Evolution of the Diatoms: VIII. Re-Examination of the SSU-Rrna Gene Using Multiple Outgroups and a Cladistic Analysis of Valve Features

Linda K Medlin

doi:10.4172/2376-0214.1000129

Abstract

The resolution of the SSU rRNA gene for phylogenetic analysis in the diatoms has been evaluated by Theriot et al. who claimed that the SSU rRNA gene could not be used to resolve the monophyly of the three diatoms classes described by Medlin and Kaczmarska. Although they used both only bolidomonads and heterokonts as outgroups, they did not explore outgroups further away than the heterokonts. In this study, the use of the multiple outgroups inside and outside the heterokonts with the rRNA gene for recovering the three monophyletic clades at the class level is evaluated. Trees with multiple outgroups ranging from only bolidophytes to Bacteria and Archea were analyzed with Bayesian and Maximum Likelihood analyses and two data sets were recovered with the classes being monophyletic. Other data sets were analyzed with non-weighted and weighted maximum parsimony. The latter reduced the number of clades and lengthened branch lengths between the clades. One data set using a weighted analysis recovered the three classes as monophyletic. Taking only bolidophytes as the only outgroup never produced monophyletic clades. Multiple outgroups including many heterokonts and certain members of the crown group radiation recovered monophyletic clades. The three classes can be defined by clear morphological differences primarily based on auxospore ontogeny and envelope structure, the presence or absence of a structure (tube process or sternum) associated with the annulus and the presence of the cribrum in those genera with loculate areolae. A cladistic analysis of some of these features is presented and recovers the three classes.

Highlights

The diatoms (Bacillariophyta) are arguably one of the most successful microalgal groups in both aquatic and terrestrial habitats with over 10,000 described species and potentially many more cryptic species [1]
In 1993, Medlin et al [5] produced the first phylogeny of the diatoms that showed two of the morphological groups of the diatoms were not monophyletic and this conflicted with the accepted perception that the group was divided into two natural groups: the centrics and the pennates, the latter divided into araphid and raphid diatoms
As more centrics were added to the tree [6,7], it became clear that there were two groups of centric diatoms: the radial centrics and the bipolar centrics with the radial Thalassiosirales falling into the bipolar clade

Summary

Introduction

The diatoms (Bacillariophyta) are arguably one of the most successful microalgal groups in both aquatic and terrestrial habitats with over 10,000 described species and potentially many more cryptic species [1] Their architecturally complex siliceous cell walls (valves and girdle bands) are unique among the algae. As more centrics were added to the tree [6,7], it became clear that there were two groups of centric diatoms: the radial centrics and the bipolar centrics with the radial Thalassiosirales falling into the bipolar clade This result has been very difficult for diatomists to accept using the morphology of the valves as the first criterion to separate taxa at higher taxonomic levels. The Golgi arrangement provides some support for the two clades, there are several exceptions in each group [10,13]

Methods

Results

Discussion

Conclusion