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VOLUME 23, NUMBER 17, SEPTEMBER 2014
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Opinion Reproductive clonality in protozoan pathogens—truth or artefact? J. D. Ramírez & M. S. Llewellyn
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FROM THE COVER 4203
New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies I. B. Baums, M. K. Devlin-Durante & T. C. Lajeunesse
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ORIGINAL ARTICLES 4216
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Population and Conservation Genetics What is genetic differentiation, and how should we measure it—GST, D, neither or both? R. Verity & R. A. Nichols Geographic differences in vertical connectivity in the Caribbean coral Montastraea cavernosa despite high levels of horizontal connectivity at shallow depths X. Serrano, I. B. Baums, K. O’Reilly, T. B. Smith, R. J. Jones, T. L. Shearer, F. L. D. Nunes & A. C. Baker Inbreeding and inbreeding depression in endangered red wolves (Canis rufus) K. E. Brzeski, D. R. Rabon JR, M. J. Chamberlain, L. P. Waits & S. S. Taylor Ecological Genomics Fine-scale population epigenetic structure in relation to gastrointestinal parasite load in red grouse (Lagopus lagopus scotica) M. A. Wenzel & S. B. Piertney Soil fungal communities of grasslands are environmentally structured at a regional scale in the Alps L. Pellissier, H. Niculita-Hirzel, A. Dubuis, M. Pagni, N. Guex, C. Ndiribe, N. Salamin, I. Xenarios, J. Goudet, I. R. Sanders & A. Guisan
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Flowering time QTL in natural populations of Arabidopsis thaliana and implications for their adaptive value E. L. Dittmar, C. G. Oakley, J. Ågren & D. W. Schemske QTL mapping of freezing tolerance: links to fitness and adaptive trade‐offs C. G. Oakley, J. Ågren, R. A. Atchison & D. W. Schemske Speciation and Hybridization Unexpected ancestry of Populus seedlings from a hybrid zone implies a large role for postzygotic selection in the maintenance of species D. Lindtke, Z. Gompert, C. Lexer & C. A. Buerkle Distinct male reproductive strategies in two closely related oak species L. Lagache, E. K. Klein, A. Ducousso & R. J. Petit Integration of conflict into integrative taxonomy: fitting hybridization in species delimitation of Mesocarabus (Coleoptera: Carabidae) C. Andú́jar, P. Arribas, C. Ruiz, J. Serrano & J. Gómez-Zurita Chromosomal variation segregates within incipient species and correlates with reproductive isolation G. Charron, J.-B. Leducq & C. R. Landry Genomics of the divergence continuum in an African plant biodiversity hotspot, I: drivers of population divergence in Restio capensis (Restionaceae) C. Lexer, R. O. Wüest, S. Mangili, M. Heuertz, K. N. Stölting, P. B. Pearman, F. Forest, N. Salamin, N. E. Zimmermann & E. Bossolini Phylogeography Phylogeography of Chinese house mice (Mus musculus musculus/castaneus): distribution, routes of colonization and geographic regions of hybridization M. Jing, H.-T. Yu, X. Bi, Y.-C. Lai, W. Jiang & L. Huang
VOLUME 23, NUMBER 17, SEPTEMBER 2014, pp.4185–4434
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Perspectives Expanding the population genetic perspective of cnidarian‐Symbiodinium symbioses S. R. Santos Species integrity in trees D. Ortiz-Barrientos & E. J. Baack Take up the challenge! Opportunities for evolution research from resolving conflict in integrative taxonomy B. C. Schlick-Steiner, W. Arthofer & F. M. Steiner
VOLUME 23 NUMBER 17 SEPTEMBER 2014
ECOLOGY
NEWS AND VIEWS
MOLECULAR
MOLECULAR ECOLOGY
FROM THE COVER: New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies. See pp. 4203–4215.
Ecological Interactions Contrasting soil fungal community responses to experimental nitrogen addition using the large subunit rRNA taxonomic marker and cellobiohydrolase I functional marker R. C. Mueller, M. M. Balasch & C. R. Kuske Assessing Symbiodinium diversity in scleractinian corals via next‐generation sequencing‐based genotyping of the ITS2 rDNA region C. Arif, C. Daniels, T. Bayer, E. Banguera-Hinestroza, A. Barbrook, C. J. Howe, T. C. Lajeunesse & C. R. Voolstra
Information on this journal can be accessed at http://wileyonlinelibrary.com/journal/mec The journal is covered by AGRICOLA, Chemical Abstracts, Current Awareness Biological Sciences and Current Contents. This journal is available at Wiley Online Library. Visit http://wileyonlinelibrary.com to search the articles and register for table of contents e-mail alerts.
