Primary endosymbiosis is a synapomorphy of the Plantae. The glaucophytes, the sister clade of the other Plantae, are unicellular algae that are similar to some of the earliest photosynthetic eukaryotes. The green plants contain chlorophyll b in addition to the chlorophyll a found in all Plantae.
learning outcomes
You should be able to:
Plot the origin and distribution of key evolutionary innovations among aquatic Plantae.
Describe the major clades of Plantae and their relationships to one another.
Explain the different possible uses of the term “plant.”
The broadest use of the term “plant” refers to eukaryotes that descended from the ancestor with the first primary endosymbiosis of chloroplasts: glaucophytes, red algae, the various lineages of green algae, and land plants. Another use of the term refers to green plants (those that contain chlorophyll b, or the green algae and land plants). The most common use of “plant” refers only to land plants.
Why doesn’t the term “algae” designate a formal taxonomic group?
“Algae” is a convenience term that is used to refer to many distantly related, photosynthetic, aquatic organisms. This includes many groups of eukaryotes outside the Plantae.
What are some of the key differences between glaucophytes, red algae, and the various clades of green algae?
Glaucophytes are unicellular and also differ from the other groups because their chloroplasts contain a small amount of peptidoglycan between the inner and outer membranes. Most red and green algae are multicellular. Red algae contain an accessory photosynthetic pigment (phycoerythrin), which often gives them a red color, especially in deeper water. The green algae lack phycoerythrin but have a second kind of chlorophyll (chlorophyll b) in addition to chlorophyll a, which is also found in glaucophytes and red algae.
What evidence supports the phylogenetic relationship between land plants and the various groups of aquatic green algae?
Both land plants and green algae contain chlorophyll b in addition to the chlorophyll a also found in glaucophytes and red algae. Land plants are more closely related to some green algae lineages than to others. The green algae most closely related to the land plants are the stoneworts; these two groups share protected embryos, a cuticle, a multicellular sporophyte, the presence of gametangia, and thick-
The green algal ancestors of the land plants lived at the margins of ponds or marshes, ringing them with a mat of dense green. It was from such a marginal habitat, which was sometimes wet and sometimes dry, that early plants made the transition onto land.