Especiación En Algas

Pseudo-cryptic speciation in coccolithophores
´ Alberto G. Saez†‡, Ian Probert§, Markus Geisen†¶, Patrick Quinn , Jeremy R. Young¶, and Linda K. Medlin†
Wegener Institute for Polar and Marine Biology, Department of Biological Oceanography, Am Handelshafen 12, D-27570 Bremerhaven, Germany; de Biologie et Biotechnologies Marines, Universite de Caen Basse Normandie, Esplanade de la Paix, 14032,Caen Cedex, France; ´ ¶Palaeontology Department, The Natural History Museum, Cromwell Road, London SW7 5BD, England; and Geological Institute, Sonneggstrasse 5, ETH, CH-8008 Zurich, Switzerland
§Laboratoire †Alfred

Communicated by W. A. Berggren, Woods Hole Oceanographic Institute, Woods Hole, MA, April 9, 2003 (received for review August 19, 2002)

Coccolithophores are a group of calcifyingunicellular algae that constitute a major fraction of oceanic primary productivity, play an important role in the global carbon cycle, and are key biostratigraphic marker fossils. Their taxonomy is primarily based on the morphology of the minute calcite plates, or coccoliths, covering the cell. These are diverse and include widespread fine scale variation, of which the biological taxonomicsignificance is unknown. Do they represent phenotypic plasticity, genetic polymorphisms, or species-specific characters? Our research on five commonly occurring coccolithophores supports the hypothesis that such variation represents pseudocryptic speciation events, occurring between 0.3 and 12.9 million years ago from a molecular clock estimation. This finding suggests strong stabilizing selection acting oncoccolithophorid phenotypes. Our results also provide strong support for the use of fine scale morphological characters of coccoliths in the fossil record to improve biostratigraphic resolution and paleoceanographic data retrieval.

arine planktonic protists often present subtle morphological differences within species that are traditionally considered cosmopolitan, e.g., Skeletonema costatum (1).[By ‘‘species’’ we refer to a reproductively isolated group, i.e., the biological species concept (2).] This raises questions about the heritability of these differences, or if heritability is accepted, about their polymorphic or fixed nature. The answers to these questions may have important implications for various aspects of the study of marine plankton: in assessing biodiversity, in evaluatingevolutionary and ecological diversification, and in interpreting the fossil record when the organisms involved have mineralized skeletons. Coccolithophores are unicellular calcifying algae, members of division Haptophyta (3). In open oceanic environments they constitute a significant fraction of the phytoplankton and have an exceptionally rich fossil record spanning the last 200 million years (4).Coccolihophores impact greatly on marine ecosystems, and hence on the global carbon cycle (5). A comprehensive species-level taxonomy based primarily on coccolith morphology has been established (6–9) and widely applied over the past decade to studies of their ecology, biogeography, and sediment fluxes. When these morphological criteria are used, most species are well differentiated and have verybroad, interoceanic distributions. However many exhibit fine-scale morphological variation: some are formally recognized as varieties, whereas others have been informally differentiated often after intensive morphometric study (e.g., ref. 10). Some authors, however, based on morphological grounds, have suggested that some of these taxa represent genuine species, or subspecies (e.g., refs. 11 and12). But overall, the biological significance of such fine-scale morphological variation has remained unclear, despite the considerable interest to palaeontologists seeking to maximize information recovery from their fossil record. Four scenarios can be envisaged to explain the morphological variation: (i) phenotypic variation as a response to environmental factors, (ii) variation associated...