RESUMO
Divergent natural selection driven by competition for limited resources can promote speciation, even in the presence of gene flow. Reproductive isolation is more likely to result from divergent selection when the partitioned resource is closely linked to mating. Obligate symbiosis and host fidelity (mating on or near the host) can provide this link, creating ideal conditions for speciation in the absence of physical barriers to dispersal. Symbiotic organisms often experience competition for hosts, and host fidelity ensures that divergent selection for a specific host or host habitat can lead to speciation and strengthen pre-existing reproductive barriers. Here, we present evidence that diversification of a sympatric species complex occurred despite the potential for gene flow and that partitioning of host resources (both by species and by host habitat) has contributed to this diversification. Four species of snapping shrimps (Alpheus armatus, A. immaculatus, A. polystictus and A. roquensis) are distributed mainly sympatrically in the Caribbean, while the fifth species (A. rudolphi) is restricted to Brazil. All five species are obligate commensals of sea anemones with a high degree of fidelity and ecological specificity for host species and habitat. We analysed sequence data from 10 nuclear genes and the mitochondrial COI gene in 11-16 individuals from each of the Caribbean taxa and from the only available specimen of the Brazilian taxon. Phylogenetic analyses support morphology-based species assignments and a well-supported Caribbean clade. The Brazilian A. rudolphi is recovered as an outgroup to the Caribbean taxa. Isolation-migration coalescent analysis provides evidence for historical gene flow among sympatric sister species. Our data suggest that both selection for a novel host and selection for host microhabitat may have promoted diversification of this complex despite gene flow.
Assuntos
Decápodes/classificação , Fluxo Gênico , Especiação Genética , Filogenia , Anêmonas-do-Mar , Animais , Brasil , Região do Caribe , Núcleo Celular/genética , DNA Mitocondrial/genética , Decápodes/genética , Ecossistema , Modelos Genéticos , Dados de Sequência Molecular , Tipagem de Sequências Multilocus , Isolamento Reprodutivo , Seleção Genética , Especificidade da Espécie , SimpatriaRESUMO
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.
Assuntos
Cnidários/parasitologia , Dinoflagellida/crescimento & desenvolvimento , Filogenia , Animais , Sequência de Bases , Análise por Conglomerados , DNA de Protozoário/química , DNA de Protozoário/genética , DNA de Protozoário/isolamento & purificação , DNA Ribossômico/química , DNA Ribossômico/genética , DNA Ribossômico/isolamento & purificação , Dinoflagellida/química , Dinoflagellida/genética , Dados de Sequência Molecular , Panamá , Reação em Cadeia da Polimerase , Polimorfismo de Fragmento de Restrição , SimbioseRESUMO
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.
Assuntos
Cnidários/parasitologia , Dinoflagellida/crescimento & desenvolvimento , Animais , Sequência de Bases , DNA de Protozoário/química , DNA de Protozoário/genética , DNA de Protozoário/isolamento & purificação , DNA Ribossômico/química , DNA Ribossômico/genética , DNA Ribossômico/isolamento & purificação , Dinoflagellida/química , Dinoflagellida/genética , Luz , Dados de Sequência Molecular , Panamá , Reação em Cadeia da Polimerase , Polimorfismo de Fragmento de Restrição , SimbioseRESUMO
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.
Assuntos
Decápodes/genética , Mitocôndrias/genética , Pseudogenes/genética , Animais , Núcleo Celular/genética , DNA/química , DNA/genética , DNA Mitocondrial/química , DNA Mitocondrial/genética , Decápodes/classificação , Complexo IV da Cadeia de Transporte de Elétrons/genética , Dosagem de Genes , Dados de Sequência Molecular , Mutação , Filogenia , Análise de Sequência de DNAAssuntos
Cnidários/fisiologia , Ecossistema , Ouriços-do-Mar/fisiologia , Animais , Eucariotos/fisiologia , Geografia , Jamaica , Densidade Demográfica , Água do MarRESUMO
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.
Assuntos
Cnidários/genética , Repetições de Microssatélites , Polimorfismo Genético , Animais , Southern Blotting/veterinária , Cnidários/classificação , Eletroforese em Gel de Ágar/veterinária , Amplificação de Genes , Masculino , Dados de Sequência Molecular , Polimorfismo de Fragmento de Restrição , Análise de Sequência de DNA/veterinária , Espermatozoides/químicaRESUMO
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.
Assuntos
Cnidários/fisiologia , Dinoflagellida/fisiologia , Simbiose , Animais , Dinoflagellida/genética , Ecologia , Fotossíntese , Polimorfismo GenéticoRESUMO
Recent taxonomic advances are challenging widely held theories of the ecology and evolution of coral reef Invertebrates and communities. Large numbers of sibling species have been discovered across a variety of higher taxa. Differences in distribution, behavior and life history characteristics among sibling species demonstrate that niche diversification is more finely tuned, and interactions among organisms more specific, than most reef ecologists believed previously. Ecological and evolutionary understanding depends on good taxonomy.
RESUMO
It is widely believed that gene flow connected many shallow water populations of the Caribbean and eastern Pacific until the Panama seaway closed 3.0 to 3.5 million years ago. Measurements of biochemical and reproductive divergence for seven closely related, transisthmian pairs of snapping shrimps (Alpheus) indicate, however, that isolation was staggered rather than simultaneous. The four least divergent pairs provide the best estimate for rates of molecular divergence and speciation. Ecological, genetic, and geological data suggest that gene flow was disrupted for the remaining three pairs by environmental change several million years before the land barrier was complete.
Assuntos
DNA Mitocondrial/genética , Decápodes/genética , Isoenzimas/genética , Filogenia , Alelos , Animais , Sequência de Bases , Decápodes/enzimologia , Feminino , Genótipo , Fenômenos Geológicos , Geologia , Masculino , Dados de Sequência Molecular , PanamáRESUMO
Coral reefs of north Jamaica, normally sheltered, were severely damaged by Hurricane Allen, the strongest Caribbean hurricane of this century. Immediate studies were made at Discovery Bay, where reef populations were already known in some detail. Data are presented to show how damage varied with the position and orientation of the substraturn and with the shape, size, and mechanical properties of exposed organisms. Data collected over succeeding weeks showed striking differences in the ability of organisms to heal and survive.