immune dysregulation in RA modulates B-cell biology and thereby contributes to the induction and perpetuation of a pathogenic humoral immune response.

Analysis of the interaction of extracellular matrix and phenotype of bladder cancer cells

Background

The extracellular matrix has a major effect upon the malignant properties of bladder cancer cells both in vitro in 3-dimensional culture and in vivo. Comparing gene expression of several bladder cancer cells lines grown under permissive and suppressive conditions in 3-dimensional growth on cancer-derived and normal-derived basement membrane gels respectively and on plastic in conventional tissue culture provides a model system for investigating the interaction of malignancy and extracellular matrix. Understanding how the extracellular matrix affects the phenotype of bladder cancer cells may provide important clues to identify new markers or targets for therapy.

Methods

Five bladder cancer cell lines and one immortalized, but non-tumorigenic, urothelial line were grown on Matrigel, a cancer-derived ECM, on SISgel, a normal-derived ECM, and on plastic, where the only ECM is derived from the cells themselves. The transcriptomes were analyzed on an array of 1186 well-annotated cancer derived cDNAs containing most of the major pathways for malignancy. Hypervariable genes expressing more variability across cell lines than a set expressing technical variability were analyzed further. Expression values were clustered, and to identify genes most likely to represent biological factors, statistically over-represented ontologies and transcriptional regulatory elements were identified.

Results

Approximately 400 of the 1186 total genes were expressed 2 SD above background. Approximately 100 genes were hypervariable in cells grown on each ECM, but the pattern was different in each case. A core of 20 were identified as hypervariable under all 3 growth conditions, and 33 were hypervariable on both SISgel and Matrigel, but not on plastic. Clustering of the hypervariable genes showed very different patterns for the same 6 cell types on the different ECM. Even when loss of cell cycle regulation was identified, different genes were involved, depending on the ECM. Under the most permissive conditions of growth where the malignant phenotype was fully expressed, activation of AKT was noted. TGFβ1 signaling played a major role in the response of bladder cancer cells to ECM. Identification of TREs on genes that clustered together suggested some clustering was driven by specific transcription factors.

Conclusion

The extracellular matrix on which cancer cells are grown has a major effect on gene expression. A core of 20 malignancy-related genes were not affected by matrix, and 33 were differentially expressed on 3-dimensional culture as opposed to plastic. Other than these genes, the patterns of expression were very different in cells grown on SISgel than on Matrigel or even plastic, supporting the hypothesis that growth of bladder cancer cells on normal matrix suppresses some malignant functions. Unique underlying regulatory networks were driving gene expression and could be identified by the approach outlined here.

Distinct profiles of Sjögren's syndrome patients with ectopic salivary gland germinal centers revealed by serum cytokines and BAFF

The formation of ectopic germinal centers (GC) has been described in Sjögren's syndrome (SS), although little is known about the molecular basis of this phenomenon. These structures are a focus of in situ autoantibody production and have been hypothesized to be involved in lymphomagenesis in SS patients. Serum cytokines also play an important role in SS pathogenesis in part via immune dysregulation and may therefore contribute to ectopic GC formation. Herein, highly multiplex cytokine screening of SS patients with (SSGC+) and without (SSGC-) GC formation was done to identify cytokine profiles that correlate with this phenomenon. Serum levels of B-cell activating factor (BAFF) were also screened as a potential biomarker of immune dysregulation in SS and SSGC formation. Univariate analysis demonstrated that serum levels of a broad spectrum of immune and inflammatory modulating cytokines are upregulated in SSGC+ and SSGC- patients relative to unaffected controls IL-1beta, IL-2, IL-6, IL-15, IFN-gamma and CCL4 (MIP-1beta). SSGC+ patients were distinguished from healthy individuals by higher levels of IL-4, IL-10, GM-CSF, IFN-alpha, CCL3 (MIP-1alpha), CCL11 (Eotaxin) and BAFF, while SSGC+ and SSGC- patients differed in CCL2 (MCP-1) expression. Discriminant function analysis (DFA), a multivariate discrimination method that uses observed differences to characterize groups when casual relationships are not well understood, was employed to identify a subset of these biomarkers that maximally discriminate among SSGC+, SSGC- and unaffected individuals. The biomarker having the strongest discriminatory power identified by DFA besides CCL11 (Eotaxin) and IFN-gamma was BAFF. The variables identified by DFA are interdependent and are often of mechanistic significance to the pathologic states they distinguish, suggesting that these factors modulate SS pathology and SSGC formation in a synergistic manner.

