Rafflesia arnoldii (Euphorbiaceae) Parasitic (host Tetrastigma) No stems, leaves or true roots...
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Transcript of Rafflesia arnoldii (Euphorbiaceae) Parasitic (host Tetrastigma) No stems, leaves or true roots...
Rafflesia arnoldii (Euphorbiaceae)Parasitic (host Tetrastigma)No stems, leaves or true rootsLargest single flower (>1 m, 10kg)Corpse flower, diclinous1/100 flowering plants are parasiticHGT can occur between parasites and hosts
Plant of the Day!
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
The mitochondria: evolved from aerobic bacteria (α-proteobacteria- rickettsias) and a host
The chloroplasts: evolved from a heterotrophic eukaryote and a cyanobacteria
>1.5 billion years
?
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
Evidence for Endosymbiotic Theory
Keeling 2004
Primary endosymbiosis
Primary endosymbiosis
amoeba
Plant ancestor
Secondary endosymbiosis
Secondary endosymbiosis
Secondary endosymbiosis
Tertiary endosymbiosis
Tertiary endosymbioses
Plant chloroplasts
Structure of Plant cp Genomes
Dempewolf et al., 2010
LSC – 84 kbSSC – 18 kbIR – 25 kb (each))
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
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
Chloroplast Gene Transfer
Plant mitochondriakbp # protein coding genes
Wheat mtDNA452,528 bp
Plant mitochondrial genomes undergo intra- or intermolecular recombination via repeated sequences subgenomic molecules (loop out) or isomeric forms (flip flop)
• 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
Mitochondrial Gene Transfer
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)