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

Globorotalia truncatulinoides is an extant species of planktic foraminifera ns commonly used for stratigraphic and paleoenvironmental analyses. It orig inated similar to2.8 m.y. ago in subtropical areas of the South Pacific, sp read to all subtropical and temperate regions of the world ocean, and expan ded its range to southern subantarctic waters between 500 and 200 Ka. The w ide geographic distribution of G. truncatulinoides is associated with a lat itudinal morphological Variability considered as an ecophenotypic variation within a single species. Here, we present the first molecular, morphologic al, and ecological evidence that G. truncatulinoides corresponds to a compl ex of four genetic species adapted to particular hydrographic conditions. T he different species are separated by significant genetic distances in seve ral ribosomal genes (SSU, ITS-1, 5.85, ITS-2). Species I and species 2 char acterize subtropical waters, species 3 is abundant exclusively in the Suban tarctic Convergence,while species 4 inhabits subantarctic waters. By using an absolute molecular crock, we deduce the time of divergence between the s ubtropical and frontal/subantarctic species at similar to 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 differen tiation identified using outline analysis. Species of the same regions are more similar in test shape but can be distinguished by coiling direction. T he 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.