MOLECULAR ECOLOGY
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MOLECULAR ECOLOGY
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Cover Illustration Acropora palmata were once dominant shallow reef building corals in the Northwestern Tropical Atlantic. Population genetic analyses reveal spatial and temporal patterns of genotypic diversity that, combined with unequal gene flow, unleash new insight into the relationship of these huge branching corals and their endosymbiotic dinoflagellates (background photo of A. palmata stand taken in Curacao at twilight by Iliana B. Baums, artwork by Todd C LaJeunesse, Pennsylvania State University).
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Molecular Ecology (2014) 23, 4203–4215
doi: 10.1111/mec.12788
FROM THE COVER
New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies I L I A N A B . B A U M S , M E G H A N N K . D E V L I N - D U R A N T E and T O D D C . L A J E U N E S S E Department of Biology, The Pennsylvania State University, 208 Mueller Laboratory, University Park, PA 16802, USA
Abstract The mutualistic symbioses between reef-building corals and micro-algae form the basis of coral reef ecosystems, yet recent environmental changes threaten their survival. Diversity in host-symbiont pairings on the sub-species level could be an unrecognized source of functional variation in response to stress. The Caribbean elkhorn coral, Acropora palmata, associates predominantly with one symbiont species (Symbiodinium ‘fitti’), facilitating investigations of individual-level (genotype) interactions. Individual genotypes of both host and symbiont were resolved across the entire species’ range. Most colonies of a particular animal genotype were dominated by one symbiont genotype (or strain) that may persist in the host for decades or more. While Symbiodinium are primarily clonal, the occurrence of recombinant genotypes indicates sexual recombination is the source of this genetic variation, and some evidence suggests this happens within the host. When these data are examined at spatial scales spanning the entire distribution of A. palmata, gene flow among animal populations was an order of magnitude greater than among populations of the symbiont. This suggests that independent micro-evolutionary processes created dissimilar population genetic structures between host and symbiont. The lower effective dispersal exhibited by the dinoflagellate raises questions regarding the extent to which populations of host and symbiont can co-evolve during times of rapid and substantial climate change. However, these findings also support a growing body of evidence, suggesting that genotype-by-genotype interactions may provide significant physiological variation, influencing the adaptive potential of symbiotic reef corals to severe selection. Keywords: climate change, Cnidarians, coevolution, contemporary evolution, coral reefs, ecological genetics, population genetics—empirical, species interactions, Symbiodinium Received 9 April 2014; revision received 28 April 2014; accepted 5 May 2014
Introduction Mutualistic associations between host and symbiont species are ubiquitous in nature (Douglas 2010). Tropical reef ecosystems are rich in animals that require dinoflagellate endosymbionts (Symbiodinium) for their survival. Phylogenetic approaches facilitated a turning point and transformed the study of these globally important symbioses (Rowan & Powers 1991). Presently, the genus Symbiodinium comprises numerous Correspondence: Iliana Baums, Fax: (814) 865-9131; E-mail:
[email protected] © 2014 John Wiley & Sons Ltd
species grouped into several divergent sub-generic clades (referred as Clades A, B, C, etc.; Rowan & Powers 1991; Pochon & Gates 2010). We now know that the ecology (e.g. host specificity, water depth), biogeography and physiology are decidedly different among Symbiodinium spp. within and between clades. However, a full understanding of the ecology and evolution of host and symbionts requires sub-species-level resolution of the partners. This is because individual genotypes are the fundamental units of selection. The resolution of individual host and symbiont genotypes over large geographical ranges provides information about population structure, gene flow and patterns of
4204 I . B . B A U M S , M . K . D E V L I N - D U R A N T E and T . C . L A J E U N E S S E association between partners: evidence that is critical to furthering our understanding of the ecological and evolutionary processes important to these mutualisms. Resolving Symbiodinium and hosts to the level of individual genotypes, or clones, can generate insights into how the symbiosis is established and maintained with potential functional consequences. Sub-species-level functional variation has been attributed to the interaction between host and mutualists in a variety of systems such as insects and bacteria (Feldhaar 2011), grasses and endophytes (Gundel et al. 2012), and legumes and Rhizobia (Parker 1995). The variation in stress responses observed among adjacent colonies of the same host species harbouring the same Symbiodinium type (Csaszar et al. 2010; LaJeunesse et al. 2010) suggests that sub-species-level variation in stress responses exists. Individual (genotype or strain)-level differences among host genets on the one hand and symbiont strains on the other, as well