3 BasicRepro2

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Published on December 7, 2007

Author: Flemel

Source: authorstream.com

Slide1:  Basic Reproduction 2: Pollination Systems Slide2:  Plant Breeding Systems General Issues to consider: A flower is an adapted leaf specialized for sex. Its function is to bring the gametes together thus forming zygotes with new genetic combinations A species’ survival is dependent on the production of offspring adapted to a particular environment. Plants have evolved a variety of adaptations that either encourage outbreeding (cross-pollination) or inbreeding (self-pollination or selfing). Each breeding system has genetic ramifications Slide3:  Basic Terms Outbreeding: Sexual reproduction between different genotypes (cross-pollination). Inbreeding: Sexual reproduction within an individual genotype (self-pollination) Sexual Reproduction: Combination of meiosis and fertilization. The sexual process is a mechanism to bring about gene recombination. Recombination is the chief source of inherited variation and provides the raw materials for species to adapt to changing environmental conditions. Slide4:  Outbreeder or Inbreeder?? Often one can tell just by looking at a flower whether it cross-pollinates or self-pollinates OUTBREEDER INBREEDER Trait Incompatibility Flower # Flower size Flower color Nectories Scent Nector guides Anther position Pollen # Style position Stigma self-incompatible self-compatible many flowers few flowers large flowers small flowers bright colors mono-colored nectaries present nectaries absent scented flowers unscented flowers nectar guides present nectar guides absent anthers far from stigma anthers close to stigma many pollen grains fewer pollen grains style exserted from flower style included in flower stigmatic area well-defined stigmatic area poorly-defined Slide5:  Outbreeding (Cross-pollination) Advantages: Disadvantages: Increases genetic variability Strong evolutionary potential Adaptation to changing conditions Successful long-term Can destroy well-adapted genotypes Relies on effective cross-pollination, seed dispersal and establishment Slide6:  Inbreeding (Self-pollination) Advantages: Disadvantages: Preserves well-adapted genotypes Insures seed set in the absence of pollinators Single colonizing individual possible Decreases (or maintains) genetic variability Evolutionary dead-end Cannot adapt to changing environmental conditions Successful short-term Self Incompatibility: How Plants Avoid Inbreeding:  Self Incompatibility: How Plants Avoid Inbreeding Evolution seems to favor genetic variability Genetic variability is promoted by outbreeding (reproduction between genetically dissimilar individuals) How do plants promote outbreeding? Self Incompatibility:  Self Incompatibility Mechanisms by which the plant affects pollen selection for fertilization; normally thought of in the prevention of self fertilization Defined as the inability of a plant producing functional gametes to achieve fertilization A genetically based mechanism which controls mating w/in angiosperm species by imposing a ‘barrier’ between pollination and fertilization Self Incompatibility:  Self Incompatibility Can be based on morphology (flower structure, sexual distinctness), development (gamete maturation time), or genetics Highly species specific Evolutionarily significant; impedes homozygosity, promotes heterozygosity Self Incompatibility-Key concepts:  Self Incompatibility-Key concepts Self incompatibility (SI) is different from sterility; fertility must be demonstrated for both male and female gametes Pre-fertilization phenomenon Widespread Ancient, ancestral Like x Like = no fertilization Like x Unlike = fertilization Types of Self Incompatibility:  Types of Self Incompatibility Imperfect Flowers- Sexual distinctness: Monoecious (Maize) vs. Dioecious (Hollies, Poplars) Perfect Flowers- Gamete Maturation Time (Dicogamy): - Protogyny - gynoecium matures 1st; stigmas receptive before anthers release pollen - Protandry - androecium matures 1st; anthers shed pollen before the stigma is receptive Protogyny:  Protogyny Stigmata Receptive Anthers pre-anthesis Stigmata Non-receptive Anthesis Protandry:  Protandry Types of Self Incompatibility:  Types of Self Incompatibility Perfect Flowers Heteromorphic: visible differences