DNA ANALYSIS: A NEW TOOL TO IDENTIFY THE ECOLOGICAL MEANING OF MORPHOLOGY IN PLANKTONIC FORAMINIFERA

The common use of the planktonic foraminiferal fossil archive as a stratigraphic and paleoceanographic tool led to a taxonomy based exclusively on morphological characters of the calcareous tests. However, it overlooks much of the biological and physiological features of these organisms. Recent application of DNA PCR amplification and sequencing methods to pelagic organisms elucidates new, molecular levels of planktonic diversity not necessarily apparent in the morphological phenotype of closely related species. Global sampling of ribosomal genes in three species of planktonic foraminifera -Orbulina universa, Globigerinella siphonifera, and Globorotalia truncatulinoides- illustrates that the classical definition of species hides higher levels of genetic and ecological differentiation: (1) each morphological species comprises a complex of three to four distinct genotypes, which diverged back in the Late Miocene according to molecular clock; (2) the individuals within a genotype are genetically identical within the srDNA, regardless of segregation by huge geographic distances, current belts, major tectonic barriers, and in spite of co-existence with other genotypes at several sites; (3) the diverse genotypes within a morphotype display specific, generally allopatric biogeographic distributions, related to different degrees in the stability of the water mass (mainly vertically mixed versus stratified systems). Thus, our molecular data suggest that the traditional taxa of planktonic foraminifera comprise clusters of three to four genetic species displaying important ecological differences without obvious shell differentiation. These differences may reflect adaptations to different food or symbiotic web structures, expressed at the molecular/physiological level rather than morphological level. The recognition of this diversity within the sediment record may allow us to greatly refine the use of foraminifera as paleoceanographic proxies. I will discuss which subtle morphological and/or chemical features of the shells may be used to discriminate between the foraminiferal genetic species. Based on molecular and stratigraphic clocks, I will also illustrate the general timing of morpho-genetic evolution in Neogene planktonic foraminifera, as well as its environmental meaning.