as the interactions between them, likely account for physiological variation. The functional diversity contributed by inter-individual combinations of host and symbiont genotypes can also be the target of selection (Rodriguez et al. 2009) and may contribute to the adaptation of mutualisms to environmental change. The spread of better-adapted host and symbiont genotypes depends on the scale over which they successfully disperse. Marked differences in life history between coral and microbe lead to expectations that gene flow and dispersal patterns differ. Branching and massive coral hosts can reproduce asexually through fragmentation over small spatial scales (tens of metres; Highsmith 1982; Willis & Ayre 1985; Foster et al. 2007) as well as sexually through planktonic larvae with much larger potential dispersal ranges (tens of metres to thousands of kilometres; Baums et al. 2005; Underwood et al. 2007). The single-celled symbionts divide and multiply asexually and occasionally reproduce sexually (LaJeunesse 2001; Santos et al. 2004). Symbiodinium cells are constantly emitted/expelled, but the effective dispersal of these cells remains largely unknown. A full understanding of the adaptive potential of host and symbionts requires resolution of the fundamental units of selection in each partner, and knowing the spatial scales over which differently adapted genotypes can disperse. The development and analysis of higher-resolution population genetic microsatellite markers has provided early insights into sub-species-level diversity of symbionts (Andras et al. 2011; Pettay et al. 2011; Pinz on et al. 2011; Thornhill et al. 2014). The initial results from multilocus genotyping recognized the existence of numerous Symbiodinium clones distributed among the colonies of a host population. Different patterns of distribution and abundance of Symbiodinium clones have been
described including that a coral colony often associates with multiple genotypes (Howells et al. 2013) to evidence that indicates most colonies harbour a dominant clonal population represented by a single multilocus genotype (MLG; Thornhill et al. 2009, 2014; Andras et al. 2011; Pettay et al. 2011). Here, we provide an analysis of gene and genotypic diversity across the entire geographic range of a coral– dinoflagellate symbiosis to identify patterns of genotype-by-genotype combinations, dispersal and gene flow that provide insight into the dynamics of animal– microbe mutualisms (Thompson & Cunningham 2002; Mihaljevic 2012). We investigated the individual-level diversity in the Symbiodinium sp. associated with the once dominant but now endangered Caribbean reefbuilding coral, Acropora palmata. A. palmata’s population genetic structure across its entire distribution range consists of just two large populations, one in the eastern and one in the western Greater Caribbean (Baums et al. 2005). Repeated genetic analyses of their dinoflagellate symbionts suggest that the coral is highly specific for the ITS-2 type A3 (LaJeunesse 2002; Thornhill et al. 2006; tentatively designated Symbiodinium ‘fitti’, nomen nudum) despite the fact that the larvae must acquire symbionts from the environment (horizontal transmission). We applied microsatellite markers developed for Symbiodinium ‘fitti’ that allow us to investigate how a combination of geographic, environmental and biotic factors affects the dispersal, formation and stability of specific combinations of host and symbiont genotypes across the Greater Caribbean.
Methods Microsatellite development Microsatellite multi-locus genotypes (MLGs) for A. palmata were of Baums et al. (2005; n = 5 loci). Symbiodinium ‘fitti’ DNA was amplified with primers of Pinz on et al. (2011; n = 10 loci, Table S1, Supporting information). Three additional loci for S. ‘fitti’ were developed here (Table S1, Supporting information). Alleles were fluorescently visualized and sized with internal standards on a PRISM 3100 Genetic Analyzer (Applied Biosystems). Briefly, 25–50 ng of template DNA was added to PCRs containing 19 Standard Taq Buffer (New England Biolabs), 2.5 mM MgCl (New England Biolabs), 0.5 mg/mL Bovine serum albumin (New England Biolabs) and 0.75 U of Taq (0.325 U for primers 31, 32 and 41; New England Biolabs). Primer concentrations of 200 nM of each primer were added to reactions involving loci A3Sym_01, 18, 27 and 28, whereas a primer concentration of 93 nM was added to reactions for primers 31, 32, 41. Primer concentrations of 50 nM of the © 2014 John Wiley & Sons Ltd
H I G H - R E S O L U T I O N G E N E T I C S O F C O R A L S Y M B I O D I N I U M 4205 tailed forward primer (see Table S1, Supporting information), 150 nM of the reverse primer and 75 nM of the dye labelled T-oligonucleotide were used for amplifying loci A3Sym_01, 02, 03, 07, 08, 09 and 48. All loci were amplified using the following thermal cycle profile: 94 °C for 2 min (1 cycle); 94 °C for 15 s, primer-specific annealing temperature (Table S1, Supporting information) for 15 s, 72 °C for 30 s (31 cycles); 72 °C for 30 min on a Mastercycler gradient thermal cycler (Eppendorf, Westbury, NY, USA). PCR primer sets designed for each S. ‘fitti’ locus were screened against DNA from Symbiodinium ITS-2 type A3 cultures. Additionally, S. ‘fitti’ primers were tested on symbiont-free Acropora egg DNA. No amplification of host DNA was observed.
