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)