Ultrastructure as a Control for Protistan Molecular Phylogeny

• Published in: The American naturalist 1999-10, Vol.154 (S4), p.125-136
• Main Author: Taylor, F. J. R. “Max”
• Format: Article
• Language: English
• Subjects: Animal morphology; Animals; Deoxyribonucleic acid; DNA; Eukaryotic cells; Evolution; Mitochondria; Molecular biology; Organelles; Phylogeny; Plant morphology; Plastids; Ribosomal DNA; Symbiosis
• ISSN: 0003-0147; 1537-5323
• DOI: 10.1086/303288

A variety of molecular sequences and treeing methods have been used in attempts to unravel early protistan evolution and the origins of “higher” eukaryotic taxa. How does one know which approach is closest to the real phylogenetic tree? Obviously it is the robustness of its resulting trees, the coherence with other data sets, both structural and molecular, that is the test. Simply put: it should make biological sense. It seems evident, comparing morphology, especially ultrastructure, with ribosomal DNA trees, that the major lineages have now been confirmed. In particular, the remarkably conservative mitochondrial crista type in protists is coherent with mitochondrial DNA sequences. Several amitochondrial groups, presumed to be primitive on the basis of SSU ribosomal DNA, show alarming positional volatility when other genes are used. In addition, the presence of mitochondrial genes in the nucleus of several amitochondrial flagellates raises doubts about them being primordially amitochondrial. Consequently, the root of the eukaryote tree is still in question. A disturbing question arises: can loss of features in parasitism mimic primitiveness not only in a morphological but also in a molecular way, evolving more rapidly and creating long branches that methodologically place them basal in the trees? Conflicting molecular phylogenies cannot be resolved by molecular data alone. Morpholological, especially ultrastructural, data are an essential component of phylogenetic reconstruction.