Table 1 Acropora palmata host (NAp) and symbiont (NSf) genotypes were obtained from colonies located throughout the Greater Caribbean Region
Population
East
Bonaire Curacao Puerto Rico SVG USVI Bahamas Dominican Republic Florida Mona Mexico Navassa Panama 12
West
Total
NPlots 2 6
3 5 4 1 1 3 25
NSf
NAp
38 132 13 8 67 105 2 173 2 37 21 66 664
34 117 13 8 61 96 2 155 2 28 21 55 592
Sampling schemes We characterized the population genetic and clonal structure of S. ‘fitti’ at various spatial scales. First, we took samples from multiple locations within a colony and then we considered reef- and region-scale clonal reproduction using randomly sampled polar plots (15 m diametre). We utilized A. palmata samples archived from around the Caribbean basin (Table 1 and Table S2, Supporting information, Baums et al. 2005). A subset of these samples (~700) was collected to examine the extent to which A. palmata propagate through fragmentation (Baums et al. 2006). In a stand of A. palmata, 24 colonies were mapped and sampled randomly using a circular transect (polar plot). Each plot consisted of nested 5-, 10- and 15-m-radius circles resulting in spatially explicit random sampling biased towards detection of genet diversity within the 5-m-radius circle. If the size of the A. palmata stand allowed it, two plots were collected per reef. Samples were collected from several reefs per region (Table 1 and Table S2, Supporting information). This design kept sampling effort constant across sites and thus allowed us to compare the contribution of clonal reproduction to the population of host versus symbionts. Additional samples were collected haphazardly to investigate gene flow patterns. Details of the sampling procedure are given in Baums et al. (2006).
Genotype diversity We refer to an assemblage of genetically identical colonies (clones) that are descendants of a single zygote as a ‘genet’. Physiologically distinct colonies that can function and survive on their own but belong to the same genet are termed ‘ramets’. Symbiodinium ‘fitti’ is a haploid, single-celled eukaryote that replicates via cell division, thus generating clonal cell lines. Different MLGs © 2014 John Wiley & Sons Ltd
Some reefs were sampled randomly in circular plots (NPlots) with constant sampling effort (n = 24 colonies per plot). Colonies sampled multiple times are listed only once, and colonies with multiple infections are excluded (Fig. 1).
of S. ‘fitti’ are referred to as strains. The microsatellite markers for the host are highly heterozygous (mean observed heterozygosity = 0.77); thus, there is a low probability of identifying two colonies as clonemates when in fact they are distinct genets (this is called the probability of identity (PI) and equals 10 7 for A. palmata). The PI for the diploid A. palmata was calculated in GENALEX version 6.4 (Peakall & Smouse 2006). The PI for S. ‘fitti’ was calculated as the sum of each allele’s frequency squared in each population and multiplied across loci. For the haploid S. ‘fitti’, the probability of identity equalled 10 5. After inspecting a plot of pairwise differences in genetic distance among all samples (Fig. S1, Supporting information), we determined that only those MLGs were clonemates that shared identical alleles at all loci (see Appendix S1 for justification of approach, Supporting information). Genet identification and calculation of diversity indices were performed in GENALEX version 6.4 and GENODIVE (Table S3, Supporting information, Meirmans & Van Tienderen 2004).
Linkage disequilibrium Tests of linkage disequilibrium (LD) were conducted using Genepop on the web 4.0 (Rousset 2008) to ascertain whether recombination occurs in Symbiodinium ‘fitti’ genotypes (Santos & Coffroth 2003). We tested the null hypothesis of random association between two loci via a log-likelihood ratio test. High values of LD are expected when clonal reproduction is high and all MLGs are considered. Once repeated MLGs are removed, LD is expected to decline in recombining
4206 I . B . B A U M S , M . K . D E V L I N - D U R A N T E and T . C . L A J E U N E S S E organisms. Indeed, S. ‘fitti’ appears to undergo sexual recombination: LD was nearly absent when considering only the 176 unique MLGs (10 of 78 tests were significant at P < 0.05 after Bonferroni correction). As expected for a species with mixed asexual and sexual reproduction, almost all pairwise locus comparisons became significant for LD when including all 664 samples with complete MLGs (71 of 78 tests were significant at P < 0.05 after Bonferroni correction).
