Bio3400 Chapter 9 Extranuclear Inheritance
  1. Extranuclear factors can often influence phenotype. One such factor is            heredity.
    • Some traits of photosynthetic organisms can be inherited through the              , which contains its own DNA.

        The four o'clock plant possess either white, green, or variegated leaves. Inheritance is uniparental: leaf color is determined only by the phenotype of the ovule source. For example, if the seeds were derived from ovules of plants with green leaves, all progeny plants bore only green leaves, regardless of the phenotype of the source of pollen. This inheritance is transmitted through chloroplasts in the cytoplasm of the female parent, since the pollen is tiny and contributes little cytoplasm to the zygote.

        Life cycle of an angiosperm (flowering plant). A plant alternates between a multicellular diploid (2n) sporophyte and a multicellular haploid (n) gametophyte generation.
      • A mature plant is a multicellular diploid sporophyte with reproductive structures.
      • Anthers contain microsporangia in which germ cells divide by meiosis to produce microspores.
      • Ovaries contain megasporangia in which germ cells divide by meiosis to yield 4 megaspores each.
      • Microspores divide by mitosis to form multicellular male gametophytes (pollen grains), which contain sperm cells.
      • One of the 4 megaspores divides by mitosis to form a multicellular female gametophyte (embryo sac), which contains an egg cell in an ovule.
      • Fertilization (pollination) occurs when a sperm fuses with an egg, producing a diploid single-celled zygote.
      • The zygote develops by mitosis to produce the sporophyte.

        The unicellular green alga Chlamydomonas has a single large chloroplast containing many copies of a circular double-stranded DNA. Its str^R (streptomycin resistance) trait exhibits uniparental inheritance: the phenotype is transmitted only through the mt^+ (mating type) parent. Reciprocal crosses yield offspring which only express the genotype of the mt^+ parent. After fertilization, the single chloroplasts of the two mating types fuse. The resulting chloroplast only retains DNA from the mt^+ parent.

        The green alga Chlamydomonas spends most of the life cycle in the haploid vegetative phase, asexually producing daughter cells by mitosis. Unfavorable conditions trigger the sexual phase, where some vegetative cells develop into isogametes, which can fuse to form a diploid resistant zygote adapted for surviving harsh conditions. When conditions become suitable again, meiosis of the zygote produces two plus mating types and two minus mating types. Mitosis of these zoospores returns the cells to vegetative colonies.
    • Another organelle capable of transmitting traits is the                , which also has its own DNA, and exhibits           inheritance.

        The poky phenotype (slow growth) in the bread mold Neurospora crassa is a mutation in cytochrome proteins that disrupts aerobic respiration. This trait is "maternally" inherited, suggesting cytoplasmic transmission via mitochondria. Neurospora mating involves fusion of conidia of opposite mating types. One of the cells contributes most of the cytoplasm and may be considered the "maternal" parent.

        The bread mold Neurospora spends most of it life cycle in a multicellular haploid stage. Following fertilization of conidia of opposite mating types, the zygote undergoes meiosis in an ascus, which retains the haploid tetrad (do not confuse with tetrad formed in prophase I). Each cell in the tetrad undergoes mitosis to produce 8 haploid ascospores. Because the 8 cells reflect the sequence of their formation following meiosis, the tetrad is "ordered" and can be subjected to ordered tetrad analysis.

        The petite phenotype (small colonies) in the yeast Saccharomyces cerevisiae is another mutation that causes abnormal aerobic respiration. A small proportion of these mutants are caused by nuclear mutations, but most have lost their mitochondrial DNA (mtDNA) and exhibits uniparental inheritance.

        Inheritance of petite phenotype in S. cerevisiae. Segregational: Some mutants are caused by nuclear mutations, and exhibit Mendelian 1:1 segregation. Neutral: all offspring are wild-type, having inherited normal mtDNA from the wild-type parent, which are replicated in the offspring. Suppressive: all offspring are petite, exhibiting "dominant" behavior to suppress wild-type mitochondrial function.
    • The origin of chloroplasts and mitochondria was probably                of free-living bacteria, and these organelles retained their prokaryote-like           , double-stranded DNA.

