# Evolution of molecular communication in the permanent Azolla symbiosis

## TL;DR

The paper explores how the water fern Azolla maintains a stable, long-term symbiosis with a cyanobacterium, despite the symbiont being extracellular.

## Contribution

It highlights the unique evolutionary integration of environmental stress responses in the Azolla-cyanobacterium symbiosis.

## Key findings

- The Azolla symbiosis has persisted for at least 60 million years without secondary loss of the symbiont.
- Stable symbiont retention may require cross-organismal integration of environmental stress responses.
- The extracellular nature of the symbiont challenges traditional models of organelle-like integration.

## Abstract

Heritable symbioses exist across eukaryotes with different degrees of intimacy. In most cases, the symbionts are obligate and require inheritance for their survival. On the host side, symbiont retention can facilitate fitness benefits. Only rarely are these symbioses interwoven to the point that host survival relies on the symbiont. In land plants, the symbiosis of the water fern Azolla with its symbiotic cyanobacterium shows such a degree of high co‐dependence. The symbiosis originated in the last common ancestor of Azolla and exists continuously for at least 60 million years with no evolutionarily stable, secondary loss of the symbiont reported. This is a feat achieved by interactions on an organellar‐like level or those considered recent organelle acquisitions. Yet, Azolla's symbiont is extracellular. How can loss of autonomy concomitant with full co‐dependence be accommodated in this extracellular symbiosis? Here, we synthesize what we know from the Azolla symbiosis on the consequences of evolutionary co‐dependence and stable symbiont retention. We discuss the need for symbiotic integration into environmental responsiveness if host survival depends on symbiont well‐being. Cross‐organismal integration of environmental stress responses may be one of the key steps that favor this evolutionarily stable permanent integration.