The emergence of eukaryotic algae within the protists: A molecular phylogeny based on ribosomal RNA sequencing

Abstract

The comparative analysis of homologous sequences of informational macromolecules (i.e. proteins or directly DNA) is providing a remarkably powerful tool to establish phylogenetic relationships among living organisms. Knowledge of the sequence of emergence of biological groups in turn illuminates a large number of key biological issues since, as stated long ago by Th. Dobzhansky, “Nothing in biology makes sense except in the light of evolution”. There are several advantages to sequence data as compared to morphological, physiological or biochemical ones for establishing phylogenetic relationships:

• the sequencing of a single set of homologous genes or gene products in different species can yield numerous characters simultaneously. In fact each nucleotide (or amino acid) is potentially susceptible of corresponding to a character difference. Thus tens or even hundreds of characters can be screened through the comparative analysis of one large gene;

• these characters evolve, to a first rough approximation, independently of each other i.e. substitution at one site is not necessarily related to substitution at another site. This is more difficult to evaluate for morphological characters whose correlated divergence may simply mean that they are under the control of the same genetic modification;

• molecular characters are easier to quantify and, eventually, to weigh than morphological ones;

• finally, and possibly most importantly, sequencing data can be obtained, at least for some widespread genes, even when morphology is exceedingly reduced or deformed ! (i.e. prokaryotes, poorly differentiated eukaryotes, parasites, and even DNA containing organelles).