Origin of land plants

Bot 201 The transition to land (by 1 lineage of green algae)

I. Many elements come together in presenting the current view for the origin of land plants

Easy & obvious features link green algae to early land plants

	Identical pigmentation types
	Identical chloroplast structure
	Identical storage products
	Very similar (but not identical) cell wall features
	Biflagellate motile cells as gametes.

Subtle differences among green algae and early land plants

	Differences in flagellar insertion for motile cells
	Differences in patterns of cytokinesis for cells
	Differences in enzymes associated with photosynthetic metabolism
	Sequence variation for gene regions that reveal ancient evolutionary divergence.

Substantial differences among green algae and early land plants

	Specialized organs for photosynthesis, nutrient acquisition
	Specialized layers to minimize water loss to atmosphere
	Specialized multicellular structures for reproduction

II. Initially, researchers expected to see a link between adult morphology and being the progenitor to early land plants.

Researchers expected to see step-wise specializations:

From an algal-like body showing heterotrichy (differentiation into upright and basal regions of the same plant)

To root-like structures and leaf-like structures in a complex, land plant form.

In this progression, these algae were held to be significant:

Chaetophora - very simple upright uninucleate algae with branching

Stigeoclonium - exhibits basal and upright portions in equal balance

Draparnaldia - the basal section is completely suppressed with upright axes branched

Fritschiella - extensive basal regions with colorless rhizoids and parenchymatous cushions as well as branched upright axes.

III. Instead, there was a likely step-wise progression in acquisition of traits that improved longevity at a variety of levels from a Coleochaete type of heterotrichous adult.

In this progression, these steps are more likely what happened:

Zygote retained in haploid female oogonium (rather than being released as in the Zygotic meiosis life history)

Diploid generation becomes multicellular and resident on haploid phase

Spores become non-motile

Acquisition of cuticle to prevent water loss in air

Emergence of large, multicell sporophyte into the dry atmosphere

Two lines emerge:

Gametophyte dominance

Sporophyte dominance

IV. Simple Coleochaete and relatives such as Nitella and Chara provide strong cases as ancestors of land plants.

These compelling data come from traditional and modern techniques:

Reproduction with specialized male gamete similar to males of Bryophytes

Phragmoplast cell division pattern

Complex life history feature of having the egg sessile in haploid plant

Subtle biochemistry distinctions

Molecular sequence data.

Evolution of land plants - this site links to a "tree" that shows relationships

	From an algal-like body showing heterotrichy (differentiation into upright and basal regions of the same plant)
	To root-like structures and leaf-like structures in a complex, land plant form.

	Chaetophora - very simple upright uninucleate algae with branching
	Stigeoclonium - exhibits basal and upright portions in equal balance
	Draparnaldia - the basal section is completely suppressed with upright axes branched
	Fritschiella - extensive basal regions with colorless rhizoids and parenchymatous cushions as well as branched upright axes.

	Reproduction with specialized male gamete similar to males of Bryophytes
	Phragmoplast cell division pattern
	Complex life history feature of having the egg sessile in haploid plant
	Subtle biochemistry distinctions
	Molecular sequence data.