Dictyotales, Phaeophyceae)1
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J. Phycol. 46, 1301–1321 (2010) Ó 2010 Phycological Society of America DOI: 10.1111/j.1529-8817.2010.00908.x SPECIES DELIMITATION, TAXONOMY, AND BIOGEOGRAPHY OF DICTYOTA IN EUROPE (DICTYOTALES, PHAEOPHYCEAE)1 Ana Tronholm2 Departamento de Biologı´a Vegetal (Bota´nica), Universidad de La Laguna, 38271 La Laguna, Canary Islands, Spain Frederique Steen, Lennert Tyberghein, Frederik Leliaert, Heroen Verbruggen Phycology Research Group and Centre for Molecular Phylogenetics and Evolution, Ghent University, Krijgslaan 281, Building S8, 9000 Ghent, Belgium M. Antonia Ribera Siguan Unitat de Bota`nica, Facultat de Farma`cia, Universitat de Barcelona, Joan XXIII s ⁄ n, 08032 Barcelona, Spain and Olivier De Clerck Phycology Research Group and Centre for Molecular Phylogenetics and Evolution, Ghent University, Krijgslaan 281, Building S8, 9000 Ghent, Belgium Taxonomy of the brown algal genus Dictyota has a supports the by-product hypothesis of reproductive long and troubled history. Our inability to distin- isolation. guish morphological plasticity from fixed diagnostic Keyindexwords:biogeography;Dictyota;Dictyotales; traits that separate the various species has severely diversity; molecular phylogenetics; taxonomy confounded species delineation. From continental Europe, more than 60 species and intraspecific taxa Abbreviations: AIC, Akaike information criterion; have been described over the last two centuries. BI, Bayesian inference; BIC, Bayesian information Using a molecular approach, we addressed the criterion; GTR, general time reversible; ML, diversity of the genus in European waters and made maximum likelihood necessary taxonomic changes. A densely sampled DNA data set demonstrated the presence of six evo- lutionarily significant units (ESUs): Dictyota dichotoma Species of the genus Dictyota J. V. Lamour., along (Huds.) J. V. Lamour., D. fasciola (Roth) J. V. with other Dictyotales, are key components of many Lamour., D. implexa J. V. Lamour., D. mediterranea coastal ecosystems (Lu¨ning 1990). Due to their effi- (Schiffn.) G. Furnari, D. spiralis Mont., and the cient chemical defense systems, which involve newly described D. cyanoloma sp. nov., which was various diterpenes as well as gaseous volatiles (Hay previously reported as D. ciliolata from the Mediter- et al. 1987, Wiesemeier et al. 2007), and their ability ranean Sea. Species distributions, based on to propagate successfully by fragmentation (Herren DNA-confirmed occurrence records, indicate that all et al. 2006), the species are able to maintain a sig- species are geographically confined to the NE nificant biomass under high grazing pressure and Atlantic Ocean with the exception of D. dichotoma therefore have an important role in the structuring and D. implexa, which also occur in South Africa of benthic communities in tropical and temperate and Bermuda, respectively. To investigate potential ecosystems. Despite their prevalence and ecological hybridization between D. dichotoma and D. implexa, importance, species-level taxonomy is long and trou- which were previously shown to be sexually compati- bled. ble in culture, we compiled and analyzed sets of Species are routinely used as fundamental units mitochondrial, plastid, and nuclear markers to of ecological surveys, conservation biology, biogeog- detect putative hybrids or introgression in natural raphy, and macroevolution, yet the empirical delimi- populations. Failure to detect natural hybrids indi- tation of species can pose serious difficulties (Wiens cates that effective pre- and postzygotic isolation 1999, Agapow et al. 2004, Sites and Marshall 2004). mechanisms are at play in natural populations and Defining species boundaries is inherently linked to the species concept applied. Despite extensive dis- 1Received 13 January 2010. Accepted 30 June 2010. putes over species concepts, most biologists agree 2Author for correspondence: e-mail [email protected]. that species are lineages, and what previous authors 1301 1302 ANA TRONHOLM ET AL. have generally disagreed about are the best criteria D. dichotoma. Caribbean representatives of the for recognizing these lineages (Mayden 1997, D. dichotoma complex were segregated and described De Queiroz 1998). As with most algal groups, the as D. pulchella Ho¨rnig and Schnetter (1988). Even morphological species concept has dominated though this tropical species shares its chromosome systematics in Dictyota. Species are recognized by number with D. dichotoma, it appears to be partially discontinuities in morphological characters, but our reproductively isolated, the sporophyte generation inability to distinguish morphological plasticity from resulting from reciprocal crosses being unable to diagnostic traits that separate the various