Mobile classification in microarray experiments

In a homogeneous group of samples, there are genes whose expression variations can be attributed to factors other than experimental errors. These factors can include natural biological oscillations or metabolic processes. These genes are rarely classified as 'interesting' based on their variability profile. However, their dynamic behaviour can tease out important clues about naturally occurring biological processes in the organism under study and can be used for group classification. Dynamical discriminate function analysis was developed on the concept that stable classification parameters (roots) can be derived from highly variable gene-expression data. Stability of these combinations implies a strongly compensatory relationship that may divulge functional interconnections.

Hypervariable genes--experimental error or hidden dynamics

In a homogeneous group of samples, not all genes of high variability stem from experimental errors in microarray experiments. These expression variations can be attributed to many factors including natural biological oscillations or metabolic processes. The behavior of these genes can tease out important clues about naturally occurring dynamic processes in the organism or experimental system under study. We developed a statistical procedure for the selection of genes with high variability denoted hypervariable (HV) genes. After the exclusion of low expressed genes and a stabilizing log-transformation, the majority of genes have comparable residual variability. Based on an F-test, HV genes are selected as having a statistically significant difference from the majority of variability stabilized genes measured by the 'reference group'. A novel F-test clustering technique, further noted as 'F-means clustering', groups HV genes with similar variability patterns, presumably from their participation in a common dynamic biological process. F-means clustering establishes, for the first time, groups of co-expressed HV genes and is illustrated with microarray data from patients with juvenile rheumatoid arthritis and healthy controls.

Statistical monitoring of weak spots for improvement of normalization and ratio estimates in microarrays

BACKGROUND

Several aspects of microarray data analysis are dependent on identification of genes expressed at or near the limits of detection. For example, regression-based normalization methods rely on the premise that most genes in compared samples are expressed at similar levels and therefore require accurate identification of nonexpressed genes (additive noise) so that they can be excluded from the normalization procedure. Moreover, key regulatory genes can maintain stringent control of a given response at low expression levels. If arbitrary cutoffs are used for distinguishing expressed from nonexpressed genes, some of these key regulatory genes may be unnecessarily excluded from the analysis. Unfortunately, no accurate method for differentiating additive noise from genes expressed at low levels is currently available.

RESULTS

We developed a multistep procedure for analysis of mRNA expression data that robustly identifies the additive noise in a microarray experiment. This analysis is predicated on the fact that additive noise signals can be accurately identified by both distribution and statistical analysis.

CONCLUSIONS

Identification of additive noise in this manner allows exclusion of noncorrelated weak signals from regression-based normalization of compared profiles thus maximizing the accuracy of these methods. Moreover, genes expressed at very low levels can be clearly identified due to the fact that their expression distribution is stable and distinguishable from the random pattern of additive noise.

Connective molecular pathways of experimental bladder inflammation

Inflammation is an inherent response of the organism that permits its survival despite constant environmental challenges. The process normally leads to recovery from injury and to healing. However, if targeted destruction and assisted repair are not properly phased, chronic inflammation can result in persistent tissue damage. To better understand the inflammatory process, we recently introduced a profiling methodology to identify common genes involved in bladder inflammation. The method represents a complementation to the classic quantification of inflammation and provides information regarding the early, intermediate, and late events in gene regulation. However, gene profiling fails to describe the molecular pathways and their interconnections involved in the particular inflammatory response. The present work introduces a new statistical technique for inferring functional interconnections between inflammatory pathways underlying classic models of bladder inflammation and permits the modeling of the inflammatory network. This new statistical method is based on variants of cluster analysis, Boolean networking, differential equations, Bayesian networking, and partial correlation. By applying partial correlation analysis, we developed mosaics of gene expression that permitted a global visualization of common and unique pathways elicited by different stimuli. The significance of these processes was tested from both biological and statistical viewpoints. We propose that connective mosaic may represent the necessary simplification step to visualize cDNA array results.

