25.2 Green Algae: Precursors of Land Plants

Green Algae and Land Plants

Green algae and land plants share a set of core traits pointing to a common ancestor. Understanding these shared features, and which algae are most closely related to land plants, helps explain one of biology's major transitions: the move from water to land.

Traits of Algae and Land Plants

These shared characteristics are the strongest evidence that green algae and land plants belong to the same lineage.

• Chloroplasts containing chlorophyll a and b give both groups their green color and drive photosynthesis. These chloroplasts originated from an ancient endosymbiotic event, where an ancestral eukaryotic cell engulfed a photosynthetic cyanobacterium.
• Cellulose cell walls provide structural support and protection. Land plants later added lignin to their walls for extra rigidity, but the cellulose foundation is the same.
• Starch stored in plastids serves as the primary energy reserve. Starch is a glucose polymer, and storing it inside plastids (rather than in the cytoplasm) is a trait specific to this lineage.
• Alternation of generations defines the life cycle. A haploid gametophyte stage produces gametes (sperm and eggs), while a diploid sporophyte stage produces spores through meiosis. Those spores develop into new gametophytes, completing the cycle.
• Flagellated sperm require water to swim to the egg for fertilization. This is a holdover from aquatic ancestry and remains a limiting factor for seedless plants on land.

Charophytes as Plant Relatives

Not all green algae are equally related to land plants. Charophytes are the closest living algal relatives of land plants, and several specific features set them apart from other green algae.

• Oogamous reproduction involves a large, non-motile egg fertilized by a smaller, flagellated sperm. This same pattern appears in bryophytes (mosses, liverworts, hornworts) and other land plants.
• Phragmoplasts form during cell division. This is a set of microtubules that guides the formation of a new cell plate between daughter cells. Other green algae divide differently, but charophytes and land plants share this mechanism.
• Plasmodesmata are channels that connect the cytoplasm of adjacent cells, allowing direct cell-to-cell communication and transport (the symplastic pathway). This is a key feature of land plant tissue organization.
• Cell wall composition in charophytes includes cellulose, pectins, and hemicelluloses, closely matching the cell wall chemistry of land plants.
• Pigment profile includes chlorophyll a, chlorophyll b, and carotenoids in their plastids, identical to land plants.

Phylogenetic analyses consistently place charophytes as the sister group to land plants, meaning they share a more recent common ancestor with land plants than any other living algae do.

DNA Analysis in Plant Evolution

Morphological traits alone can be misleading because of convergent evolution, where unrelated organisms independently evolve similar features. DNA sequence comparisons offer a more reliable way to determine evolutionary relationships.

• Researchers compare sequences from nuclear, chloroplast, and mitochondrial genomes to build phylogenetic trees that map how groups are related and how distantly they diverged.
• A monophyletic group includes an ancestor and all of its descendants. Identifying monophyletic groups is the goal of modern classification because they reflect true evolutionary history.
• DNA evidence confirms that charophytes are the closest living relatives of land plants, consistent with the morphological evidence described above.
• Molecular clock analyses use the rate of DNA mutations to estimate when key divergence events occurred, such as the transition to land, the origin of vascular tissue, and the evolution of seeds and flowers.

Advances in sequencing technology continue to refine these relationships, sometimes overturning classifications that were based on morphology alone.

Adaptations for Terrestrial Life

Moving onto land posed major challenges: drying out, exchanging gases in air instead of water, and reproducing without being submerged. Early land plants evolved several key structures to cope.

• Cuticle: A waxy coating on exposed plant surfaces that reduces water loss through evaporation.
• Stomata: Tiny pores (usually on leaf surfaces) that open and close to regulate gas exchange ($CO_2$ in, $O_2$ out) while minimizing water loss.
• Archegonia: Flask-shaped female reproductive organs that produce and protect the egg. The enclosed structure keeps the egg from drying out.
• Antheridia: Male reproductive organs that produce and release sperm.
• Sporangia: Structures where spores are produced by meiosis and held until release.
• Gametangia: The general term for any gamete-producing organ (archegonia and antheridia are both types of gametangia).

These adaptations allowed descendants of charophyte-like ancestors to survive and reproduce on land, setting the stage for the enormous diversity of land plants that followed.