Clonal structure Considering all genotypes sampled in random plots, we performed a spatial autocorrelation analysis assuming 15 even distance classes ranging from 1 to 15 m (the radius of the plots) in GENALEX version 6.5. Spatial autocorrelation analysis describes the spatial extent or neighbourhood size of genets by calculating the pairwise genetic distances among samples for each of a number of distance classes and correlates those with the geographic distance between the samples. The resulting correlogram depicts the change in correlation (r) between genetic and geographic distance over the
range of distance classes. A spatial heterogeneity test (Banks & Peakall 2012) was used to test the null hypothesis of no difference between the correlograms derived from S. ‘fitti’ and from A. palmata (999 permutations).
Genetic population structure Clustering methods that use unique MLGs were employed to estimate the number of different populations in the data set albeit the symbiont is haploid, violating model assumptions (Pritchard et al. 2000). We then used analysis of molecular variance (AMOVA) to compare the amount of variation distributed within and between populations when grouping S. ‘fitti’ populations by the Structure results versus grouping S. ‘fitti’ populations according to the previously identified eastern and western host population (Table 2). A similar AMOVA was performed on all genotypes, and pairwise PhiPT values were calculated among populations (Peakall & Smouse 2006). Results were qualitatively similar to the unique MLG data set (Table S4, Supporting information).We also constructed a median-joining
Table 2 Population differentiation of Symbiodinium ‘fitti’ across 10 populations in the Greater Caribbean (A) Source
d.f.
SS
MS
Var.
%
Φ
Among population Within population Total
9 166 175
185.20 408.72 593.92
20.58 2.46
1.07 2.46 3.53
30 70 100
0.30**
(B) Source
d.f.
SS
MS
Var.
%
Φ
Between regions Among population Within population Total
1 8 166 175
38.77 146.43 408.72 593.92
38.77 18.307 2.46
0.14 0.99 2.46 3.59
4 27 69 100
0.04** 0.29** 0.32**
(C) ΦPT
Bonaire
Curacao
SVG
USVI
Bahamas
Florida
Mexico
Navassa
Puerto Rico
Curacao SVG USVI Bahamas Florida Mexico Navassa Panama Puerto Rico
0.127 0.407 0.168 0.382 0.410 0.397 0.351 0.226 0.126
0.258 0.165 0.235 0.320 0.305 0.214 0.142 0.135
0.362 0.414 0.491 0.462 0.425 0.330 0.385
0.334 0.393 0.409 0.203 0.255 0.159
0.182 0.421 0.245 0.353 0.325
0.383 0.339 0.441 0.340
0.375 0.349 0.411
0.272 0.247
0.211
Dominican Republic (n = 1) and Mona (n = 2) were excluded because of small sample sizes. Given are the results of an AMOVA when considering all populations separately (A) and according to the east/west regions of the host (B). Pairwise comparisons (C) of population differentiation (ΦPT) were significant between all populations (P < 0.05). d.f., degrees of freedom; SS, sum of squares; MS, mean sum of squares; Var., estimated variance; Φ, fixation index. **P < 0.001. © 2014 John Wiley & Sons Ltd
H I G H - R E S O L U T I O N G E N E T I C S O F C O R A L S Y M B I O D I N I U M 4207
Multiple alleles To assess the prevalence of multiple infections per A. palmata colony, we considered a data set containing all samples for which we attempted genotyping of primers 1, 3, 7, 9, 18, 27 and 28 (n = 759). Any samples that showed multiple alleles at those loci were not run for the remaining loci and thus the analysis would be skewed if we were to include samples that had complete or nearly complete MLGs for all 13 loci. In this data set, we found n = 124 samples with multiple alleles at one or more loci (16%). We tested the hypothesis of no difference in the proportion of multiple infections per colony per region (Bonferroni adjusted) and between the eastern and western Greater Caribbean using a Pearson chi-Square test. See Appendix S2 (Supporting information) for further details and justification of approach.