Zooxanthellae of the Montastraea annularis species complex: patterns of distribution of four taxa of Symbiodinium on different reefs and across depths

Corals of the Montastraea annularis complex host several different dinoflagellates in the genus Symbiodinium. Here we address two questions arising from our previous studies of these associations on an offshore reef. First, do the same taxa and patterns of association (Symbiodinium A and B found in higher irradiance habitats than Symbiodinium C) occur on an inshore reef? Second, does M. franksi at the limits of its depth range host only Symbiodinium C, as it does at intermediate depths? In both surveys, a new Symbiodinium taxon and different patterns of distribution (assayed by analyses of small ribosomal subunit RNA genes [srDNA]) were observed. Inshore, a taxon we name Symbiodinium E predominated in higher irradiance habitats in M. franksi and its two sibling species; the only other zooxanthella observed was Symbiodinium C. Offshore, M. franksi mainly hosted Symbiodinium C, but hosted Symbiodinium A, B, C, and E in shallow water and Symbiodinium E and C in very deep water. Symbiodinium E may be stress-tolerant. Observed srDNA heterogeneity within samples of Symbiodinium B, C, and E is interpreted as variation across copies within this multigene family. Experimental bleaching of Symbiodinium C supported this interpretation. Thus sequences from natural samples should be interpreted cautiously.

Repopulation of Zooxanthellae in the Caribbean corals Montastraea annularis and M. faveolata following experimental and disease-associated bleaching

Caribbean corals of the Montastraea annularis species complex associate with four taxa of symbiotic dinoflagellates (zooxanthellae; genus Symbiodinium) in ecologically predictable patterns. To investigate the resilience of these host-zooxanthella associations, we conducted field experiments in which we experimentally reduced the numbers of zooxanthellae (by transplanting to shallow water or by shading) and then allowed treated corals to recover. When depletion was not extreme, recovering corals generally contained the same types of zooxanthellae as they did prior to treatment. After severe depletion, however, recovering corals were always repopulated by zooxanthellae atypical for their habitat (and in some cases atypical for the coral species). These unusual zooxanthellar associations were often (but not always) established in experimentally bleached tissues even when adjacent tissues were untreated. Atypical zooxanthellae were also observed in bleached tissues of unmanipulated Montastraea with yellow-blotch disease. In colonies where unusual associations were established, the original taxa of zooxanthellae were not detected even 9 months after the end of treatment. These observations suggest that zooxanthellae in Montastraea range from fugitive opportunists and stress-tolerant generalists (Symbiodinium A and E) to narrowly adapted specialists (Symbiodinium B and C), and may undergo succession.

Evidence for three major clades within the snapping shrimp genus Alpheus inferred from nuclear and mitochondrial gene sequence data

The snapping shrimp genus Alpheus is among the most diverse of caridean shrimps, and analyses of taxa separated by the Isthmus of Panama have been used to estimate rates of molecular evolution. Although seven morphological groups have been informally suggested, no formal phylogenetic analysis of the genus has been previously attempted. Here we infer the phylogenetic relationships within Alpheus using sequence data from two nuclear genes, glucose-6-phosphate isomerase and elongation factor-1alpha, and from the mitochondrial gene cytochrome oxidase I. Three major clades corresponding to previously noted morphological features were identified. Discrepancies between earlier informal morphological groupings and molecular analyses largely consisted of species whose morphologies were not entirely typical of the group to which they had been assigned. The traditional placements of shrimp with highly sessile lifestyles and consequently simplified morphologies were also not supported by molecular analyses. Phylogenies for Alpheus suggest that specialized ecological requirements (e.g., symbiotic associations and estuarine habitats) and modified claw morphologies have evolved independently several times. These new analyses also support the sister species status of transisthmian pairs analyzed previously, although very similar pairs were not always resolved with the more slowly evolving nuclear loci. In addition, six new cryptic species were identified in the course of these studies plus a seventh whose status remains to be determined.

