“But other times things change, and all you have to do is find a way to change with them.”- Jeanne Ray, writer
If you think of the word ‘countless’, what do you imagine? Stars? But you would be surprised to know there are millions of bacteria in the oceans, more than all the known stars in the universe! Microbes are everywhere, invisible but ubiquitous. They are wonderful tiny organisms except that they don’t follow the age-old theme in Biology – “Cell to Tissue to Organ to Organ System to Organism”. Why didn’t they become one of the giant living beings to eventually roam on our planet? Or did they and no one took notice of it?
Multicellularity means a group of cells should stick together, perform a specific function that the others cannot, and should keep interacting with one another all the time. When you think about multicellularity, all you can picture are plants and animals! But, microbes boast of multicellularity through filaments, biofilms, and as unique multicellular magnetotactic prokaryotes (MMPs).
Trace it back to the origins, bacteria show the way
Cyanobacteria are widely accepted as the first known multicellular prokaryote. They are often seen as colonies of specialized cells named ‘heterocysts’ that line up together as filaments with the sole purpose of Nitrogen fixation. This emergent structure of Cyanobacteria paved the way for saving these organisms from predation and facilitated acquiring food from all directions around the cluster of cells. Hence, instead of being a loner in the pond, it took the leap of faith to become multicellular.
Figure 1- Nostoc under the lens, can you spot two versions of cells here?
Another mode that microbial communities adopt to live together is by secreting a biofilm, an extracellular matrix made up of proteins, carbohydrates, nucleic acids, and lipids.By living together, bacterial communities pass on nutrients, enzymes and even leftovers of the dead and at the same time, prevent the spread of toxic substances like antibiotics. They are now thought of as multicellular collectives like a flock of birds rather than a singular entity of multicellularity. Hence, by forming intricate networks, microbes can cooperate with each other to carry out tasks that are costly to perform individually and yet at other times can cheat for diverting resources to oneself.
Figure 2 – Not the surface of the sun, but intricate connections and morphology of biofilm in Bacillus subtilis.
In the year of 2007, scientists collected samples from the Araruama lagoon in Rio de Janeiro, Brazil, and found the real star, a prokaryote but true multicellular, Candidatus Magnetoglobus multicellularis– MMP. If you imagine a bacteria, a spiral ball of flagellated bacterial cells together is not what you would think of! They observed an aggregate of 10-40 genetically identical gram-negative bacteria containing magnetosomes under the microscope. The MMP had its cells arranged next to each other to contact both the external environment and an acellular internal compartment. The individual units were bound to each other, moved together through coordinated flagellar movement, and divided by doubling.They lost their viability if disintegrated into individual cells, pointing to their need to be multicellular.
To be or not be? From being a loner to believing in partnership
In the search for an ideal model organism for the origin of multicellularity, scientists have studied a number of eukaryotes. Looking at the class Chlorophyceae, Chlamydomonas, a unicellular member along with its multicellular relative, Volvox, have provided insights into the multicellular origins of organisms. Chlamydomonas alternates between a swimming phase, with a pair of flagella, and an aflagellate reproductive phase. It is a likely single-celled ancestor of Volvox that separates the two alternating functions into ‘distinct cell types’. So, Volvox is now thought to have evolved 200 million years ago from its unicellular ancestors to have terminally differentiated somatic cells and germ cells for reproduction separately.
Figure 3- (a) Unicells of Chlamydomonas. (b) Multicellular arrangement of cells to form a multicellular unit of Volvox.
Choanoflagellates, the collared flagellates are small unicellular protists, found in both freshwaters and the ocean. They have finger-like projections that surround their flagellum used for movement and taking in food. These organisms have parallels to the choanocyte cells of sponges, and are proposed to be the closest living relatives of metazoans. They are proposed to have diverged from a unicellular eukaryote capable of forming colonies around 600 million years ago. By using phylogenetic and molecular analysis, MBRTK1, a receptor tyrosine kinase (RTK) was discovered for the first time outside metazoans in the choanoflagellate Monosiga brevicollis. RTKs are molecules present at the cell membrane known to play essential roles in cell-cell signalling and cell adhesion- functions that guide crosstalk between cells. In another species of choanoflagellate Salpingoeca rosetta, it was found that it could no longer form an orderly rosette-structure owing to mutations of two proteins, Jumble and Couscous pointing to a pre-metazoan version of cell-adhesion.
