World’s Smallest Genome of a Cellular Organism?

A one page paper in Science reports on what I think is one of the most exciting findings in microbial genomics in years. The reports describes the sequencing and analysis of the genome of a bacterial endosymbiont of an aphid. This bacteria, known as Carsonella, has a TINY genome – only 160 kbp in length. This is ~ 3 fold smaller than the previously known smallest genome – that of Nanoarchaeum equitans which has a genome of 490 kbp.

I think almost certainly this symbiont should be considered an organelle. It is missing many cellular functions found even in the most reduced symbionts. Thus in essence it may not be the smallest genome of a cellular organism. But who cares how we define it. If it is a new organelle – that is amazing. If it is a tiny cellular genome – that is amazing too.

One thing that strikes me as strange is the fact that the paper is only one page long. It contains so little detail on what was done and what was found in the genome that the story is woefully incomplete. This I would guess is somehow related to a rush to publish but also likely due to it being published in Science, which has severe page restrictions.

This paper has been getting ENORMOUS press coverage for valid reasons. But I agree with Craig Venter (see the New Scientist article) that this genome is not of much relevance to efforts to create a “minimal” genome. This is because the ideal minimal genome is one that can support independent life. Carsonella, is far from independent and thus represents a really wild evolutionary story, but nothing of much relevance to minimal genome studies.

Some related links:

• Commentary by Siv Andersson
• NPR Story
• Nancy Moran’s Home Page (one of the corresponding authors of the paper)
• World Science article

Nakabachi, A., Yamashita, A., Toh, H., Ishikawa, H., Dunbar, H., Moran, N., & Hattori, M. (2006). The 160-Kilobase Genome of the Bacterial Endosymbiont Carsonella Science, 314 (5797), 267-267 DOI: 10.1126/science.1134196

Author: Jonathan Eisen

I am an evolutionary biologist and a Professor at U. C. Davis. (see my lab site here). My research focuses on the origin of novelty (how new processes and functions originate). To study this I focus on sequencing and analyzing genomes of organisms, especially microbes and using phylogenomic analysis View all posts by Jonathan Eisen