Abstract
A key innovation emerging in complex animals is irreversible somatic differentiation: daughters of a vegetative cell perform a vegetative function as well, thus, forming a somatic lineage that can no longer be directly involved in reproduction. Primitive species use a different strategy: vegetative and reproductive tasks are separated in time rather than in space. Starting from such a strategy, how is it possible to evolve life forms which use some of their cells exclusively for vegetative functions? Here, we develop an evolutionary model of development of a simple multicellular organism and find that three components are necessary for the evolution of irreversible somatic differentiation: (i) costly cell differentiation, (ii) vegetative cells that significantly improve the organism's performance even if present in small numbers, and (iii) large enough organism size. Our findings demonstrate how an egalitarian development typical for loose cell colonies can evolve into germ-soma differentiation dominating metazoans.
Highlights
In complex multicellular organisms, different cells specialise to execute different functions.These functions can be generally classified into two kinds: reproductive and vegetative
We found that irreversible somatic differentiation (ISD) does not evolve when cell differentiation is not associated with any costs, see Fig 2A
Reversible somatic differentiation strategies (RSD) do not experience a similar tradeoff, as germ-role cells can be generated from soma-role cells
Summary
Different cells specialise to execute different functions. The majority of the theoretical models addressing the evolution of somatic cells focuses on the evolution of cell specialization, abstracting from the developmental process how germ (reproductive specialists) and soma are produced in the course of the organism growth. We developed a theoretical model to investigate conditions for the evolution of the irreversible somatic differentiation, in which vegetative soma-role cells are, in principle, capable to re-differentiate and produce reproductive germ-role cells. For a given combination of differentiation costs (cg , cs ) and a composition effect profile (determined by four parameters: x0 , x1 , b, and α), we screen through a number of stochastic developmental strategies D and identify the one providing the largest growth rate to the population. We searched for those parameters under which ISD strategies lead to the fastest growth and are evolutionary optimal, see model details in Appendix A.1
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