Mitochondrial pseudogenes are pervasive and often insidious in the snapping shrimp genus Alpheus

Here we show that multiple DNA sequences, similar to the mitochondrial cytochrome oxidase I (COI) gene, occur within single individuals in at least 10 species of the snapping shrimp genus Alpheus. Cloning of amplified products revealed the presence of copies that differed in length and (more frequently) in base substitutions. Although multiple copies were amplified in individual shrimp from total genomic DNA (gDNA), only one sequence was amplified from cDNA. These results are best explained by the presence of nonfunctional duplications of a portion of the mtDNA, probably located in the nuclear genome, since transfer into the nuclear gene would render the COI gene nonfunctional due to differences in the nuclear and mitochondrial genetic codes. Analysis of codon variation suggests that there have been 21 independent transfer events in the 10 species examined. Within a single animal, differences between the sequences of these pseudogenes ranged from 0.2% to 20.6%, and those between the real mtDNA and pseudogene sequences ranged from 0.2% to 18.8% (uncorrected). The large number of integration events and the large range of divergences between pseudogenes and mtDNA sequences suggest that genetic material has been repeatedly transferred from the mtDNA to the nuclear genome of snapping shrimp. Unrecognized pseudogenes in phylogenetic or population studies may result in spurious results, although previous estimates of rates of molecular evolution based on Alpheus sister taxa separated by the Isthmus of Panama appear to remain valid. Especially worrisome for researchers are those pseudogenes that are not obviously recognizable as such. An effective solution may be to amplify transcribed copies of protein-coding mitochondrial genes from cDNA rather than using genomic DNA.

The future of coral reefs

Coral reefs, with their millions of species, have changed profoundly because of the effects of people, and will continue to do so for the foreseeable future. Reefs are subject to many of the same processes that affect other human-dominated ecosystems, but some special features merit emphasis: (i) Many dominant reef builders spawn eggs and sperm into the water column, where fertilization occurs. They are thus particularly vulnerable to Allee effects, including potential extinction associated with chronic reproductive failure. (ii) The corals likely to be most resistant to the effects of habitat degradation are small, short-lived "weedy" corals that have limited dispersal capabilities at the larval stage. Habitat degradation, together with habitat fragmentation, will therefore lead to the establishment of genetically isolated clusters of inbreeding corals. (iii) Increases in average sea temperatures by as little as 1 degrees C, a likely result of global climate change, can cause coral "bleaching" (the breakdown of coral-algal symbiosis), changes in symbiont communities, and coral death. (iv) The activities of people near reefs increase both fishing pressure and nutrient inputs. In general, these processes favor more rapidly growing competitors, often fleshy seaweeds, and may also result in explosions of predator populations. (v) Combinations of stress appear to be associated with threshold responses and ecological surprises, including devastating pathogen outbreaks. (vi) The fossil record suggests that corals as a group are more likely to suffer extinctions than some of the groups that associate with them, whose habitat requirements may be less stringent.

Variable investment, the Continuous Prisoner's Dilemma, and the origin of cooperation

Cooperation is fundamental to many biological systems. A common metaphor for studying the evolution of cooperation is the Prisoner's Dilemma, a game with two strategies: cooperate or defect. However, cooperation is rare all or nothing, and its evolution probably involves the gradual extension of initially modest degrees of assistance. The inability of the Prisoner's Dilemma to capture this basic aspect limits its use for understanding the evolutionary origins of cooperation. Here we consider a framework for cooperation based on the concept of investment: an act which is costly, but which benefits other individuals, where the cost and benefit depend on the level of investment made. In the resulting Continuous Prisoner's Dilemma the essential problem of cooperation remains: in the absence of any additional structure non-zero levels of investment cannot evolve. However, if investments are considered in a spatially structured context, selfish individuals who make arbitrarily low investments can be invaded by higher-investing mutants. This results in the mean level of investment evolving to significant levels, where it is maintained indefinitely. This approach provides a natural solution to the fundamental problem of how cooperation gradually increases from a non-cooperative state.