species has produce viable tetraspores. severely confounded species delimitation. The lack The application of a biological species concept in of understanding of the variability of morphological Dictyota yielded an orderly and simple classification. characters has led to an erroneous taxonomy However, the ability of different strains to produce whereby specimens on the fringes of the morpho- viable offspring under artificial conditions, though logical spectrum have habitually been described as meaningful in itself, does not necessarily reflect nat- different species (De Clerck and Coppejans 1999) ural conditions. It is obvious that the evolution of or where different species were lumped into a single reproductive isolation is far more complex than entity on the basis of superficial similarity. From gametic compatibility alone (Coyne and Orr 2004). continental Europe, more than 60 species and intra- Various pre- and postzygotic reproductive barriers specific taxa have been described over the last two may interact to reduce or completely prevent the centuries (Ho¨rnig and Schnetter 1988, De Clerck formation of hybrid offspring between two sexually 2003). It is generally accepted that this number is a compatible species. It is therefore of interest to test gross overestimation of the real taxonomic diversity if the observed gametic compatibility of the D. dicho- caused by misinterpretation of morphological plas- toma complex extends to the formation of hybrids ticity. There is currently no consensus as to how in natural populations also. The detection of many species are present. Along the Atlantic coast hybrids or introgression in natural populations has of mainland Europe, reports are limited to two become feasible by applying molecular techniques. species: D. dichotoma, a common species ranging as Differential inheritance patterns of DNA from dif- far north as southern Norway (Hoek 1982), and ferent genomic compartments offer an effective tool D. spiralis, which is probably common along the Por- to detect putative hybrids or the lack thereof (Coyer tuguese coast but known only from scattered locali- et al. 2002a, Fraser et al. 2009, Niwa et al. 2009, ties in more northern reaches (Newton 1931, Gayral Tellier et al. 2009). In brown algae, mtDNA appears 1966, Ardre´ 1970, Hardy and Guiry 2003, Araujo to be maternally inherited (Motomura 1990, Coyer et al. 2009). The Mediterranean Sea is thought to et al. 2002b, Peters et al. 2004, Kato et al. 2006), be harbor a much higher diversity than the Atlantic it that paternal leakage has recently been demon- coast, but there is no consensus on how many strated in a hybrid zone between two closely related species are present there. Fucus species (Hoarau et al. 2009). The inheritance In an attempt to clarify the taxonomy of Dictyota pattern of chloroplast DNA is likely correlated with in the North Atlantic, Schnetter and coworkers the morphological differentiation of male and employed a biological species concept in combina- female gametes. Inheritance is biparental in isoga- tion with karyological observations and morphologi- mous species (Peters et al. 2004, Kato et al. 2006); cal analyses. Schnetter et al. (1987) and Ho¨rnig and all oogamous brown algae examined to date inherit Schnetter (1988) recognized three Mediterranean chloroplasts maternally. It is therefore assumed that species, D. fasciola, D. spiralis, and the morphologi- in Dictyota chloroplasts are maternally transmitted. cally extremely variable D. dichotoma. Slender growth Besides the conceptual issues related to delimit- forms of the latter species were named D. dichotoma ing species, several problems regarding the diversity var. intricata (C. Agardh) Grev. While there was rela- of Dictyota in Europe persist. First, the presence of tively little doubt about the identities of D. fasciola only three species along the coasts of mainland and D. spiralis, the studies of Schnetter and cowork- Europe and the Mediterranean Sea was never unani- ers applied a very broad concept of D. dichotoma. mously accepted. Taxa such as D. linearis and Previously, phycologists had recognized several D. mediterranea, which were considered synonyms of, species in the D. dichotoma complex, D. divaricata respectively, D. dichotoma and D. fasciola by Ho¨rnig J. V. Lamour., D. linearis (C. Agardh) Grev., and et al. (1992a,b), continue to be recognized by sev- D. pusilla J. V. Lamour. being the most commonly eral authors (Furnari et al. 1999, Pena Martı´n et al. applied names. Chromosome counts and crossing 2004, Rull Lluch et al. 2005, Serio et al. 2006). Sec- experiments revealed a haploid chromosome num- ond, the geographic distribution of the various spe- ber of n = 16 and full interfertility for all European cies remains uncertain. Literature reports suggest specimens of the D.