Organelle Genetics

1. 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

2. Examples of non-Mendelian inheritance: • Variegated-shoot phenotypes in four o’clocks (Correns, 1908) Mixed chloroplasts White/green Mutant chloroplast White non-photosynthetic Normal chloroplast Green photosynthetic

3. Maternal inheritance The female phenotype in a cross is always expressed in its offspring • 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 inheritance of organellar (chloroplast and mitochondrial) DNA.

4. Maternal inheritance:with occasional paternal transmission

5. 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

6. Origins of mitochondria & chloroplasts Both 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)

7. 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

8. 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)

9. 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

10. 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 gene Examples: • 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

11. Why aren’t all mitochondrial mutations lethal?Heteroplasmy

12. Heteroplasmic Cells Homoplasmic Cell 70% mutant mitochondria =severe symptoms? 30% mutant mitochondria =mild symptoms 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. http://www.mitoresearch.org/mitodiseases.html

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

14. Commonly Affected Systems in Mitochondrial Disorders http://www.mitoresearch.org/treatmentdisease.html

15. Maize mitochondrial genome“NB” genotypefrom the inbred B37 line 58 identified genes 33 known proteins 21 tRNAs (for 14 diff aa)* 3 rRNAs *A tRNA is carried on a 2 kb linear plasmid 569,630 nt From Clifton et al. 2004, Plant Physiology

16. Exons Introns rRNAs tRNAs ORFs pseudo ctDNA ??? What is in the NB maize mitochondrial genome ? Based on genome complexity (one copy of large repeats removed) 520 kb for maize NB 359 kb for rice From Clifton et al. 2004, Plant Physiology

17. Half the Arabidopsis mitochondrial DNA sequences are both non-coding and novel - 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) Marienfeld, Unseld and Brennicke,1999, TIPS 4:495-502