One of the important points that evolution highlights is that all organisms come from a common ancestor. Phylogenetic trees are developed in order to show the relationships of organisms. These trees are continually challenged and analyzed so that a more comprehensive representation of the relationship between organisms can be created. In examining how different organisms are related, different methods are used so that they can be surveyed at different levels. One would be surprised at how the most complex animals can be related to the simplest microorganisms.
An established hypothesis about animal evolution is that animals are descended from a unicellular ancestor. This unicellular organism has the same structure to that of modern sponge choanocytes and choanoflagellates. However, in a recent study entitled Pluripotency and the origin of animal multicellularity conducted by scientists at The University of Queensland, it was reported that the earliest multicellular animals can be classified more as a collection of convertible cells that are similar to a stem cell – somatic cells that can divide and differentiate, and can have further specializations. Since sponges are known to only be an aggregate of cells with no ability to differentiate further, this raises a question as to how close the relationship between animals and choanoflagellates is.
How are Choanoflagellates and Sponge Choanocytes Closely Related to Animals?
In 1841, a French biologist named Félix Dujardin was able to report and propose that choanoflagellates and sponge choanocytes may have a close relationship due to the similarities between their structures. Many independent studies that used multiple unlinked sequence analyses (18S rDNA, nuclear protein-coding genes, mitochondrial genes, etc.) have supported the hypothesized relationship. Through genome sequencing, it was also revealed that choanoflagellates are the most closely related living thing to animals. The sponge choanocytes, more commonly known as “collared cells”, comprise similar primary structures to that of choanoflagellates. They are also found in animal groups such as ribbon worms. This is why it was suggested that the last common ancestor of sponge choanocytes, choanoflagellates, and animals may have the same morphology.
Pluripotency and the origin of animal multicellularity: A Summary
The last common ancestor of animals has an ontogenetic life cycle, but it also possesses the smallest amount of epithelial and mesenchymal cell types, or cells that can differentiate into different types. An organism that has an ontogenetic life cycle should be able to regulate spatial and temporal gene expression, and have a diversified set of signaling pathways, transcription factors, enhancers, promoters and non-coding RNAs. Unicellular holozoans are the closest single-celled relatives of animals, and from recent analyses, they appear to have the same gene regulatory mechanisms of an organism that undergoes an ontogenetic life cycle. Due to this discovery, it was recommended that early stem metazoans are more complex than previously thought.
Different methods were used to find out if choanocytes and choanoflagellates accurately represent the ancestral animal cell type. The first method is through comparison of cell-type specific transcriptomes, and then phylostratigraphy – where they find out the evolutionary age of all protein-coding genes, and lastly, the investigation of the cell types in development. Cell-type specific transcriptomes were compared from a specific sponge species against transcriptomes found in a choanoflagellate, a filasterean, and an ichthyosporean, protists that are closely-related to animals. The three somatic cell types of sponges are then chosen because they are hypothesized to be homologous in structure to that of the last common ancestor of modern metazoans, choanozoans, or holozoans.
Analysis shows that the three somatic cell types of sponges are unique; the most unrelated being the choanocytes. It also showed that archaeocytes display an important role in gene upregulation that controls cell proliferation, transcription, and translation – indicating that they act as pluripotent stem cells. Choanocyte and pinacocyte transcriptomes show genes that have roles in cell adhesion, signalling, and polarity, and are able to act like epithelial cells.
For the phylostratigraphic aging of the three cell types, results show that there is no significant difference in their ages – suggesting that its gene regulatory networks have the same evolutionary age. When the expression profiles of transcription factor genes of unicellular holozoans were compared with the specific sponge species used in the study, there was no proof of a conserved and co-expressed gene regulatory network. However, the role of Myc, a proto-oncogene with partner Max, is present in the archaeocytes of the sponge species for the study, the same with other metazoan stem cells. This proto-oncogene is also present in choanoflagellates, filastereans, and ichthyosporeans. The role of Myc therefore provides a recommendation that it was a possible cardinal feature of the first metazoan cell.
The results of investigating the development of the different cell types show that archaeocytes are the most important in the development and maintenance of body plan of the sponge species used in the study, as well as other sponge species. Choanocytes appear in the later part of development and exist in metastable state. In times, choanocytes only last a few hours and they differentiate back to archaeocytes.
The results from the analyses mentioned show that the ancestral metazoan cell type has the ability to change between multiple cell states, resembling the behavior of modern stem cells capable of transdifferentiation. This does not support the long-established hypothesis that animal cells came from non-differentiating cells that are seen in modern choanocytes and choanoflagellates.
Why is this important?
The study shows us that there is yet to investigate in the evolution of living things. The presence of genomic innovations allow new methods to be introduced so that more extensive analyses can be developed. With technology always leaning towards advancement, more tools are allowed to be generated for research, making way for breakthroughs in the various fields of science, especially in the field of evolution and genetics – thus helping humans continually understand what we really are, and where we really come from.
References:
Leadbeater, B. S., & Kelly, M. (2001). Evolution of animals choanoflagellates and sponges. Water and Atmosphere Online. 9 (2): 9–11.
King, N., & Carroll, S.B. (December 2001). A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution. Proceedings of the National Academy of Sciences of the United States of America. 98 (26): 15032–7. Bibcode:2001PNAS...9815032K. doi:10.1073/pnas.261477698. PMC 64978. PMID 11752452
King, N., Westbrook, M.J., Young, S.L., Kuo, A., Abedin, M., Chapman, J., et al. (February 2008). The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans. Nature. 451 (7180): 783–8. Bibcode:2008Natur.451..783K. doi:10.1038/nature06617. PMC 2562698. PMID 18273011
Cantell, C., Franzén, Å., & Sensenbaugh, T. (October 1982). Ultrastructure of multiciliated collar cells in the pilidium larva of Lineus bilineatus (Nemertini). Zoomorphology. 101 (1): 1–15.doi:10.1007/BF00312027 https://www.labroots.com/trending/cell-and-molecular-biology/14972/changing-transition-multicellular-life?fbclid=IwAR3viSI4ZKOHolrPwy8PZkuG1-9NcXi4HPMNJKSzwIrqdGHWSuQ5RRYLAXg
Sogabe, S., Hatleberg, W., Kocot, K. Say, T., Stoupin, D., Roper, K., Selene Fernandez-Valverde, S., Degnan, S. & Degnan, B. (n.d.). Pluripotency and the origin of animal multicellularity. doi: https://doi.org/10.1101/564518
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Nikki S. Matta is an undergraduate student of the BS Biology program in UPLB. As much as she loves reading about theories in evolutionary biology, she also loves to explore the many genres of TV and film in her free time.
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