Division of labour and the evolution of multicellularity

TL;DR

This paper presents a quantitative model showing how multicellularity can evolve spontaneously from unicellular ancestors through division of labour, driven by fitness advantages from cellular compartmentalization without pre-existing genetic roles.

Contribution

The study introduces a mathematical model demonstrating that multicellularity can emerge from unicellular organisms via spontaneous division of labour, driven by fitness benefits and changes in phenotype space.

Findings

Multicellularity arises from unicellular ancestors through spontaneous cellular differentiation.

Division of labour in aggregates provides a fitness advantage without genetic pre-disposition.

Aggregation increases phenotype space dimensionality, creating new fitness maxima.

Abstract

Understanding the emergence and evolution of multicellularity and cellular differentiation is a core problem in biology. We develop a quantitative model that shows that a multicellular form emerges from genetically identical unicellular ancestors when the compartmentalization of poorly compatible physiological processes into component cells of an aggregate produces a fitness advantage. This division of labour between the cells in the aggregate occurs spontaneously at the regulatory level due to mechanisms present in unicellular ancestors and does not require any genetic pre-disposition for a particular role in the aggregate or any orchestrated cooperative behaviour of aggregate cells. Mathematically, aggregation implies an increase in the dimensionality of phenotype space that generates a fitness landscape with new fitness maxima, and in which the unicellular states of optimized metabolism become fitness saddle points. Evolution of multicellularity is modeled as evolution of a hereditary parameter, the propensity of cells to stick together, which determines the fraction of time a cell spends in the aggregate form. Stickiness can increase evolutionarily due to the fitness advantage generated by the division of labour between cells in an aggregate.