Summaries of the Ten Algal Divisions

Summaries of the Ten Algal Divisions
  Cyanophyta and Prochlorophyta
Endosymbiosis and Origin of Eukaryotic Algae
The phylogenetic reconstruction adopted in this book is intended to be more or less speculative because most of the evidence has been lost and many organisms have left no trace in the fossil records. Normally, systematic groups and categories arranged in a hierarchical system on the basis of similarities between organisms replace it. Each of these natural groups consists of a set of organisms that are more closely related to each other than to organisms of a different group. This interrelationship is inferred from the fundamental similarities in their traits (homologies) and is thought to reflect fundamental similarities in their genomes, as a result of common descent. Historically, the major groups of algae are classified into Divisions (the equivalent taxon in the zoological code was the Phylum) on the basis of pigmentation, chemical nature of photosynthetic storage product, photosynthetic membranes’ (thylakoids) organization and other features of the chloroplasts, chemistry and structure of cell wall, number, arrangement, and ultrastructure of flagella (if any), occurrence of any other special features, and sexual cycles.

Recently, all the studies that compare the sequence of macromolecules genes and the 5S, 18S, and 28S ribosomal RNA sequences tend to assess the internal genetic coherence of the major divisions such as Cyanophyta and Procholophyta and Glaucophyta, Rhodophyta, Heterokontophyta, Haptophyta, Cryptophyta, Dinophyta, Euglenophyta, Chlorarachniophyta, and Chlorophyta. This confirms that these divisions are non-artificial, even though they were originally defined on the basis of morphology alone. Table 1.4 attempts to summarize the main characteristics of the different algal divisions.