THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

 

17. Protists: Advent of the Eukaryotes

 

17.7. The Road to Plants

 

Another key lineage on the protist phylogenetic tree, the red and green algae, marks the path that has led to the evolution of plants. Both red and green algae contain similar chloroplasts, and molecular analysis indicates a single endosymbiotic origin for both, confirming a common ancestry. The red algae appear to have arisen before the evolutionary lineage that led from green algae to plants.

 

 

Rhodophyta Are Ancient Photosynthesizers

Red algae, members of the phylum Rhodophyta, possess red pigments called phycobilins that give them their characteristic color, as shown in figure 17.16. Almost all of the 4,000 species of red algae are multicellular and live in the sea, where they grow more deeply than any other photosynthetic organism. Red algae have complex bodies made of interwoven filaments of cells. The laboratory media agar is made from the cell walls of red algae. The life cycle of red algae is complex, usually involving alternation of generations, like in plants. None of the red algae have flagella or centrioles, suggesting that red algae may be one of the more ancient groups of eukaryotes.

 

 

Figure 17.16. Red algae.

These red algae have their cellulose cell walls heavily impregnated with calcium carbonate, the same material of which oyster shells are made. Because they are hard and occur on coral reefs, they are called coralline algae.

 

Chlorophyta Are the Direct Ancestors of Plants

Green algae, members of the phylum Chlorophyta, are of special interest because the ancestor of terrestrial plants was a member of this group. Green algae chloroplasts are similar to plant chloroplasts, and like them, they contain chlorophylls a and b.

Green algae are an extremely varied group of more than 7,000 species, mostly mobile and aquatic like Chlamydomonas, shown in figure 17.17a, but a few (like Chlorella) are immobile in moist soil or on tree trunks. Although most green algae are microscopic and unicellular, some are intermediate, colonial, or truly multicellular. Some of the most elaborate colonies are seen in Volvox, a species which forms a hollow sphere made of 500 or more cells (figure 17.17b). The two flagella of each cell beat in time with all the others to rotate the colony, which has reproductive cells at one end. While Volvox borders on multicellularity, the green algae, Ulva (sea lettuce), shows true multicellularity. The sexual life cycle in Ulva alternates between a multicellular haploid structure called a gameto-phyte, and a multicellular diploid structure called a sporophyte (figure 17.18). The sporophyte is similar in appearance to the gametophyte.

 

 

Figure 17.17. Green algae.

(a) Chlamydomonas is a unicellular mobile green algae. (b) Volvox forms colonies, an intermediate stage on the way to multicellularity.

 

 

Figure 17.18. A green algae life cycle: Ulva.

Individuals of this green algae exhibit a life cycle that is somewhat unique among algae in that they alternate between a haploid form called the gametophyte and a diploid form called the sporophyte, which are identical in appearance and consist of flattened sheets two cells thick. In the haploid (n) gametophyte, gametangia give rise to haploid gametes, which fuse to form a diploid (2n) zygote. The zygote germinates to form the diploid sporophyte. Sporangia within the sporophyte give rise to haploid spores by meiosis. The haploid spores develop into haploid gametophytes.

 

 

Key Learning Outcome 17.7. The red and green algae share a common ancestor, but the green algae gave rise to terrestrial plants.