Reproduction: How can the division of labor of cells prevail?

October 18, 2021

In complex organisms, two basic types of cells are found: on the one hand, reproductive cells, which are responsible only for the reproduction of the living being, and on the other hand, cells specialized for the vegetative functions of the body. This division of cells is irreversible, that is, the vegetative cell type produces only vegetative cells that are no longer involved in the process of reproduction. Why has this division of labor evolved, what are the advantages? This is what scientists at the Max Planck Institute for Evolutionary Biology in Plön are researching with the help of mathematical models.

The life cycle of an organism in the model begins with a single reproductive cell. In each round, all cells divide and the daughter cells each develop into one of the different cell types. Once the organism is mature, it reproduces, with each reproductive cell (green) becoming a new organism and each somatic cell (white) dying.

The vast majority of multicellular animals (metazoans) exhibit a very specific pattern of cell differentiation: Each cell that performs vegetative body functions forms a somatic lineage, i.e., it gives rise to cells that perform the same vegetative function - somatic differentiation is irreversible. Since reproductive cells cannot arise from such somatic cells, somatic cells have no chance to pass on their offspring to the next generation of organisms. This type of organism evolution has paved the way for deeper specialization of somatic cells and thus for the amazing complexity of multicellular animals.

From the perspective of a cell in an organism, however, the guaranteed extinction of its lineage seems to be the worst possible evolutionary outcome. From the perspective of the organism, in turn, the extinction of vegetative cell lineages at the end of their life cycle is actually a waste of resources.
So how and why did this principle of irrversible somatic differentiation of cells arise?

Mathematical models show the conditions of possible developments

The researchers, led by Yuanxiao Gao, Ph.D., and Yuriy Pichugin, Ph.D., who is now a researcher at Princeton, developed a model to examine the conditions under which the strategy of irreversible somatic differentiation maximizes the organism's growth rate compared with strategies in which this differentiation does not occur or is reversible. While in some cases the model can be solved with paper and pencil, in most cases large-scale computer simulations are required.
The simulations revealed several clear results:
1. irreversible somatic differentiation develops only when differentiation is associated with high costs. Otherwise, reversible differentiation develops, meaning somatic cells can change back to reproductive cells.
2. it develops when the presence of already few somatic (vegetative) cells contributes to increased growth of the organism. This means that the specialization of the vegetative cells, which can concentrate entirely on one function without having to deal with reproduction, has a positive effect on the body in question.
3. for irreversible somatic differentiation to occur, the size of the organism must be sufficient.

Thus, the results show clear factors that contribute to irreversible somatic differentiation providing an evolutionary advantage. This advantage leads to faster growth relative to other developmental strategies and to displacement of these strategies.

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