Plant evolution: multicellular already a billion years ago?

Microscopic image of a multicellular, filamentous Klebsormidiophyceae algae, which is adapted to life on land due to thick cell walls. © Tatyana Darienko

Leaves, flowers, roots...: When did the basis of this plant complexity arise - multicellularity? Researchers have now discovered this question using the “molecular clock” method. The clues were provided by the phylogenomic reconstruction and the temporal classification of the developmental history of a group of green algae related to land plants. It is therefore apparent that the common ancestor of both evolutionary lines was a multicellular algal form that can be dated back to almost a billion years.

From trees to daisies, they grow in countless versions on our planet and provide food and oxygen: land plants form the basis of terrestrial life. Because of their importance, they are also of particular interest to evolutionary biology: researchers want to gain insight into how and when these living creatures arose on our planet. It is assumed that land plants emerged from algae-like plants that were able to adapt to land life around 500 million years ago and subsequently developed increasingly complex forms. So far, however, the origin of a more ancient and fundamental plant trait has remained unclear: multicellularity.

Special algae in sight

A research team from the University of Göttingen has now investigated this question by studying an interesting group of green algae. The so-called Klebsormidiophyceae are classified as streptophytes, which also include land plants. It is therefore assumed that they had common ancestors who once lived in primeval waters. The interesting thing about the Klebsormidiophyceae is that some species are unicellular, while others form multicellular, thread-like or branched plants. In addition, some of these algae have conquered the land: In addition to aquatic species, there are extremely robust inhabitants of terrestrial surfaces.

As part of the study, the researchers first collected samples of these previously little-researched Klebsormidiophyceae algae. The aim of the comprehensive search was to create a global distribution map of these organisms, depicting their adaptability, ecological importance and hidden diversity. The team discovered different species in the hottest to the coldest regions of the earth's living environment. These were representatives from bodies of water, moors and post-mining landscapes and from various substrates: including soils, rocks, tree bark, sand dunes, city walls and building facades. The researchers then took a close look at the characteristics of these different Klebsormidiophyceae algae. “It is fascinating that the tiny, robust organisms are so diverse in their morphology and also extremely well adapted to life in sometimes harsh environments,” says Tatyana Darienko from the University of Göttingen.

However, the genetic test results became the core aspect of the study. The researchers created new genetic sequences for 24 Klebsormidiophyceae species, and existing data from 14 species were also included in their studies. The genetic information was then examined using special analytical and comparative methods. “Our approach, known as phylogenomics, was to reconstruct evolution considering entire genomes or large parts of genomes,” explains co-author Iker Irisarri. “This powerful method can be used to uncover evolutionary connections with a very high level of precision.”

“Molecular clocks” point to early development

Specifically, the procedure can provide information about the relationships between species and show groupings. What is particularly exciting is that the researchers can also use the genetic data to gain clues as to when the splits from common ancestors occurred. The corresponding method is called a “molecular clock”. This dating method is based on the following principle: the more different the DNA sequences of two species, the longer they have been evolving separately. The times of the splits can be estimated using certain clues. The team now used over 800 new markers for the study, which led to their analysis results.

As the team reports, their approach resulted in a family tree of the Klebsormidiophyceae algae, grouping them into three orders. This made it possible to draw conclusions about the origin of these organisms: "By delving deeply into phylogenomics and using our 'molecular clock', we revealed the ancestor of the Klebsormidiophyceae," says first author Maaike Bierenbroodspot from the University of Göttingen. According to dating, this organism lived over 830 million years ago. The reconstruction of this alga showed that it was already a multicellular creature, the researchers report.

As they explain, based on this finding it was possible to draw conclusions about the previous evolutionary history of the streptophytes, which include the Klebsormidiophyceae and also land plants. It therefore stands to reason that the common ancestor of the Klebsormidiophyceae and the evolutionary lineage of land plants was also multicellular. “We provide evidence that the first multicellular streptophytes probably lived about a billion years ago,” the authors write. Finally, senior author Jan de Vries from the University of Göttingen says: “The genetic potential for multicellularity was present in streptophytes earlier than previously assumed. “This suggests a long-ago origin of this crucial feature, almost a billion years ago,” said the scientist.

Source: University of Göttingen, specialist article: Current Biology, doi: 10.1016/j.cub.2023.12.070

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