A PRELIMINARY PHYLOGENY OF DIATOMS BASED ON 28S RIBOSOMAL-RNA SEQUENCE DATA

A molecular phylogeny of the diatoms has been constructed from the nuc leotide sequence of two domains of the nuclear-encoded large subunit r ibosomal RNA in four centric and four pennate species. Distance and pa rsimony analyses support the monophyly of the diatoms and their closen ess to the chrysophytes. The Centrales (radially symmetrical) appear t o be paraphyletic: the centric Lithodesmium undulatum Ehrenberg (Litho desmiales, Lithodesmiaceae) is associated with the pennate lineage. In the Pennales (bilaterally symmetrical), the araphids also appear to b e paraphyletic. These phylogenetic inferences are in close congruence with those previously obtained from 18S rRNA data (Bhattacharya et al. 1992; Medlin et nl. 1993). The evolutionary relationships of diatom f amilies inferred from the ribosomal RNA data are discussed in relation to the fossil record.