EVOLUTION OF THE DIATOMS (BACILLARIOPHYTA) .2. NUCLEAR-ENCODED SMALL-SUBUNIT RIBOSOMAL-RNA SEQUENCE COMPARISONS CONFIRM A PARAPHYLETIC ORIGIN FOR THE CENTRIC DIATOMS

A phylogeny of the diatoms was inferred from comparisons of nuclear-en coded small subunit ribosomal RNA coding regions using maximum likelih ood, weighted maximum parsimony, and neighbor-joining distance methods with Jukes and Canter, Kimura, Gamma, van de Peer, and LogDet evoluti onary models. Analyses of 30 taxa in 11 orders recovered two clades (C lades I and II). Neither of these clades correspond to the three class es of diatoms presently recognized or to the traditionally recognized radially symmetrical centric diatoms or bilaterally symmetrical pennat e diatoms. All analyses show that the centric diatoms are a paraphylet ic lineage. Tests of alternative phylogenies that address existing hyp otheses regarding diatom systematics with the maximum likelihood and m aximum parsimony methods support the two clades. Clade I is defined by centric diatom orders with specialized tubes, termed labiate processe s, located peripherally in the cell wall. Clade II contains (1) bi(mul ti)polar centric diatoms with centrally located labiate processes, (2) centric diatoms with other central tubes termed strutted processes, a nd (3) pennate diatoms. Morphological evidence from fossil assemblages and cytological architecture support the results of the molecular ana lyses, whereas morphological features of extant diatoms are too derive d to resolve the deeper branches in the tree.