THE CRYPTIC GENETIC TYPES OF NEOGLOBOQUADRINA PACHYDERMA AND THEIR EVOLUTIONARY HISTORY

DNA sequence analysis of planktonic foraminifera shows that shell morphology is not a true measure of species diversity. Molecular phylogenies generated from small subunit ribosomal RNA gene sequences amplified from living specimens reveal a high degree of cryptic diversity within morphologically characterised species. The morphospecies Neogloboquadrina pachyderma, which currently dominates the high latitude assemblages, has two distinct morphological variants which are traditionally differentiated by their opposite coiling direction. The left coiling morphotype frequents the cold polar and upwelling waters while the right coiling variant inhabits the subpolar and transitional zones. Their different ecologies are mirrored by substantial genetic difference and they should be considered as distinct species. However, coiling direction is not an absolute discriminator between them as all populations of N. pachyderma examined include a small but significant percentage which coil in the opposite direction. Even coiling direction is therefore not a sufficient guide to genetic type. Molecular analyses of both coiling variants have revealed a high degree of further genetic diversity which is particularly manifest in the left coiling morphotype. Individual genetic types within this morphotype are associated with different regions of the global ocean and exhibit specific adaptations. A combination of molecular, biogeographic, fossil and paleoceanographic evidence indicates that this cryptic genetic diversity arose through a stepwise progression of diversification associated with the onset of Northern Hemisphere glaciation and the glacial-interglacial climate dynamics of the Quaternary period. Both allopatric and ecological evolutionary processes have played an important role in the diversification of these high latitude planktonic protists.