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. 2008 Jan;21(1):104-110.
doi: 10.1111/j.1420-9101.2007.01466.x. Epub 2007 Nov 23.

Organism size promotes the evolution of specialized cells in multicellular digital organisms

M Willensdorfer  1
Affiliations

Organism size promotes the evolution of specialized cells in multicellular digital organisms

M Willensdorfer. J Evol Biol. 2008 Jan.

Abstract

Specialized cells are the essence of complex multicellular life. Fossils allow us to study the modification of specialized, multicellular features such as jaws, scales, and muscular appendages. But it is still unclear what organismal properties contributed to the transition from undifferentiated organisms, which contain only a single cell type, to multicellular organisms with specialized cells. Using digital organisms I studied this transition. My simulations show that the transition to specialized cells happens faster in organism composed of many cells than in organisms composed of few cells. Large organisms suffer less from temporarily unsuccessful evolutionary experiments with individual cells, allowing them to evolve specialized cells via evolutionary trajectories that are unavailable to smaller organisms. This demonstrates that the evolution of simple multicellular organisms which are composed of many functionally identical cells accelerates the evolution of more complex organisms with specialized cells.

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Figures

Figure 1
Figure 1
Multicellularity in DISCOs. (a) The first five cell division of a DISCO with a genome that encodes for two X cells (green shaded regions) and one Y cell (blue shaded region). The first three cell divisions produce the somatic X and Y cells. Every further division produces offspring which is released into the environment. (b) The merit of this multicellular organism during the first and second 10 000 generations of the -X and +X simulations. The genome encodes for five (out of nine) logic functions as indicated by the nine-digit binary sequence. D, X, and Y cells can utilize these functions (second column) only according to their cell type specificity (first column) and receive a corresponding merit (third column) which is used to calculate the merit of the organism (fourth column). Note that Y cells are not able to increase the merit of the organism during the first 10 000 generations and that X cells are not able to increase the merit of the organism during the -X simulations. Cells that do not increase merit are disadvantageous, since they increase the number of cell divisions that are required to reach maturity. A more detailed description is available in the SOM.
Figure 2
Figure 2
Number of simulations that evolved specialized cells as a function of time. Simulations are grouped according to the evolutionary paths cf, si, and fc (see Figure legend and main text) that led to the evolution of new cell types that utilize new functions. Noticeable differences between simulations with unicellular (-X) and undifferentiated multicellular (+X) ancestors exists only with respect to evolutionary path cf along which specialized cell (Y cells) appear before the genome encodes for the specialized functions.
Figure 3
Figure 3
Time, t, in number of generations between the appearance of specialized cells (Y cells) and the appearance of specialized functions. The data is grouped according to the size of the DISCO immediately before the appearance of Y cells. The plot contains data from the +X (organism size greater than one) and the -X (organism size equals one) simulations. The pannel in the middle shows the number of simulations that evolved Y cells via fc, si, and cf, respectively. The correlation between t and the size of the organism for evolutionary path cf is evident (Kendall's tau statistic: z = 5.15, p = 2.64 · 10−7). It shows how organism size affects the evolution of specialized cells by reducing the detrimental effect of temporarily unsuccessful evolutionary experiments with individual cells. To increase expressiveness I added small random noise to the organism size and used different plot regions for fc and cf.
Figure 4
Figure 4
Number of simulations that evolved organisms with (white bars) and without (black bars) specialized cells after 200, 500, 5000, and 10000 generations, grouped according to the size of the organism. Large organisms show a significant bias (see p-values of a Pearson's chi-squared test) towards discovering specialized cells earlier than small organisms.
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