Transcript Organelle Evolution

Plant of the Day!
Rafflesia arnoldii
(Euphorbiaceae)
Parasitic (host Tetrastigma)
No stems, leaves or true roots
Largest single flower (>1 m,
10kg)
Corpse flower, diclinous
1/100 flowering plants are
parasitic
HGT can occur between
parasites and hosts

Organelle Genome Evolution
Describe the origin of the organelle genomes
Describe the evidence for gene transfer between
the organelle and nuclear genomes and discuss
hypotheses concerning the mechanism of gene
transfer
Discuss hypotheses concerning the maintenance
of organelle genomes
endosymbiotic theory
>1.5 billion years
?
The mitochondria: evolved from aerobic bacteria (αproteobacteria- rickettsias) and a host

The chloroplasts: evolved from a heterotrophic
eukaryote and a cyanobacteria

Evidence for Endosymbiotic Theory
-Circular molecule
No histones
Protein synthesizing machinery (ribosomes,
tRNA, rRNA)
Some antibiotics block protein synthesis within
the mitochondria and chloroplasts
Structurally similarity
Reproduce through fission
-Strong phylogenetic evidence
-
-
Plant ancestor
Primary
endosymbiosis
Primary
endosymbiosis
amoeba
Secondary
endosymbiosis
Secondary
endosymbiosis
Secondary
endosymbiosis
Tertiary
endosymbioses
Tertiary
endosymbiosis
Keeling 2004
Plant chloroplasts
Structure of Plant cp Genomes
LSC – 84 kb
SSC – 18 kb
IR – 25 kb (each))
Dempewolf et al., 2010
Gene transfer and comparative
analysis
Comparisons of nuclear, organelle and
candidate prokaryotic ancestor genomes?
-Ancient transfer events
Comparison of organelle genes and nuclear
genes of the same species?
-Recent transfer events
Chloroplast Gene Transfer
NUPTS (nuclear plastid DNA) :
– Arabidopsis has 11kb from 17 insertions
– rice chromosome 10 has a 33kb and a
131kb insertion and 26 more over 80bp
each
Chloroplast Gene Transfer
InfA (translation initiation factor)
transferred to nuclear genome (~24x)
Mutational decay/loss of cp sequence
~18% of protein coding genes in
Arabidopsis are from the plastid
(transit peptide coding region)
Endosymbiotic gene replacement
Millen et al 2001
Chloroplast Gene Transfer
Rate estimates from tobacco chloroplasts
– 1 transfer in 5 million leaf cells
– 1 transfer in 16 000 pollen grains
Higher rates of transfer in the pollen?
Degradation of the organelle genomes in pollen could
make DNA fragments available for uptake
Plant mitochondria
kbp
# protein coding genes
Wheat mtDNA
452,528 bp
Plant mitochondrial genomes undergo intra- or intermolecular
recombination via repeated sequences subgenomic molecules
(loop out) or isomeric forms (flip flop)
Mitochondrial Gene Transfer
• NUMTS:
– complete mt genome sequences
in cat nuclei
– >296 in humans, from 106 to
14.5 kbp each (older NUMTs
more abundant-tandem repeats)
– rice chromosome 10 has 57
NUMTS ranging from 80-2552bp
Do larger nuclear genomes have more NUMTS?
Mitochondrial Gene Transfer
• rate estimates:
– 1 plasmid transfer to nucleus in 20,000
yeast cells
(integration rare)
Mitochondrial Gene Transfer
Evidence for parallel transfer and
loss
e.g. rps10 gene independently
transferred to nucleus
numerous times
Adams et al 2000
Mechanisms
• Bulk DNA – recombination between escaped
organelle DNA and nuclear DNA
– Expt. transfer in yeast
– Non-coding sequence frequently transferred
– Whole organelle sequences transferred
MUST HAPPEN
• cDNA intermediates
– Nuclear copies of organelle genes often lack
organelle-specific introns and edited sites
MAY HAPPEN
Why are the organelle
genomes maintained?
• Hydrophobicity -hydrophobic proteins are
poorly imported (excess of membrane
embedded proteins)
• Fitness advantage if coding sequence and
regulation are in same location
• Other constraints (e.g. RNA editing, genetic
code)