# Germline fate determination by a single ARGONAUTE protein in Ectocarpus

## TL;DR

A single ARGONAUTE protein in brown algae controls germline development and key life cycle transitions through RNA-based regulation.

## Contribution

Demonstrates that a single AGO protein can regulate complex developmental processes in brown algae, revealing a minimalist RNA silencing system.

## Key findings

- A single AGO protein in Ectocarpus is essential for transitioning from vegetative growth to sexual reproduction.
- AGO functions with microRNAs to regulate gene expression posttranscriptionally during germline specification.
- Brown algae use a minimalistic AGO-based system for developmental regulation despite their complex multicellular organization.

## Abstract

ARGONAUTE (AGO) proteins are central regulators of gene expression in multicellular eukaryotes, mediating developmental processes through interactions with small RNAs. While extensively studied in plants and animals, the diversity of AGO function across independently evolved multicellular lineages remains poorly understood. Using brown algae as a comparative model, we demonstrate that a single AGO protein can direct key developmental transitions and germline specification likely through posttranscriptional regulation. These findings reveal a minimalist yet effective RNA silencing system and provide insights into the evolutionary versatility of AGO proteins in coordinating complex developmental programs across eukaryotes.

ARGONAUTE (AGO) proteins are a highly conserved family of RNA-binding proteins that play central roles in gene regulation and developmental processes across eukaryotes. Although AGO family members have been extensively studied in animals and plants, where they are typically encoded by multiple genes, their function in brown algae, a diverse and complex group of multicellular algae, remains largely unknown. Here, we show that the genomes of several brown algae encode only a single AGO protein, containing the conserved functional domains characteristic of the family. Using the model brown alga Ectocarpus and a combination of cell biology, genetic, and transcriptomic approaches, we demonstrate that AGO is essential for the transition from vegetative growth to sexual reproductive development and for germline establishment. Our results further suggest that AGO functions in concert with microRNAs to regulate target genes primarily at the posttranscriptional level, likely through translational repression. Ectocarpus thus represents a rare example of a complex multicellular organism that relies on a single AGO protein to regulate key developmental processes, pointing to a minimalistic model of RNA-based regulation in brown algae.