Figure 4- (a) A funnel like structure of an individual cell of a choanoflagellate, Salpingoeca rosetta which resembles collar cells of sponges. (b) Resembling a circularized flower design pattern or rose, Salpingoeca rosetta get their name after their seemingly multicellular lifestyle (Image source)
Capsaspora owczarzaki is a unicellular protist with distinct stages in life, starting as an individual amoeba with filopodia, followed by an aggregative stage with a multicellular organization. This organism is also a sought-after model to trace the origins of life’s mystery of sticking together as they are very close to the choanoflagellates and metazoans. Not only cell-adhesion proteins, tyrosine kinases are identified in this amoeba, but also brachyury homolog (a transcription factor that in animals is involved in gastrulation).
Figure 5- Belonging to Filastera clade, Capsaspora owczarzaki shows a number of amoeboid cells together with protruding filopodia.
Another group of organisms from the phylum Placozoa named Trichoplax adhaerens are multicellular organisms with a mosaic of cells. They are sometimes considered a living fossil to the bilateral animals and even metazoans due to their simple morphology. They are flat-disc like without a body axis and have only five types of somatic cells. They live freely in the seawater and divide by fission or budding but there are some indications of bisexual reproduction in their genes. Surprisingly, the genome sequencing of Trichoplax showed that this organism shares characteristics with sponges and cnidaria. It caught the attention of many researchers with its unique conservation of genes involved in embryonic development, cell specialization, and differentiation.
Figure 6 – Trichoplax adhaerens cells are clumped together in large numbers.
When and why did it all start?
One question that rattles all the new findings are the timescales of ‘when’ this organism or ‘when’ that gene began to redefine its genome for a transition. A group of scientists grew unicellular Saccharomyces cerevisiae in tubes, spun them in a centrifuge every day to pass on the clustered cells which sank the fastest for the next round of the experiment. Two weeks later, the snowflake yeast appeared, the reason behind this was a single mutation in a transcription factor. These snowflake yeast cells adhere to one another, reproduce as clusters by fragmentation and produce clonal populations of itself, a hallmark of multicellularity starting from a single cell for ensuring the organism finally consists of genetically similar cells.
Figure 7 – Snowflake yeast (Source and a real-time video of multiplying snowflake yeast forming individual clumps)
All of these results are only the tip of the iceberg as they are beginning to provide answers on how life took this leap. Was it one giant step to become multicellular or a series of small yet measured steps to never revert back to unicellular beings? Why did the prokaryotes choose to be loners more often than clusters? How did eukaryotes manage to be unicells, clusters, and both at times? Was it one mutation or many? Was it a chance event or a planned re-tuning of genes? Many of these phenomena are yet to be investigated in detail.
Yet, scientists are starting to believe that microbes might have been the first ones to experiment with several versions of multicellularity. By accumulating features of the multicellular organization during specific parts of their life cycle, microbes have learnt to thrive better!
Thank you for reading, stick together and show your gratitude to our prokaryotic ancestors!
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Author:
Ananya Dash is a final year student of Integrated M.Sc in Systems Biology at the University of Hyderabad. Of all the wonders of life, the invisible world of cells and microbes inspire her the most. When she gets a chance to share her learnings, she is found telling the unheard stories of science to curious minds in school classrooms, on her blog, and in myriad ways when opportunities find their way to her. She is interested to pursue scientific research in the future and making STEM accessible to all.
Editors:
Sumbul Jawed Khan received her Ph.D. in Biological Sciences and Bioengineering from the Indian Institute of Technology Kanpur, where she studied the role of microenvironment in cancer progression and tumor formation. During her post-doctoral research at the University of Illinois at Urbana-Champaign, she investigated the gene regulatory networks that are important for tissue regeneration after damage or wounding. She is committed to science outreach activities and believes it is essential to inspire young people to apply scientific methods to tackle the challenges faced by humanity. As an editor, her aim is to simplify, translate, and excite people about current advances in science.
Amrita Anand is in her 4th year of Ph.D. in Genetics and Genomics at the Baylor College of Medicine, Houston. She studies the reprogramming potential of certain key factors in the regeneration of mouse inner ear hair cells. She has been actively pursuing Science communication over the last three years as she enjoys bridging the gap between scientists and non-experts. As an editor, she wants to make science more accessible to the public and also hopes the hard work behind the science gets due credit.
Illustrator:
Aryaa Apotikar is in her third year of bachelor’s in Life Science and Biochemistry at St. Xavier’s College, Mumbai. She has a keen interest in cell and molecular biology and has dreamt of being an illustrator since the age of 5. She is a voracious reader and the amalgamation of reading and illustrating brought together the idea of being a scientific illustrator. She has been an active graphics editor in Lignum Vitae, her college magazine, and other websites.
