Flip Flops in Plant Ancestry
Are green algae the ancestors of all land plants? Do paleontologists even know that these fossils are green algae?
by Margaret Helder, PhD
In science, as in many disciplines, some studies attract popular attention while other studies—perhaps equally significant—do not. The public is definitely interested in origins, however. So it was that a paper in Nature Ecology & Evolution entitled “A one-billion-year-old multicellular chlorophyte” attracted immediate attention.1 New Scientist, for example, reported that “Billion-year-old fossil seaweeds could be ancestors of all land plants.”2 Other publications made similar statements.
There are just two problems with such statements. Nobody can be sure that the fossils are green algae. And even if they are, they are not actually believed to be related to land plants.
Identity of the Fossils
What scientists found, preserved in mudstone, were tiny (scarcely visible) branched filaments. Some cells appeared to show individual nuclei inside, as well as some larger areas which might have contained numerous nuclei (areas termed coenocytic). There were obvious tip ends to the branches and at the bottom a holdfast structure which definitely would not have been useful on mudstone, but rather for growing on rocks or other solid structures. These algae must have been washed in to their point of burial in the mud. Most filamentous green algae with holdfasts today are much larger, several inches or cm in length and of course they grow attached to rocks or cement or whatever.
The authors are well aware that the description of this new fossil could fit several kinds of organism. Thus they report: “Although coenocyte or syncytium exists in many groups of eukaryotes, a filamentous siphonocladous construction is characteristic of only a handful of extant eukaryote groups, including some filamentous fungi (for example Neurospora), xanthophytes (for example, Vaucheria in reproductive stage), rhodophytes (for example, Griffithsia) and chlorophytes (for example, Rhizoclonium).3
Definitions of the above technical terms are as follows:
- Coenocyte or syncytium denotes a large part of a body with many nuclei and no cell walls separating the nuclei.
- Siponocladous refers to filamentous organisms with internal cell walls which separate a coenocytic organization into individual cells at some stage of the lifecycle.
- Fungi include molds such as Neurospora.
- Xanthophytes are golden algae with no chlorophyll b but only chlorophyll a and exhibiting other accessory pigments different from those in green algae.
- Rhodophytes are the red seaweeds (see my article from 10 Feb 2020). They have chlorophyll a and other accessory pigments different from the groups above.
- Chlorophytes are most green algae and they have the same pigments as the land plants.
Identification by Elimination
The authors then rule out most of these groups as possibilities on the basis of morphology. What is left are the green algae, which they then select as the correct identity. However, they rule out fungi on the basis that the latter exhibit a perforation in walls separating adjacent cells. The fossil specimens have no such perforations between adjacent cells. Nevertheless, there is one group of fungi, the Mucorales, which do not exhibit perforated septa in most of their representatives. The Mucorales include dainty Pilobolus on manure and Rhizopus, the black bread mold.
It is evident therefore that identification of these fossils as green algae, is a little tenuous (i.e., iffy). One would not necessarily want to assign too much theoretical weight to the identity of these specimens as green algae. Happily, this is not a concern because the described characteristics suggest that they are not ancestral to anything, even if they are green algae!
They classified the fossil, therefore, as a new species of an extinct green alga called Proterocladus, in this case P. antiquus. They describe it as a partially coenocytic (siphonocladalean) green alga growing on the sea bottom sediments. There are branched green algae today (like Stigeoclonium) but they grow attached to solid surfaces. The authors place this alga, based on its morphology, in the green algal family Ulvophyceae, characterized by such famous freshwater algae as Cladophora (or maybe it is infamous for the nasty smell of decaying masses of the alga).
The interesting thing is that in former generations, algae with descriptions such as Proterocladus would have been considered related to ancestors of land plants. Not any more! Molecular clock and electron microscope studies have totally upset the former ideas of ancestry of land plants.
Willing Suspension of Disbelief
Since land plants exhibit the same pigment composition in their chloroplasts as do the green algae, most scientists have considered it obvious that the green algae are ancestral to the land plants. But there is a huge diversity within the green algae. Which ones specifically might be more closely connected to the land plants?
Before the advent of biochemical comparisons and molecular clocks, botanists looked at morphology and lifestyle. Unlike the large seaweeds, both brown and red, most green algae show little variety of cell types within their structure. So, for example, we see unbranched filaments like the Zygnematales, wherein all cells look the same (there is not even a holdfast cell). Then there are unbranched filaments like Ulothrix which nevertheless exhibit a simple holdfast at one end. There are also filaments like Stigeoclonium which are branched with a holdfast at the bottom end. Then there is Fritschiella which exhibits branched filaments coming from a tissue-like base, attached to a firm surface. This organism was considered the most promising ancestor candidate at one time.