        Mitochondria and chloroplasts arose independently about 2 billion years ago by endosymbiosis when free-living prokaryotes were engulfed by primitive eukaryotic cells. The engulfed cells specialized in aerobic respiration and photosynthesis, respectively, and developed a mutually beneficial relationship with their host.

        Chloroplast DNA (cpDNA) is double-stranded and fairly large, ranging from 100 to 225 kb in length, The genes carried on the DNA encode products involved in photosynthesis and translation.

        Mitochondrial DNA (mtDNA) is also double-stranded but smaller than chloroplast DNA. Introns and gene repetitions are usually absent.

        In humans, mtDNA encodes many parts needed for protein synthesis and cellular respiration; while nuclear DNA codes the rest (arrows entering the organelle). For example, replication is dependent on enzymes encoded by nuclear DNA. Mitochondrial ribosomes also differ from cytoplasmic ribosomes.

        Ribosomes in mitochondria from different species exhibit various sedimentation coefficients, different from the cytoplasmic ribosome coefficient of 80S of all eukaryotes. The 80S coefficient for Tetrahymena is probably coincidental. This supports the endosymbiotic hypothesis of the origin of mitochondria.
    • Mutation in an organelle may result in               , where an individual has cells with a mixture of normal and abnormal organelles.

      Myoclonic Epilepsy and Ragged Red Fiber disease (MERRF) is a mitochondrial mutation with maternal inheritance, and exhibits ataxia (lack of muscular coordination). (a): The muscle fiber exhibits heteroplasmy, with mild proliferation of mutant mitochondria, showing "ragged-red" fibers when stained with dye.
      (b) Marked proliferation where most mitochondria have the mutant gene.
  2.             heredity results from the            association between a host cell and an invading microorganism or particle.

      Paramecia can undergo sexual exchange of DNA through conjugation where two mating cells exchange haploid micronuclei, resulting in new, recombinant micronuclei with identical genotypes. Autogamy is a similar process involving a single cell.

      In autogamy a Paramecium loses the genes from one micronucleus since only one of the 8 meiosis products survives. A heterozygous cell will become a homozygote, and a population of heterozygotes will produce a 1:1 ratio of cells homozygous for each allele.

      Killer Paramecia possess toxic kappa particles (symbiotic bacteria) and depend on a dominant nuclear K allele for their maintenance. Both cells from a conjugation between homozygous strains are heterozygous for K. They can become Killers only if, during conjugation, they received some cytoplasm containing kappa particles, otherwise they remain sensitive. The Killer phenotype persists only if the kappa particles are supported by at least one dominant K allele, since kk cells are sensitive even if they inherit the cytoplasm from a Killer.
  3. Gene products in the cytoplasm of the egg may also determine an offspring's phenotype due to maternal         (maternal            ). The effect may be temporary or permanent.

      The wild-type dominant allele yields brown eyes in the moth Ephestia kuehniella since it can synthesize a precursor pigment molecule, kynurenine. The a mutation interrupts synthesis of kynurenine and yields red eyes.
    • In an Aa x aa cross, the aa larvae exhibit wild-type brown eyes if the Aa parent is female. These aa larvae gradually develop red eyes as they grow into adults.
    • The Aa oocyte contains sufficient kynurenine in the cytoplasm to be distributed to the cells of young larvae, who start out life with brown eyes. As the pigment is diluted among many cells as the larva grows, red eyes emerge.

      Shell coiling in the hermaphroditic snail Limnaea peregra may be right-handed (dextral) or left-handed (sinistral). The coiling depends on the genotype of egg donor parent, regardless of the phenotype of that parent.
    • If that parent is DD or Dd, the offspring will be dextral.
    • The dominant allele (D) produces a gene product that influences spindle orientation in the first cleavage division of the zygote, which in turn affects subsequent cell divisions to produce permanent coiling in that direction.