defined by 13 microsatellite loci; Fig 1a) across the distributional range of this coral (Table 1 and Table S2, Supporting information). This symbiont type was recently designated a provisional binomial Symbiodinium ‘fitti’ and is awaiting formal description by the authors. PCR amplification of these loci was often sensitive enough to distinguish between two co-occurring S. ‘fitti’ strains when the DNA concentration of the minor strain was >5% depending on locus and allele size differences (Fig. S2, Supporting information). We acknowledge that additional strains may have occurred
(a) –90º
–80º
–70º
Florida Keys
126
–60º >200 101-200 51-100
Bahamas
10-50 100 km. Thus, clonal propagation occurred over smaller spatial scales in host populations then in populations of the symbiont. Gene flow among locations across the Greater Caribbean was assessed for S. ‘fitti’ using an expanded, haphazardly sampled data set but including only unique MLGs (n = 176). Symbiont gene flow was moderately influenced by geographic distance (Mantel test between geographic distance and genetic distance, r2 = 0.44, P < 0.01). Much stronger genetic differentiation was observed in S. ‘fitti’ compared to its host (Baums et al. 2005) based on the analysis of 176 strains recovered from ten regions (Table 2, ΦPT = 0.30, P < 0.001). Struc-
© 2014 John Wiley & Sons Ltd
ture identified K = 7 as the most likely number of S. ‘fitti’ clusters (Fig. 4a) compared to two host clusters (Baums et al. 2005). S. ‘fitti’ strains from Florida, the Bahamas, Panama and Mexico comprised separate cohesive genetic units. The Virgin Islands and St. Vincent and the Grenadines belonged mostly to the same inferred population. Navassa had admixed genotypes. One set of genotypes (i.e. the ‘blue’ cluster in Fig. 4a, K = 7) contributed to populations in Puerto Rico, Bonaire and Curacßao. An additional cluster (‘yellow’) was found at low frequency but with high individual membership probabilities in Curacßao (Fig 4a). A network analysis of S. ‘fitti’ (Fig 4b) differentiated northern (Bahamas, Florida) and southwestern (Mexico, Panama) populations. Pairwise comparisons of ΦPT values were significant in all cases (ranging from 0.126 between Bonaire and Puerto Rico to 0.491 between Florida and St. Vincent and the Grenadines, Table 2c).
4210 I . B . B A U M S , M . K . D E V L I N - D U R A N T E and T . C . L A J E U N E S S E West
East
West
BA
VI
SV
FL
N A PR
K=7
K=4
Prob of membership
K=2
(a)
BO
CU
PA
ME
(b) Bahamas (BA) Bonaire (BO) Curacao (CU) Florida (FL) Mexico (ME) Navassa (NA)
relatively stable over time and space even at the genotype/strain level, and only occasional displacement of one strain by another was observed. In cases where strains did co-infect colonies, we found evidence of sexual recombination between them. Such temporal and spatial uniformity of many A. palmata host-symbiont genotype combinations, especially at the edges of A. palmata’s northern range, may be explained by either a lack of symbiont diversity in these regions or environmental selection for certain pairings. By recognizing that genotypic diversity (numbers of strains) and effective gene flow among symbiont populations are considerably less than the host, we may begin to infer the ecological and evolutionary response of these symbioses to climate change with greater precision.
Panama (PA) Puerto Rico (PR) St. Vincent/Grenadines (SV) Virgin Islds (VI)
30 10
Fig. 4 The genetic structure of Symbiodinium ‘fitti’ (based on 13 microsatellite loci, n = 176 distinct multilocus genotypes, MLGs) obtained from locations throughout the Greater Caribbean. The results from Structure analyses (a) of unique MLGs at K-values of 2, 4, and 7 show increasing population differentiation that mostly relates to regions. Initially, the Florida Keys and Bahamas cluster as distinct from the rest of the Caribbean (K = 2). At the optimal K = 7 (Evanno et al. 2005), S. ‘fitti’ MLGs from the Bahamas were distinguished from the Florida Keys, Yucatan Mexico arose as unique from the rest of the Greater Caribbean, and there is indication that some proportion of the populations in Panama (light brown membership) and Curacao (yellow membership) are distinctive. Host MLGs (Acropora palmata) are divided into just two populations (West and East, Baums et al. 2005). Given is the probability of membership (y-axis, from 0–1) in each cluster, K for each multilocus genotype (x-axis). (b) A median-joining network analysis shows the genetic variation and relative similarity among S. ‘fitti’ MLGs from all sample locations. Circle size is proportional to the number of times a MLG was observed and circle colour corresponds to geographic origin. MLGs were never found in more than one location. Locations with