The evolution of mutualisms: exploring the paths between conflict and cooperation

Mutualisms are of fundamental importance in all ecosystems but their very existence poses a series of challenging evolutionary questions. Recently, the application of molecular analyses combined with theoretical advances have transformed our understanding of many specific systems, thereby contributing to the possibility of a more general understanding of the factors that influence mutualisms.

Molecular determination of species boundaries in corals: genetic analysis of the Montastraea annularis complex using amplified fragment length polymorphisms and a microsatellite marker

Analyses of DNA have not been widely used to distinguish coral sibling species. The three members of the Montastraea annularis complex represent an important test case: they are widely studied and dominate Caribbean reefs, yet their taxonomic status remains unclear. Analysis of amplified fragment length polymorphisms (AFLPs) and a microsatellite locus, using DNA from sperm, showed that Montastraea faveolata is genetically distinct. One AFLP primer yielded a diagnostic product (880 bp in M. faveolata 920 bp in M. franksi and M. annularis) whose homology was established by DNA sequencing. A second primer revealed a 630 bp band that was fixed in M. faveolata, and rare in M. franksi and M. annularis; in this case homologies were confirmed by Southern hybridizations. A tetranucleotide microsatellite locus with several alleles exhibited strong frequency differences between M. faveolata and the other two taxa. We did not detect comparable differences between M. annularis and M. franksi with either AFLPs (12 primers screened) or the microsatellite locus. Comparisons of AFLP patterns obtained from DNA from sperm, somatic tissues, and zooxanthellae suggest that the technique routinely amplifies coral (animal) DNA. Thus analyses based on somatic tissues may be feasible, particularly after diagnostic differences have been established using sperm DNA.

The evolution of interspecific mutualisms

Interspecific mutualisms are widespread, but how they evolve is not clear. The Iterated Prisoner's Dilemma is the main theoretical tool to study cooperation, but this model ignores ecological differences between partners and assumes that amounts exchanged cannot themselves evolve. A more realistic model incorporating these features shows that strategies that succeed with fixed exchanges (e. g., Tit-for-Tat) cannot explain mutualism when exchanges vary because the amount exchanged evolves to 0. For mutualism to evolve, increased investments in a partner must yield increased returns, and spatial structure in competitive interactions is required. Under these biologically plausible assumptions, mutualism evolves with surprising ease. This suggests that, contrary to the basic premise of past theoretical analyses, overcoming a potential host's initial defenses may be a bigger obstacle for mutualism than the subsequent recurrence and spread of noncooperative mutants.

Landscape ecology of algal symbionts creates variation in episodes of coral bleaching

Reef-building corals are obligate, mutualistic symbioses of heterotrophic animals and phototrophic dinoflagellates (Symbiodinium spp.). Contrary to the earlier, widely accepted belief that corals harbour only one symbiont, we found that the ecologically dominant Caribbean corals Montastraea annularis and M. faveolata can act as hosts to dynamic, multi-species communities of Symbiodinium. Composition of these communities follows gradients of environmental irradiance, implying that physiological acclimatization is not the only mechanism by which corals cope with environmental heterogeneity. The importance of this diversity was underlined by analysis of a natural episode of coral bleaching. Patterns of bleaching could be explained by the preferential elimination of a symbiont associated with low irradiance from the brightest parts of its distribution. Comparative analyses of symbionts before and after bleaching from the same corals supported this interpretation, and suggested that some corals were protected from bleaching by hosting an additional symbiont that is more tolerant of high irradiance and temperature. This 'natural experiment' suggests that temporal and spatial variability can favour the coexistence of diverse symbionts within a host, despite the potential for destabilizing competition among them.