Viral Evolution in Endosymbionts

February 11, 2012

Seth BordensteinDepartments of Biological Sciences &Pathology, Microbiology, and Immunologys.bordenstein@vanderbilt.edu

Symbiosis: “The living together of dissimilar organisms” de Bary 1879

Gene loss / Genome reduction

How do bacterial genomes change during symbiosis?

Single nt mutations

Insertions of DNA from foreign sources;Duplications

The Genome Sizes of Life

Rule 1: Genome Size Variation Correlates with Gene Number Variation

Genome Size (Mb)

Prot

ein

Codi

ng G

enes

Rule 2: Bacterial Ecology Alters Genome Evolution

Diagram modified from Mira, A., Ochman, H. & Moran, N.A. 2001. Trends Genet 17, 589-96.

DNA Inflow DNA Outflow

Eukaryotic cell • Mutation bias for

DNA deletions

• Relaxed selection on redundant genes between bacteria and host

• Reduced exposure to novel gene pools

Bacteriophage TransposonPlasmid

Rule 3: Bacterial Mobile DNAs AreThe Agents of Open Source Evolution

Ecology Affects Mobile DNA Abundance

• Exposure to other microbes• Horizontal Gene Transfer• Mobile DNA

• Reduced exposure to other microbes

• Genome Reduction• Genome Stability

Free-living Bacteria Obligate Intracellular Bacteria

Diagram modified from Mira, A., Ochman, H. & Moran, N.A. 2001. Trends Genet 17, 589-96.

DNA Inflow DNA Outflow

Eukaryotic cell

DNA Inflow?Mobile DNA?

Three Questions

• How variable are mobile DNA contents in Bacteria?

• Does this variation statistically associate with phylogeny, genome size, or ecology?

• How do mobile elements survive and flourish in obligate, intracellular bacteria?

The Genomes of LifeBACTERI

A2622

GenomesARCHAEA

147 Genomes

EUCARYA166

Genomes

How variable are mobile DNA contents in Bacteria?

% Transposon % Prophage % PlasmidNewton & Bordenstein 2011

Is Mobile DNA Gene Density Independent

of Phylogeny?

16S rRNA Tree & Mobile DNA %

K statistic

Full dataset, K = 0.0024K > 1 is significant association

Mobile DNA % is scattered randomly across the tree

Newton & Bordenstein 2011

Do Constraints in Ecology Vary with Mobile DNA

Gene Density?

Facultative

Intracellular world

Facultative Extracellular

Free-living world

ObligateIntracellular

Three Bacterial Ecotypes

Facultative Extracellular Obligate0

1

2

3

4

Mob

ile D

NA

Den

sity

(%)

> >

P < 0.001

P < 0.005

N = 270N = 74 N = 40

Newton & Bordenstein 2011

FacultativeIntracellular world

Free-living world

Obligate

Facultative Extracellular