The evolution of the diatoms (Bacillariophyta). I. Origin of the group and assessment of the monophyly of its major divisions

Abstract

The diatoms are one of the best characterised algal groups. Despite this, little is known of the evolution of the group from the earliest cell to the myriad of taxa known today. Relationships among taxa at the family or generic level have been recognised in some diatoms. However, relationships at higher taxonomic levels are poorly understood and have often been strongly influenced by the first appearances of key taxa in the fossil record. An independent assessment of relationships among the diatoms at these higher taxonomic levels has been made using rRNA sequence data to infer phylogenetic relationships. In this paper we present an analysis of 18S rRNA data from several chosen centric, araphid and raphid pennate taxa. The phylogenetic inferences from these 18S rRNA sequences are supported by evidence from the fossil record and evidence from ontogenetic data. Ribosomal RNA data indicate that both the centric and araphid pennate lineages may not be monophyletic.

Evolution of the diatoms: insights from fossil, biological and molecular data

“…Simonsen 1979) that the pennate diatoms evolved from the centric forms because they appear later in the geological record (which had been noted also by Fritsch 1935, p. 642), is supported unambiguously by molecular phylogenies (see below, this section). Likewise, Simonsen's view that centric diatoms are paraphyletic (though he did not use this terminology) is also clearly demonstrated by molecular data, which were presented first by Medlin et al (1993). However, molecular analyses have also brought several surprises.…”

“…The Lithodesmiales possess a central rimoportula, although this has a characteristic bilabiate internal morphology, which one easily can envisage having been formed by the closure of a single open tube. This fact led Medlin et al (1993) and Medlin & Kaczmarska (2004) to suggest that the central strutted process could be homologous with the bilabiate process of the Lithodesmiales. Kaczmarska et al (2006) have suggested further that the external tubes of the marginal strutted processes could be formed by a fragmentation, then encircling and fusing of the marginal fringe in the Lithodesmiales to form hollow tubes above the pore structure of the fultoportula.…”

“…However, molecular analyses have also brought several surprises. The first molecular phylogenies (Medlin et al 1993) showed the araphid pennate diatoms to be paraphyletic, undermining the class-level distinction of Fragilariophyceae and Bacillariophyceae by Round et al (1990). Furthermore, among the taxa studied by Medlin et al (1993), the centric diatom, Skeletonema Greville was most closely related to the pennate diatoms, with high bootstrap support in phylogenetic analyses.…”

“…The first molecular phylogenies (Medlin et al 1993) showed the araphid pennate diatoms to be paraphyletic, undermining the class-level distinction of Fragilariophyceae and Bacillariophyceae by Round et al (1990). Furthermore, among the taxa studied by Medlin et al (1993), the centric diatom, Skeletonema Greville was most closely related to the pennate diatoms, with high bootstrap support in phylogenetic analyses. Sörhannus et al (1995) confirmed that centric and araphid taxa were paraphyletic, using an analysis of partial sequences from the 28S large-subunit (LSU) rRNA coding region from eight diatoms.…”

“…Medlin et al (1993Medlin et al ( , 1997b suggested that the diplonty of diatoms is plesiomorphic, on the basis that basal heterotrophic heterokonts are also diplonts. The main basis for this idea was an outgroup comparison, in which Medlin et al (2000) claimed that all oomycetes are diploid and implied that the labyrinthulids might be also.…”