Global scale same-specimen morpho-genetic analysis of Truncorotalia truncatulinoides: A perspective on the morphological species concept in planktonic foraminifera

Genetic analyses of planktonic foraminifera have unveiled significant levels of cryptic diversity, thus calling into question the usefulness of the morphological species concept for paleoceanographic reconstructions. Here, we present single-specimen combined genetic and morphological analyses performed on living Truncorotalia truncatulinoides collected across the world oceans. A combined morphogenetic analysis allows us to (1) detect five different genetic types (Types I to V) within the morphospecies T. truncatulinoides, (2) statistically analyze shape variations among these genotypes, and (3) assess the biogeographic patterns and the links between surface ocean properties and the distribution of morphological and genetic diversity within T. truncatulinoides. Of the five genetic types, Type I appears to inhabit the warm (sub)tropical waters of the South Hemisphere, Types II and V are found in the warm (sub)tropical waters of the Atlantic and NW Pacific, respectively, and Types III and IV appear to be restricted to the productive subtropical and the cold subpolar frontal zones of the Southern Ocean, respectively. Same-specimen morphogenetic comparisons reveal significant differences in test morphology between the warm (sub)tropical cluster of genotypes (Types I, II, and V) and the colder subpolar cluster of genotypes (Types III and IV). These results indicate that changes in shell conicalness, observed across the subtropical fronts in the Southern Ocean and for a long time interpreted as ecophenotypic variation, reflect genetic differentiation, with large, highly conical left (Indian Ocean) or right-coiled (Pacific Ocean) specimens north of the North Subtropical Front representing genetic Type I, and small, axially compressed and biconvex left-coiled specimens south of this front representing genetic Types III and IV. Our morphogenetic data are consistent with the scenario of a late Pleistocene invasion of the Southern Ocean by newly evolved T. truncatulinoides genotypes, specifically adapted to cold water masses. Finally, we build a model based upon test outline analyses, which correctly assigns up to 75% of the specimens to their corresponding cluster of genotypes. Application of this model to sediment samples may contribute to the reconstruction of migrations of the Subtropical Front during the late Pleistocene. (C) 2011 Elsevier B.V. All rights reserved.