Section 17Organelle GeneticsGenetics of Mitochondriaand ChloroplastsFlourescence micrograph ofalga Olisthodiscus. Chlorophyllautofluoresces red. DNAstained with DAPI fluoresceswhite.•Chloroplasts and cpDNA•Mitochondria and mtDNA•Nucleus and nuDNAPiotr SlonimskiRuth SagerNick GillhamTony LinnaneDavid WilkieChlamydomonas chloroplastYeast mitochondriaOriginated as intracellular symbionts:Eukaryotic cell --phagocytosis of alpha-proteobacterium--> cell + symbiont --genes lost or transferred to nucleus--> cell + mitochondriaCell + mitochondrion --phagocytosis of cyanobacterium cell + symbiont--geneslost or transferred to nucleus cell + chloroplastIn subsequent evolution, mito and cp retained some traits of their symbioticancestors:• self-replication• some genes• protein-synthesizing machinery.BUT organelles never evolved• mechanism to ensure that every copy of the genome replicated once per cellcycle• mitotic apparatus to partition copiesThey never evolved the machinery necessary for Mendelian inheritance.• Mitochondria and chloroplasts are self-replicating organelles.They are produced only by growth and division of pre-existing mitochondria orchloroplasts. They cannot be formed de novo or from other organelles or pre-existingmembranes. They grow by the insertion of molecules in to their membranes.(a) Mitochondria and chloroplasts contain DNA genomes with a small number offunctional genes.mitochondrial DNA = mtDNAchloroplast DNA = cpDNA• Very few genes remain from the symbiont ancestors.Compare and contrast to nuclear genes (orders of magnitude):number C valueof genes in kbpeukaryote nuclear genome 104 - 105 104 - 108euk. mitochondria & chloroplasts 40 - 102 1 - 102prokaryote cell genome 500 - 104 500 – 104Number of genes ranges from 37 genes in human mito to ≈130 genes in plantmitochondria.• Most proteins, and all lipids, etc. in organelles are synthesized in the cytoplasm andimported.• Genes required for aerobic respiration/electron transport (mitochondria) andphotosynthesis (chloroplasts).• Organelle genomes almost always have all their genes on a single circular DNAmolecule.• Each cell contains many organelle DNA molecules, order of 102 – 104 (morein big eggs), hence many copies of each gene.• These are packaged in 1 - 103 organelles (more in big eggs).e.g. alga Chlamydomonas, haploid: ca. 100 cpDNA molecules in one chloroplaste.g. yeast: ca. 50-100 mtDNA molecules in 1 to 50 mitochondria (depending ongenotype and physiological state of cells; mito fuse and divide)e.g. mammalian cells in culture: ca. 103 mtDNA molecules in several hundredmitochondria1909 Erwin Baur and Carl Correns found first cases of non-Mendelianheredity, in plants. But only Baur interpreted them correctly.Pelargonium (geranium) Studied inheritance of wild type green and mutantwhite (no chlorophyll, no photosynthesis) variegated leaves:Violated two of Mendel's laws:• Some plants inherited genes from only one parent, usually female,sometimes male.• Alleles segregated during vegetative (asexual) growth.Laws of Organelle Genetics• Vegetative segregation: alleles oforganelle genes segregate duringmitotic as well as meioticdivisions.• Uniparental inheritance: organellegenes are often transmitted fromonly one parent.Mechanisms of VegetativeSegregation• Many copies of genome per celland per organelle.• Genomes selected ca. randomly forreplication, so some may replicatemore than others.• Genomes partitioned ca. randomlywhen organelle divides.• Organelles partitioned ca.randomly when cell divides.MECHANISMS OF VEGETATIVE SEGREGATION•Homoplasmic: cell, organelle, or organism has only one allele of an organellegene (cf. homozygous)•Heteroplasmic: cell, organelle, or organism has ≥ 2 alleles of an organelle gene(cf. heterozygous)•Alleles in heteroplasmic cell can be in different proportions (frequencies), e.g.1/100, 23/100, etc.• Random replication: Organelle DNA molecules are selected randomly(stochastically) for replication, so one allele often replicates more than theother, just by chance.• Random partitioning of genomes: When organelle divides, genomespartitioned randomly with respect to genotype, so one or both organelles areoften homplasmic.• Random partitioning of organelles: When cell divides, organelles arepartitioned randomly between daughter cells, so one or both daughters areoften homoplasmic.Intracellular random genetic drift (Thrailkill, Birky, Lückermann, and Wolf 1980mitochondria; Birky et al. 1981 chloroplasts): random changes in allelefrequencies in cell.Intracellular selection (Birky 1973): Some molecules can replicate more oftenthan others, not only by chance but also because they are smaller and canreplicate faster, or because they confer increased fitness on their mitochondria.homoplasmicgreenhomoplasmicgreenhomoplasmicwhiteWhen all cells are homoplasmic, expected ratio homoplasmic green:homoplasmic white = 3:1Frequency green genomes = f(G) = 0.75 f(W) = 0.25homoplasmicgreenhomoplasmicgreenhomoplasmicwhiteIntracellular selection for green plastids: replication stochastic, not strictly random. Red outcomesfavored. f(G)O > 0.75Inheritance in animals and many plants is exclusively from female parent, therefore oftencalled maternal inheritance. But this isn't the way to state a general rule of organelleinheritance; uniparental inheritance is better.Uniparental InheritanceExtraembryonictissueEmbryoMaternalMaternal Paternal MaternalMaternalMaternalPaternalUPI HAS MANY MECHANISMSNo organelles in gameteRandom partitioningOrganellesexcludedfromzygoteInput bias +random replicationMixtureSelective silencing(degradation)Selective silencing in Chlamydomonas reinhardtiiSager: reciprocal crosses using cpDNA genedetermining sensitivity vs. resistance tostreptomycin:mt+ str-r × mt- str-sall tetrads 2 mt+ : 2 mt-most tetrads 4 str-r : 0 str-smt+ str-s × mt- str-rall tetrads 2 mt+ : 2 mt-most tetrads 4 str-s : 0 str-r• A number of inherited diseases in humans are due to mitochonrial mutations.• Homoplasmic mutant cells (with only mutant mito genomes) are unable to doaerobic respiration, so they die.• Heteroplasmic cells survive, but have reduced respiration -> defects in tissuesrequiring hi level of respiration, e.g. muscle, eye.• Mitochondrial mutations may be involved in aging.Doug WallaceMITOCHONDRIAL DISEASESMITOCHONDRIAL GENES USED TO TRACK HUMAN GENEALOGIESDescribed in text; will discuss after Spring