Complex Diversity and Diverse Complexity
There are other organisms too, like Chara, which exhibits whorls of cells and elaborate reproductive structures, and Coleochaete whose growth is flat and tissue-like. One algal group that everybody knew with certainty was not related to the land plants, was Zygnematales like Spirogyra. These algae exhibit no differentiation and absolutely no flagellated cells. They undergo sexual reproduction by means of cell contents oozing together in a conjugation tube. The rest of the green algae and land plants except for most seed plants, all exhibit flagellated motile cells in the form of sperm cells.
Closer Looks at Confusing Ancestor Candidates
With the advent of electron microscopy, botanists discovered that cell division in land plants and a few green algae was concluded by means of a dividing wall (phragmoplast). Most of the green algae however exhibited a more pinching style of cell division, called a phycoplast. So the search was on for green algae with phragmoplasts. Some Ulothrix appearing algae turned out to possess a phragmoplast and so these were renamed Klebsormidium and moved from the bulk of the green algae to the line leading to the land plants. Thus similar appearing species are placed in two completely separate lines of descent. Other characteristics that scientists sought among the green algae were plasmodesmata (a narrow thread of cytoplasm that passes through end cell walls connecting adjacent cells), and multiple biochemical compounds.
Such a search eliminated the previously promising candidates and centered on relatives of Chara (Charales) and relatives of Coleochaete (Coleochaetales) and most importantly the Zygnamatales, formerly considered the least promising candidate but now the most promising. These latter three groups are now clustered into the Streptophytes,4, 5 a sister group to the green algae proper (Chlorophytes). And according to studies of genetic divergence within the Streptophytes, the closest living relatives to land plants are the Zygnamatales. Some Zygnematales have indeed been found with phragmoplasts, but so have some Chlorophyte algae. Thus the possession of this feature seems hardly diagnostic, it is the biochemical features which make the Zygnematales seem so important. 6
Land Plants Are a Cut Above
Land plants exhibit important features that they require to survive (and which Streptophyte algae cannot claim). These include
- an epidermis and cuticle to prevent drying of extensive tissues exposed to the air
- stomates to allow for exchange of gases
- alternation of generations and retaining of the fertilized zygote inside the female sexual organ
It seems amazing that anyone would consider the Streptophytes – much less the Chlorophytes like the new fossil species – as suitable ancestors of land plants. As far as the Zygnematales are concerned, their identification as ancestral to land plants seems preposterous. Advocates of this idea perhaps display an attitude of willing suspension of disbelief, similar to that which an audience must assume in order to enjoy a Shakespearean play!
The main feature of the Streptophyte green algae connecting them to land plants is supposed to be multicellularity (many cells of diverse descriptions).7 Yet the Zygnematales, supposedly closest to the land plants, exhibit only one cell type in unbranched filaments and absolutely none of the motile cells that are so characteristic of the land plants and the other green algae (Chlorophytes).
It takes an active imagination to picture that there could be evolutionary possibilities in this situation!
- Qing Tang, Ke Pang, Xunlai Yuan and Shuhai Xiao. 2020. A one-billion-year-old multicellular chlorophyte. Nature Ecology and Evolution. February 24.
- Michael Le Page. 2020. New Scientist https:///www.newscientist.com/article/2234683-billion-year-old0fossil-seaweeds-could-be-amcestprs-of-all-land-plants/
- Qing Tang et al. p. 4
- Sabina Wodniok, Henner Brinkmann, Gernot Glockner, Andrew J. Heidel, Herve Philippe, Michael Melkonian and Burkhard Becker. 2011. Origin of land plants: Do conjugating green algae hold the key? BMC Evolutionary Biology 11 #104. https://doi.org/10.1186/1471-2148-11-104
- One Thousand Plant Transcriptomes Initiative. 2019. One thousand plant transcriptomes and the phylogenomics of green plants. Nature 574 #7780 pp. 679-685. https://doi.org/10.1038/s41586-019-1693-2
- J. M. Lopez-Bautista, D. A. Waters and R. L. Chapman. 2003. Phragmoplastin, green algae and the evolution of cytokinesis. Int. J. Syst. Evol. Microbiol. November 53 (Pt 6): 1715-1718.
- Alexander M. C. Bowles, Ulrike Bechtold and Jordi Paps. 2020. The Origin of Land Plants is Rooted in Two Bursts of Genomic Novelty. Current Biology30: 1-7. https://doi.org/10.1016/j.cub.2019.11.090
Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.