Page 32 - Algae

General Overview 15

FIGURE 1.21 Life cycle of Fucus sp.: 1, FIGURE 1.22 Life cycle of Ulva sp.: 1, sporophyte;
0
0
sporophyte; 2, anteridium; 2 , oogonium; 3, 2, male zoospore; 2 , female zoospore; 3, young male
0
0
sperm; 3 , egg; 4, zygote; 5, young sporophyte. gametophyte; 3 , young female gametophyte; 4, male
0
R!, meiosis. gametophyte; 4 , female gametophyte; 5, male
0
gamete; 5 , female gamete; 6–8, syngamy; 9,
young sporophyte. R!, meiosis.
SUMMARIES OF THE TEN ALGAL DIVISIONS
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 ultra-
structure 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 ribo-
somal 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.