Molecular phylogeny of the planktic foraminifera

Planktic foraminifers are an important group of marine zooplankton whose fossil record has been used extensively to reconstruct past climatic and oceanographic changes. Knowledge of the molecular phylogenetics of the planktic foraminifera will allow us to determine whether a link exists between individual water masses and specific foraminiferal genotypes and help unravel the relationship between planktic foraminiferal evolution and the history of ocean circulation. However, the isolation of foraminiferal genomic DNA has proved technically complex due to major difficulties in obtaining uncontaminated DNA from living specimens. We have minimized this problem by extracting genomic DNA from gametogenic foraminifers, and successfully amplified an approximately 1,000 base pair (bp) fragment of the foraminiferal small subunit (SSU) ribosomal (r)RNA gene for five planktic species. A molecular phylogeny was constructed which shows that the planktic foraminifera form a distinct monophyletic group within the eukaryotic SSU rDNA tree, probably representing one of the earliest splits among free-living aerobic eukaryotes. This separation preceded the rapid eukaryote diversification, represented by the ''crown'' group, dated at 1100-1000 million years before present (Knoll, 1992). The monophyly of the planktic foraminiferal group conclusively confirms the foraminiferal origin of the amplified DNA. The planktic foraminiferal SSU rRNA gene also contains a number of highly variable regions of which some are unique to the foraminifera. These areas should permit genetic comparisons at the species and morphotype level, allowing the interpretation of the relationships between genotype, phenotype and environmental change.