# What makes a megaplasmid?

## TL;DR

This paper explores the characteristics and significance of large plasmids, called megaplasmids, in microbial biology and evolution.

## Contribution

The paper provides a perspective on the overlooked diversity and biological roles of megaplasmids in microbial systems.

## Key findings

- Megaplasmids are widespread and diverse, often encoding key traits for host microorganisms.
- Improved sequencing technologies have revealed the complexity and ecological importance of megaplasmids.
- Megaplasmids may have distinct characteristics beyond just their large size.

## Abstract

Naturally occurring plasmids come in different sizes. The smallest are less than a kilobase of DNA, while the largest can be over three orders of magnitude larger. Historically, research has tended to focus on smaller plasmids that are usually easier to isolate, manipulate and sequence, but with improved genome assemblies made possible by long-read sequencing, there is increased appreciation that very large plasmids—known as megaplasmids—are widespread, diverse, complex, and often encode key traits in the biology of their host microorganisms. Why are megaplasmids so big? What other features come with large plasmid size that could affect bacterial ecology and evolution? Are megaplasmids 'just' big plasmids, or do they have distinct characteristics? In this perspective, we reflect on the distribution, diversity, biology, and gene content of megaplasmids, providing an overview to these large, yet often overlooked, mobile genetic elements.

This article is part of the theme issue ‘The secret lives of microbial mobile genetic elements’.