Delimitación de las especies de dictyota (phaeophyceae), combinando estudios moleculares, morfológicos y ecológicos

Resumen

Representatives of the brown algal genus Dictyota form an important component of seaweed assemblages in tropical and subtropical seas. Although being a conspicuous component of these floras, identifying the species of this genus is problematic due to the high morphological plasticity and the absence of diagnostic features that characterise the individual species. These confusing and overlapping patterns of phenotypic variation have seriously confounded estimates of species diversity over the last two centuries. In the Canary Islands, Dictyota has a relevance not only in terms of diversity but also in biomass, where they form large populations in eulittoral pools and shallow sublittoral. The main goal of this thesis was to delineate the species of Dictyota in the Canary Islands, from a combined molecular, morphological and ecological approach. In order to obtain a broader overview of this genus, the geographical framework was extended to other Macaronesian archipelagos, the European Atlantic, Mediterranean and to a lesser extent the Caribbean Sea. In Chapter 2 the occurrence of D. dichotoma, the type species of the genus, in the Canary Islands is confirmed by nuclear ribosomal sequence data (LSU rDNA). An exhaustive analysis of several quantitative characters reveal significant morphological variation both seasonally and between life-cycle phases. A temporal displacement of favourable and resting periods across the distribution range of the species is detected, the Canarian population showing the favourable period in the coldest season (winter) and the resting period in the warmest season (autumn). Phenological data indicate that the optimum reproductive stage occur in winter, while the optimum vegetative stage in summer. Fertile thalli were always dominant, sporophytes outnumbering gametophytes throughout the year. Sexual reproduction has a relevant role, as gametophytes made up to 25% of the total of the population. In Chapter 3 species boundaries of continental European Dictyota are delineated using a densely sampled DNA dataset of nearly 400 sequences (psbA). Six evolutionarily significant units (ESUs) are identified: D. dichotoma, D. fasciola, D. implexa, D. mediterranea, D. spiralis and the newly described D. cyanoloma sp. nov. Phylogenetic relationships are interpreted using a multigene dataset (LSU rDNA, psbA, rbcL, cox1, cox3, nad1) containing a single representative per species. Species distributions, based on DNA-confirmed occurrence records, indicate that all species were geographically confined to the NE Atlantic Ocean with the exception of D. dichotoma and D. implexa which also occur in South Africa and Bermuda, respectively. Potential natural hybridization between D. dichotoma and D. implexa is investigated by comparison of the genetic signatures, derived from chloroplast (psbA), mitochondrial (cox1) and nuclear markers (LSU rDNA). Failure to detect natural hybrids indicate that effective pre- and postzygotic isolation mechanisms are at play in natural populations and support the by-product hypothesis of reproductive isolation. In Chapter 4 cryptic diversity is reported from the Canary Islands in two sympatric Dictyota species, D. dichotoma and D. cymatophila sp. nov. Gene sequence data (LSU rDNA, psbA, rbcL, cox1, cox3, nad1) demonstrate that D. dichotoma and D. cymatophila do not represent sister species. Rather, D. cymatophila and D. dichotoma have converged on a nearly identical morphology. Multivariate analyses reveal sinificant differences between the two species in the sizes of medullary cells and cortical cells as well as branching angles of axes. Spatial and temporal niche partitioning is taking place between both species. They co-occur in eulittoral pools and the shallow sulittoral in Tenerife. Even though D. cymatophila is more dominant in wave-exposed places and D. dichotoma in less exposed areas, the spatial distribution of both species overlap in intermediate habitats. Dictyota dichotoma reaches its highest density in winter and early spring and disappears nearly completely in autumn, while D. cymatophila dominates the study site from July until November. The timing of gamete release also differ between both species, D. dichotoma releasing gametes twice every lunar cycle while the release of gametes in D. cymatophila occurred roughly every other day. In Chapter 5 species boundaries within the tropical to warm-temperate D. ciliolata - D. crenulata complex are refined using a Generalized Mixed Yule Coalescent modelling (GMYC) method. Distribution ranges based on molecular data were reassess and the realized geographical range of the respective species is interpreted in relation to their thermal tolerance and major tectonic and climatic events during the Cenozoic. Considerable conflict between traditional and DNA-based species definitions is found. Dictyota crenulata consists of four pseudocryptic species, which have restricted distributions in the Atlantic Ocean and Pacific Central America. In addition, several non-dentate Dictyota species are found to fall within the D. crenulata clade. In contrast to D. crenulata, the pantropical distribution of D. ciliolata is confirmed by DNA data. The origin and rapid diversification of Dictyota is inferred to be in the Paleocene_Eocene, possibly as a response to increased herbivory. Estimated mean node ages indicate that the D. crenulata clade diverged gradually from the late Oligocene onwards. Dictyota ciliolata diverged from its sister species ca. 12 Ma. Amphi-Atlantic distributions of several species provide evidence for long distance dispersal. However, biogeographical asymmetry between D. ciliolata and the D. crenulata-complex can be best explained by differences in thermal tolerance range. The nearly circumtropical range of D. ciliolata results from its tolerance towards relatively low minimal temperatures which allows it to disperse over cold water barriers. In Chapter 6 the pseudocryptic species of the Dictyota ciliolata - D. crenulata complex, are morphologically characterised and the classification is adapted, based on the molecular phylogenetic results presented in Chapter 5, and the re-examination of herbarium and fresh specimens. A comparative morphological analysis of the dentate species of Dictyota is provided. A higher taxonomic rank is given to the variety D. crenulata var. canariensis raising it here to D. canariensis. The taxonomic statuses of D. kohlmeyeri, D. menstrualis and D. plectens are discussed and their assignment as synonyms of D. ciliolata are addressed. The following names are accommodate to the genetic lineages: D. crenulata#1, which is restricted to the Pacific coast of Central America, is considered as genuine D. crenulata; D. canariensis is applied to D. crenulata#2, which is restricted to Macaronesia; D. jamaicensis is applied to D. crenulata#3, which has an amphi-Atlantic distribution; and D. crenulata#4 is described as the new species D. pleiacantha from the Canary Islands. In Chapter 7 an identification key to the North Atlantic, Mediterranean and Caribbean species of Dictyota is provided, based on the results of this thesis. It comprises 16 species, of which Dictyota sp. and the D. pfaffii - D. humifusa complex are still under study. In the construction of this key, emphasis is placed on the use of characters which are easily recognisable in the field and in the laboratory. Reproductive features are only used to separate D. fasciola and D. spiralis. Although results of Chapter 3 indicate that the recognition of varieties is not genetically supported in D. dichotoma, two morphologies are referred to in order to help identifying the species.