# Evolution and coexistence in response to a key innovation in a long-term evolution experiment with Escherichia coli

## TL;DR

A new ability to consume citrate in E. coli led to stable coexistence with non-citrate-consuming bacteria through cross-feeding, creating a more complex ecosystem.

## Contribution

Demonstrates how a novel evolutionary trait can reshape ecosystem dynamics and promote coexistence through cross-feeding.

## Key findings

- Citrate-consuming E. coli coexisted with non-citrate-consuming clades via cross-feeding on byproducts.
- The ecosystem evolved from one resource (glucose) to five shared or partitioned resources.
- Evolution of citrate use altered environmental conditions and coexistence dynamics.

## Abstract

Evolution of a novel function can greatly alter the effects of an organism on its environment. These environmental changes can, in turn, affect the further evolution of that organism and any coexisting organisms. We examine these effects and feedbacks following evolution of a novel function in the long-term evolution experiment (LTEE) with Escherichia coli. A characteristic feature of E. coli is its inability to consume citrate aerobically. However, that ability evolved in one of the LTEE populations. In this population, citrate-utilizing bacteria (Cit+) coexisted stably with another clade of bacteria that lacked the capacity to utilize citrate (Cit−). This coexistence was shaped by the evolution of a cross-feeding relationship in which Cit+ cells released the dicarboxylic acids succinate, fumarate, and malate into the medium, and Cit− cells evolved improved growth on these carbon sources, as did the Cit+ cells. Thus, the evolution of citrate consumption led to a flask-based ecosystem that went from a single limiting resource, glucose, to one with five resources either shared or partitioned between two coexisting clades. Our findings show how evolutionary novelties can change environmental conditions, thereby facilitating diversity and altering both the structure of an ecosystem and the evolutionary trajectories of coexisting organisms.

Evolution does not produce novelties from scratch. It works on what already exists, either transforming a system to give it new functions or combining several systems to produce a more elaborate one.

–Francois Jacob