EXTREME DIFFERENCES IN RATES OF MOLECULAR EVOLUTION OF FORAMINIFERA REVEALED BY COMPARISON OF RIBOSOMAL DNA-SEQUENCES AND THE FOSSIL RECORD

Foraminifera have one of the best known fossil records among the unice llular eukaryotes. However, the origin and phylogenetic relationships of the extant foraminiferal lineages are poorly understood. To test th e current paleontological hypotheses on evolution of foraminifera, we sequenced about 1,000 base pairs from the 3' end of the small subunit rRNA gene (SSU rDNA) in 22 species representing all major taxonomic gr oups. Phylogenies were derived using neighbor-joining, maximum-parsimo ny, and maximum-likelihood methods. All analyses confirm the monophyle tic origin of foraminifera. Evolutionary relationships within foramini fera inferred from rDNA sequences, however, depend on the method of tr ee building and on the choice of analyzed sites. In particular, the po sition of planktonic foraminifera shows important variations. We have shown that these changes result from the extremely high rate of rDNA e volution in this group. By comparing the number of substitutions with the divergence times inferred from the fossil record, we have estimate d that the rate of rDNA evolution in planktonic foraminifera is 50 to 100 times faster than in some benthic foraminifera. The use of the max imum-likelihood method and limitation of analyzed sites to the most co nserved parts of the SSU rRNA molecule render molecular and paleontolo gical data generally congruent.