key concept 27.1 Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes

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More than a billion years ago, when a cyanobacterium was first engulfed by an early eukaryote, the history of life was altered radically. The chloroplasts that resulted from primary endosymbiosis of this cyanobacterium (see Figure 26.2) were obviously important for the evolution of plants and other photosynthetic eukaryotes, but they were also critical to the evolution of all life on land. Until photosynthetic plants were able to move onto land, there was very little on land to support multicellular animals or fungi, and almost all life was restricted to the oceans and fresh waters.

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work with the data

The Phylogeny of Land Plants

Original Paper: Qiu, Y.-L. et al. 2006. The deepest divergences in land plants inferred from phylogenetic evidence. Proceedings of the National Academy of Sciences USA 103: 15511–15516.

In addition to the morphological characters of land plants shown on the phylogeny in Figure 27.1, DNA sequences are widely used to study and reconstruct the evolutionary history of plants. These sequences are many tens of thousands of nucleotides long and have been collected from a large number of species. The full data set used by Yin-Long Qiu and his colleagues (available at treebase.org) includes DNA sequences from 67 genes. The table below provides sample sequences from a chloroplast gene that has been used to reconstruct the relationships of representative plant species; the table shows 27 nucleotide positions for 10 species.

Nucleotide position (character state)
Species 1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Outgroup (chlorophyte alga) T A T T A T G A T T C C A A A T A T T A T A A T C T A
Stonewort T A T T T A A A T T A C T A A T A A T A T A A T C T A
Liverwort A C T T T T A A T G A T T C A G A A T A T A A T C T A
Moss A C T T T T A A T A T T T T A A T A T A A A A T C T T
Hornwort A C T T T T A A T G T T T T A A T A C A G A A A C T T
Lycophyte A C T C C C G G T G T T C T G A T A C A A G G A C C T
Fern C C T C C G A G C G T T C T T A G A T A A G G A C C T
Pine tree A C C C C G C G C G T T C T G A T G C G A G G A T C T
Rice A C C C C G C G C G T T C T G A T G C G A G G A T A T
Tobacco A C C A C G C G C G T T C T G A T G C G A G G A T A T

QUESTIONS

Question 1

Construct a phylogenetic tree of these 10 species using the parsimony method (see Key Concept 21.2 and the examples in Table 21.1 and Figure 21.5 for instructions). Use the outgroup to root your tree. Assume that all changes among nucleotides are equally likely.

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Question 2

How many changes (from one nucleotide to another) occur along each branch on your tree?

Changes for each branch are indicated on the tree diagram provided with the answer to Question 1. Note that there are alternative (equally parsimonious) reconstructions for character numbers 6, 14, and 16.

Question 3

Which nucleotide positions (i.e., which character states) exhibit homoplasy (convergence or reversal of the character state)?

Characters 7 and 19 exhibit homoplasy.

Question 4

Which group on your tree represents the streptophytes? The land plants? The vascular plants? The euphyllophytes?

These groups are labeled in the figure provided as the answer to Question 1.

A similar work with the data exercise may be assigned in LaunchPad.

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Figure 27.1 The Evolution of Plants In its broadest definition, the term “plant” includes the glaucophytes, red algae, and green plants—all the groups descended from a common ancestor that had chloroplasts (A). Some biologists restrict the term “plant” to the green plants (those with chlorophyll b) or, even more narrowly, to the land plants (B). Three key characteristics that emerged during the evolution of land plants—protected embryos, vascular tissues, and seeds—led to their success in the terrestrial environment.

Question

Q: Which two names of plant groups are used in this figure only as convenience terms, and do not refer to clades?

The terms “algae” and “nonvascular plants” are both convenience terms, as neither of those groups forms a monophyletic group.

focus your learning

  • The major clades of Plantae evolved from the first photosynthetic eukaryote.

  • Key evolutionary innovations evolved among the aquatic Plantae.

  • Land plants fall into ten major clades.

Primary endosymbiosis is a shared derived trait—a *synapomorphy—of the group known as Plantae (Figure 27.1). Although Plantae is Latin for “plants,” in everyday language—and throughout this book—the unmodified common name “plants” is usually used to refer only to the land plants. However, the first several clades to branch off the tree of life after primary endosymbiosis are all aquatic. Most aquatic photosynthetic eukaryotes (other than those secondarily derived from land plants) are known by the common name algae. This name, however, is just a convenient way to refer to these groups, which are not all closely related. Many of the photosynthetic groups discussed in Chapter 26 (which acquired chloroplasts through secondary endosymbiosis) are also commonly called algae.

*connect the concepts As noted in Key Concept 21.1, derived traits shared by a group of organisms that offer evidence of their common ancestry are called synapomorphies.