Charophytes + Zygnematophytes

Sister groups to land plants

The page includes the clade of green algae that is a sister group to the land plants (embryophytes). The clade that includes these green algae plus land plants has been called the Phragmoplastophyta, because they create a phragmoplast during cell division: a scaffold-like structure that forms during late cytokinesis and aids in cell plate construction and formation of a new cell wall. This clade of algae includes the Charophyceae, Coleochaetophyceae, and Zygnematophyceae (Lecointre & Guyader, 2006)

Ecology & Form

These are the most closely related algae to land plants
They are mostly freshwater or terrestrial algae
They consist of unicellular, colonial, filamentous, or parenchymatous forms
They exhibit branched apical growth like ancestral land plants

Phragmoplast

a scaffold-like structure that forms during late cytokinesis and aids in cell plate construction and the formation of a new cell wall
This structure is only found in the land plants and these algae (Phragmoplastophyta)

Charophyte sporogenesis (creation of spores)

In the charophytes, the diploid zygote undergoes several rounds of karyokinesis without cytokinesis, creating multiple copies of its DNA inside the cell (Hopkins & McBride, 1976).
This creates a cell with many nuclei, which then undergoes cytokinesis. This is sometimes called successive cytokinesis
To create final products, there is an internal or endogenous process of wall formation to create 4-32 zoospores
These spores are flagellated and released into the water to grow into new gametophytes

Diversity

Zygnematophyceae

This group has several thousand different species in two families
Unicellular and unbranched filamentous members
Sometimes called the Conjugatales, since they perform "conjugation" during reproduction
Evidence indicates that these are the most closely related algae to the land plants (Cheng et al., 2019)
Examples include Spirogyra, called the water-silk, and Zygnema are common as freshwater pond "scum", as well as the unicellular desmids such as Micrasterias

Above: Zygnema

Above: Close-up of spiraling chloroplast of Spirogyra

e.g., Spirogyra

Different than land plants, these algae exhibit a haplontic life cycle (=no multicellular sporophyte generation)
Filaments of Spirogyra line up during reproduction
Each cell of the filament forms a conjugation papilla, which then connects with the adjacent cell
This forms a conjugation tube between the cells, creating a ladder-like appearance of the strands
Gametes are formed inside of each cell
One set of gametes then migrates to the other side, where they fuse during fertilization
The result is a zygote or zygospore, which will act like a resisting spore

Above: Conjugation (reproduction) of Spirogyra from right to left

e.g., Desmids

Desmids consist of unicellular, colonial, and filamentous green algae
They are highly symmetrical and come in a diversity of forms
Desmids constitute the largest group of charophytes, comprising around 40 genera and 5,000-6,000 species
They are found mostly in freshwater, particularly acidic waters (pH 4.8-6.0) such as bogs
They are useful as bioindicators for water and habitat quality

Above: the desmid Micrasterias

Above: the desmid Cosmarium

The below algae were originally thought to be the closest relatives to land plants, due to their parenchymatous and apical growth

Coleochaetales

- Algae that are filamentous or parenchymatous, like land plants
- Coleochaete orbicularis is a flat, disk-like algae that lives as an epiphyte on basal portions of marsh plants
- Coleochaete has sterile cells that surround the zygotes after fertilization, but unlike land plants, it has a haplontic lifecycle with meiosis taking place directly in the zygote (and not in diploid cells resulting from mitosis of the zygote)
- Some species produce special parenchyma cells that connect to the embedded zygote (Graham, 1984; Graham & Wilcox, 2000). These cells resemble transfer cells found in land plants and may aid in transferring sugars from the mother to the zygote
- This might be a form of matrotrophy or “maternal feeding” which is found in land plants

Above: Coleochaete

Charophyceae (Charales)

- Parenchymatous, with apical branching
- Chara, also called a stonewort, is found in freshwater, particularly in limestone areas
- This plant-like alga has complex, upright growth with whorled branches
- The alga coats the cells with calcium carbonate, giving them a stony feel; therefore, the name stonewort
- During reproduction, they produce antheridia with sperm, and oogonia with eggs.

Above: the stonewort, Chara

Additional Resources

The Drought Alert System of Terrestrial Plants has an Underwater Origin (In Defense of Plants, 12Mar2019)
How Did Plants Conquer Land? These Humble Algae Hold Clues (New York Times, Nov. 2019)

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