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Pleistocene adaptive radiation in Globorotalia truncatulinoides: genetic, morphologic, and environmental evidence

¹ Colomban de Vargas, Jan Pawlowski. Département de Zoologie et Biologie Animale, Université de Genève, CH-1224 Chêne-Bougeries, Switzerland
² Sabrina Renaud, Heinz Hilbrecht. Geological Institute, ETH, CH-8092 Zürich, Switzerland
³ Present address: Department of Organismic and Evolutionary Biology (OEB), Harvard University, 16 Divinity Avenue, Cambridge, Massachusetts 02138. cvargas{at}oeb.harvard.edu
⁴ Present address: FRE 2158 CNRS, UCB Lyon I, F-69622 Villeurbanne, France

Globorotalia truncatulinoides is an extant species of planktic foraminiferans commonly used for stratigraphic and paleoenvironmental analyses. It originated 2.8 m.y. ago in subtropical areas of the South Pacific, spread to all subtropical and temperate regions of the world ocean, and expanded its range to southern subantarctic waters between 500 and 200 Ka. The wide geographic distribution of G. truncatulinoides is associated with a latitudinal morphological variability considered as an ecophenotypic variation within a single species. Here, we present the first molecular, morphological, and ecological evidence that G. truncatulinoides corresponds to a complex of four genetic species adapted to particular hydrographic conditions. The different species are separated by significant genetic distances in several ribosomal genes (SSU, ITS-1, 5.8S, ITS-2). Species 1 and species 2 characterize subtropical waters, species 3 is abundant exclusively in the Subantarctic Convergence, while species 4 inhabits subantarctic waters. By using an absolute molecular clock, we deduce the time of divergence between the subtropical and frontal/subantarctic species at 300 Ka, which is in agreement with stratigraphic data and suggests an adaptive radiation of the species allowing it to colonize the nutrient-rich and cold subantarctic waters. This genetic dichotomy is associated with a morphological differentiation identified using outline analysis. Species of the same regions are more similar in test shape but can be distinguished by coiling direction. The evolutionary patterns recognized here by combining DNA and morphological analyses from plankton-tow specimens mirror and allow a new interpretation of the data available from Recent sediments. They highlight the importance of adaptation and heterochronic processes, leading to cryptic speciation, in planktic foraminifera.

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