Evolutionary changes arise gradually via the slow accumulation of mutations. However, large changes can also occur suddenly, via “major transitions” that give rise to fundamentally new forms of organismal complexity. The eukaryotic cell, for example, arose from the endosymbiotic fusion of two prokaryotes. Although this new cell was vastly more complex than either one of the original cells, fusion forced each cell to sacrifice its autonomy for the new greater whole. Additionally, to reduce conflicts that could destroy the nascent organism, each parent cell was forced to cooperate with its new partner, to divide tasks, to coordinate gene expression and chromosome segregation, and to abandon duplicate genes or functions. We remain unable to explain why or how this major transition occurred, and how it remained robust in the face of parasitic cells that did not cooperate with the others. Thus, while we are increasingly able to describe and predict evolutionary change within and between species using phylogenetic inference, we lack a framework for experimentally understanding the evolution of the major transitions that gave rise to much of life’s complexity.

To address this knowledge gap, we will exploit a powerful experimental system using bacterial cells that proliferate without their cell wall, called L-forms, which can be fused to form polyploid cells containing chromosomes that express complementary, and essential functions. This fusion gives rise, for the first time, to a synthetic mutualism in a single cell with a division of labour, and which will allow us to elucidate the dynamics, stability and genetic underpinnings of transitions in complexity. Our results will transform our understanding of the evolution of complex cells. Additionally, they will help identify underlying principles required to sustain designer cells that are robust in the face of environmental change.


Project number


Main applicant

Dr. D.E. Rozen

Affiliated with

Universiteit Leiden, Faculteit der Wiskunde en Natuurwetenschappen, Institute of Biology Leiden - IBL

Team members

Mr. T van Dijk


14/05/2018 to 01/09/2021