Chronos, Chairos and Co Evolution in Rhizobium Legume Symbioses

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Chronos, Chairos and Co- Evolution in Rhizobium
Legume Symbioses
Kristina Lindström Viikki Biocenter University of
Helsinki Finland
15th INFC Cape Town 25.01.2007
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RHIZOBIA HAVE TWO LIVES
ENERGY
O2
NITROGENASE
N2
N2 4H2
2NH3H2
FREE LIVING AND SYMBIOTIC
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GENOME COMPOSITION
ACCESSORY GENOME genes for niche adaptation
(rhizobium symbiotic and stress tolerance
genes) strain specific expansions
CORE GENOME ribosomal genes other housekeeping or
core genes
These genomic components are evolving and natural
selection acts upon them (Darwin). But how do
they evolve?
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RHIZOSPHERE BACTERIA (CULTURABLE AND
UNCULTURABLE)
GENE POOL
GENOME GRADIENT
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CHAIROS EVOLUTION accessory genome
CHRONOS EVOLUTION core genome
CHRONOS EVOLUTION Slow changes of core genome
over time (speciation) CHAIROS EVOLUTION Fast
changes of accessory genome (niché adaptation)
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MECHANISMS

• CHRONOS EVOLUTION Slow changes of core genome
  over time (speciation)
• mutation in sequence
• mutation in structure
• genome rearrangements
• recombination

• CHAIROS EVOLUTION Fast changes of accessory
  genome
• (niche adaptation)
• conjugation
• transposition
• transduction

• EXAMPLES OF GENES
• antibiotic resistance
• pathogenicity
• T3SS
• symbiotic

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APPLICATION OF THIS CONCEPT TO TAXONOMY
THE EVOLUTIONARY SPECIES CONCEPT (BASED ON
CHRONOS EVOLUTION) VS. THE ECOLOGICAL SPECIES
CONCEPT (BASED ON CHAIROS EVOLUTION )
THE TAXONOMY OF RHIZOBIA IS BASED ON CHRONOS
EVOLUTION
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BACTERIAL GENERA KNOWN TO FORM NITROGEN-FIXING
NODULES
Alpha-proteobacteria Agrobacterium Allorhizobium
Azorhizobium Blastobacter Bradyrhizobium Devosia H
erbaspirillum Mesorhizobium Methylobacterium Ochro
bactrum Phyllobacterium Rhizobium Sinorhizobium
Beta-proteobacteria Burkholderia Cupriavidus
(Ralstonia) Gamma-proteobacteria?
Proteobacteria
Bern and Goldberg
The only one until 1982
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GENUS AND SPECIES NAMES ARE GIVEN ACCORDING TO
THE PHYLOGENY OF CORE GENES AND REFLECT THEIR
EVOLUTION AND THE SPECIATION OF THE ORGANISMS

• SYMBIOTIC PROPERTIES CAN BE INDICATED IN BIOVAR
  DESIGNATIONS
• Rhizobium leguminosarum biovar viciae
• (nodulates vetches and peas)
• Rhizobium leguminosarum biovar trifolii
• (nodulates clovers)

THE SYMBIOTIC GENES ARE IMPORTANT ACCESSORY GENES
FOR RHIZOBIA THEY CONFER ADAPTATION TO SPECIFIC
ECOLOGICAL NICHES THE HOST PLANTS
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A CROSS DISCIPLINARY SPECIES CONCEPT
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A CROSS DISCIPLINARY SPECIES CONCEPT
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CURRENT SPECIES DEFINITIONS
SAME SPECIES SUFFICIENTLY SIMILAR DIFFERENT
SPECIES SUFFICIENTLY DIFFERENT
(16S PHYLOGENY) RELATIVE DNADNA HOMOLOGY ABOVE
70 WITHIN SPECIES DISTINCTIVE PHENOTYPIC FEATURE
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BACTERIA ARE NOT INDIVIDUALS BUT CLONES
A CLONE Static, and consisting of identical
cells A PROKARYOTIC POPULATION A mixture of
genetically diverging clonal cell lines on which
natural selection acts A TAXONOMIC
CATEGORY Consists of evolving populations THE
EVOLUTIONARY SPECIES CONCEPT (ESC) Incorporates
the biology and the phylogeny of evolving
populations. If lateral gene transfer occurs
within a taxonomic category, it adds to its
coherence, if it occurs between taxa, the
category diverges and may give rise to new taxa
SPECIATION the result is a new species
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Agrobacterium and Rhizobium Webmaster Pablo
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