Organelle Genetics

• Maternal inheritance• Non-Mendelian inheritance

– Ratios do not fit those proposed by Mendel

• Cytoplasmic inheritance• Nonchromosomal inheritance

Terms are not necessarily equivalent!

Plasmon = genetic elements of the cytoplasm

Examples of non-Mendelian inheritance:• Variegated-shoot phenotypes in four o’clocks (Correns, 1908)

Normal chloroplastGreenphotosynthetic

Mutant chloroplastWhitenon-photosynthetic

Mixed chloroplastsWhite/green

Maternal inheritance

• Experiments were performed by Correns on the four o'clock plant Green, variegated (white and green) or white leaves

Normal flowers develop at different locations on the plant

Crosses were made among the flowers associated with:

Female Male Phenotype Progeny Phenotype Phenotype

Green Green, variegated or white Green

Variegated Green, variegated or white Variegated

White Green, variegated or white White The progeny cross always exhibited the color of the leaf of the female Trait expresses maternal inheritance

Most plants, including corn, Arabidopsis, wheat and tomato show strict maternal inheritanceof organellar (chloroplast and mitochondrial) DNA.

The female phenotype in a cross is always expressed in its offspring

Maternal inheritance: with occasional paternal transmission

Plants showing "strict" maternal inheritance (albomaculatus)

Test for male transmission in Antirrhinum majus:

Normal x yellow-green (prasinizans) 41,203 normal:13 variegated

yellow-green (prasinizans) x Normal 42,285 yellow-green:8 varieg

Mutation in female parent unlikely. a) Mutation rate is low.

Only 1/436,000 plants was spontaneously variegated. b) Same phenotype seen for all variegated progeny

Biparental inheritance

• Inheritance of variegation suggested both parents contribute, but ratios are non-Mendelian.

• Pollen transmission of plastids found for several species

– e.g. Pelargonium (geranium), Oenothera, Medicago, Phaseolus, Acacia

• Some plants have regular paternal transmission of plastids through pollen.

• Nuclear genes can affect “strength” of pollen transmission of organelles.

– e.g. Oenothera, Pelargonium (geranium), Phaseolus, Hypericum

Origins of mitochondria & chloroplastsBoth mitochondria and chloroplasts are believed to be derived from

endosymbiotic bacteria.

Endosymbiotic bacteria = free-living prokaryotes that invaded ancestral eukaryotic cells and established a mutually beneficial relationship.

Mitochondria - believed to be derived from a photosynthetic purple bacterium that entered a eukaryotic cell > billion years ago.

Chloroplasts - believed to be derived from a photosynthetic cyanobacterium.

Many required mitochondria and chloroplast proteins also are coded by nuclear genes.

numtDNA = nuclear mtDNA (mtDNA transposed to the nucleus)

Mitochondrial genome size and genic content

Size and gene content of mitochondrial genomes compared with a Proteobacterial (Rickettsia) genome

Circles and lines represent circular and linear genome shapes, respectively. For genomes >60 kbp, the DNA coding for genes with known function (red) is distinguished from that coding for unidentified ORFs and intergenic sequences (blue).

M. W. Gray, G. Burger, and B. F. Lang Science 1999: 1476-1481. Mitochondrial Evolution

mtDNAs --- Overview

• mtDNAs occur in (almost) all aerobic eukaryotic cells & generate energy for cells by oxidative phosphorylation (producing ATP).

• Most mtDNA genomes are circular and supercoiled (linear mtDNAs occur in some protozoa and some fungi).

• mtDNAs lack histone proteins.

• Copy number is high, multiple genomes per mitochondria and many mitochondria per cell (can easily PCR).

• Sizes of mtDNA varies widely.

• Humans and other vertebrates ~17 kb(all of mtDNA codes gene products)

• Yeast ~80 kb

• Plants ~100 kb to 2 Mb

(lots of non-coding mtDNA)

Human Mitochondrial Genomes

• Human Nuclear Genome: About 30,000 genes on 23 chromosomes (3.3 billion base pairs/haploid cell)

• Mitochondrial Genome contains 37 genes:– 13 code for some of the proteins involved in

oxidative respiration– 22 tRNA genes– 2 rRNA genes

– 16,569 base pairs, circular, very compact, filled with genes

Human Mitochondrial Diseases

Are maternally inherited: only offspring of affected mothers are affected

Show deficiency in mitochondrial function

Are caused by a mutation in a mitochondrial geneExamples:

• myoclonic epilepsy and ragged red fiber disease (MERRF)

• Deafness, dementia, seizures

• Point Mutation in a mitochondrial tRNA

• Leber’s Hereditary Optic Neuropathy (LHON)

• Sudden bilateral blindness

• Point mutation in small subunit of NADH dehydrogenase

• Kearns-Sayre Syndrome (KSS)

• Symptoms in eyes, muscles, heart, brain

• Deletion mutation in mtDNA

Why aren’t all mitochondrial mutations lethal?

Heteroplasmy

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HETEROPLASMY: Normal mitochondria with normal DNA vs Mitochondria with mutant DNA "Homoplasmic Cell. Healthy people have homoplasmic cells -- that is, each cell has normal mitochondrial DNA. People with mitochondrial DNA mutations have heteroplasmic cells. Each cell has a mixture of good and bad mitochondria.

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70% mutant mitochondria =severe symptoms?

30% mutant mitochondria =mild symptoms

Heteroplasmic CellsHomoplasmic Cell

http://www.mitoresearch.org/mitodiseases.html

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http://www.mitoresearch.org/mitodiseases.html

http://www.mitoresearch.org/treatmentdisease.html

Commonly Affected Systems in Mitochondrial Disorders

Maize mitochondrial genome“NB” genotype

from the inbred B37 line

569,630 nt58 identified genes33 known proteins21 tRNAs (for 14 diff aa)* 3 rRNAs

*A tRNA is carried on a 2 kb linear plasmid

From Clifton et al.2004, Plant Physiology

What is in the NB maize mitochondrial genome ?

Based on genome complexity (one copy of large repeats removed)520 kb for maize NB359 kb for rice

Exons

Introns

rRNAs

ctDNA

tRNAs

???

ORFs

pseudo

From Clifton et al.2004, Plant Physiology

Half the Arabidopsis mitochondrial DNA sequences are both non-coding and novel

Marienfeld, Unseld and Brennicke,1999, TIPS 4:495-502

- In sugar beet, 55% of mt genome has no recognizable function or database homology- Only 21% of sugar beet mtDNA is shared with Arabidopsis (Kubo et al. NAR 2000)