Rapid transition towards the Division of Labor via evolution of developmental plasticity

Abstract

A crucial step in several major evolutionary transitions is the division of labor between components of the emerging higher-level evolutionary unit. Examples include the separation of germ and soma in simple multicellular organisms, appearance of multiple cell types and organs in more complex organisms, and emergence of casts in eusocial insects. How the division of labor was achieved in the face of selfishness of lower-level units is controversial. I present a simple mathematical model describing the evolutionary emergence of the division of labor via developmental plasticity starting with a colony of undifferentiated cells and ending with completely differentiated multicellular organisms. I explore how the plausibility and the dynamics of the division of labor depend on its fitness advantage, mutation rate, costs of developmental plasticity, and the colony size. The model shows that the transition to differentiated multicellularity, which has happened many times in the history of life, can be achieved relatively easily. My approach is expandable in a number of directions including the emergence of multiple cell types, complex organs, or casts of eusocial insects.

Figure 1. Evolution in major loci.

(A) An example of the model's dynamics with . Shown are at top: the average values of (red) and (blue), middle: the average fertility (red) and viability (blue), and bottom: the number of colonies in the system. (B) The equilibrium values of for different and (blue),8, 16, 32 and 64 (pink). , so that . (C) The relative equilibrium population size for the same values of parameters as in (b).

Figure 2. Examples of the model dynamics with .

First column: the dynamics of the main (solid lines) and modifier (dashed lines) allelic effects and the population size. Second column: fertility and viability for pro-some and proto-germ cells; each cell in the population is represented by a circle. Data are saved every 2000 generations. First row: . Second row: . Third row: .

Figure 3. The areas of the 3-dimensional parameter space where complete germ-soma differentiation was observed (filled cubes).

. For , and (lightly colored subcube), the major locus effects and evolved very close to but the modifier effects and were around .

Figure 4. Conditions for local stability of an equilibrium with no gene supression () and optimum value of major locus effects () when the colony size is very large () for 3 different values of (shown on the graph).

The equilibrium is stable for and values on the left of the corresponding curve. The dashed curve corresponds to no costs of gene supression ().