in flower morphology coincide with crossability differences (vs. Homomorphic) Characteristics affected: style length, filament length, pollen size, exine sculpturing, etc… - Does not affect the presence or absence of other SI mechanisms Heteromorphic SI-Distyly:  Heteromorphic SI-Distyly Types of Self Incompatibility:  Types of Self Incompatibility Perfect Flowers - Homomorphic: the mating types can not be recognized by morphological features - Numerous mating types controlled by “S” alleles (“S”, historical from idea that no seed = sterility) - Parental S alleles have to be different for successful fertilization Homomorphic SI:  Homomorphic SI Sporophytic SI - Proteins coded by the S alleles present within the sporophytic tissues of the parents interact; e.g. exine (male sporophyte genotype) and the female stigma/style Gametophytic SI - Proteins coded by the S alleles present within gametophytic and sporophytic tissues interact; e.g. intine/pollen tube (male gametophyte genotype) and the female stigma/style Sporophytic SI:  Sporophytic SI Less common then gametophytic SI Reaction controlled by S loci (w/ dominance) Dominance relationships are S1>S2>S3>S4... Extremely polymorphic Incompatibility is controlled by the diploid genotype of the sporophyte Thus, pollen will not germinate on stigma of a flower that contains either of the two alleles in the sporophyte parent that produced the pollen Sporophytic SI:  Sporophytic SI All pollen grains produced by a S1S2 plant Genotypes of pistils Stigma Style Gametophytic SI:  Gametophytic SI More common then sporophytic SI Reaction controlled by S loci (w/o dominance) Extremely polymorphic Incompatibility is controlled by the haploid genotype in pollen Thus, pollen will not grow in any pistil that contains the same allele Gametophytic SI:  Gametophytic SI All pollen (incompatible and compatible) will germinate and grow into style However, growth of incompatible pollen tubes will be arrested w/in style Reaction based on S-locus-encoded ribonucleases (RNase) synthesized in style RNase appears to enter all pollen tubes but destroys RNA in only incompatible pollen Gametophytic SI:  Gametophytic SI All pollen grains produced by a S1S2 plant Genotypes of pistils Stigma Style Overcoming SI Systems:  Overcoming SI Systems Temperature treatment Irradiation Grafting Double pollination Bud pollination Slide24:  Variants in the Reproductive Process Apomixis:  Apomixis Substitute for sexual reproduction Development of an embryo w/o sex Identified in over 300 species across 37 families. Not rare but relatively uncommon Apomixis:  Apomixis Must bypass two different processes: Meiosis: no reduction of chromosome # Fertilization: development of embryo w/o fusion of sperm and egg Reproductive Behavior:  Reproductive Behavior Obligate Apomict: a plant which reproduces only by asexual reproduction. Facultative Apomict: a plant which has the potential to reproduce either sexually or asexually. Both process may occur simultaneously or one may be predominant. Within a genus or species, all reproduction strategies may be possible Stimulus for Embryo Development:  Stimulus for Embryo Development Autonomous: requires no further stimulation - embryo and endosperm may be developed before flowers open Pseudogamy (False-fertilization): development of 2n gametophyte requires pollination and/or pollen tube growth. Sometimes a sperm nucleus fuses with polar nuclei, but never with egg Cytology in Apomicts:  Cytology in Apomicts Meiosis in mega/microsporogenesis usually irregular However, in some cases only the female gametophyte is affected and normal pollen is produced. Genetics of Apomicts:  Genetics of Apomicts Apomixis is genetically controlled in most cases studied Control is complex (multigenic) and recessive to sexual reproduction Polyploidy may affect apomictic reproduction Hybridization tends to increase apomixis Use of Apomixis in Breeding:  Use of Apomixis in Breeding Production of exact replicas of diploid Fix hybrid vigor Fix heterozygous status Identifying Apomixis:  Identifying Apomixis Pollination control Seed w/o pollination, apomixis present Progeny analysis Uniform, all progeny maternal like Cytological observations Unexpected chromosome # from cross with 2 ploidy levels (2N x 4N= 2N or 4N) Developmental observations Megaspore, embryo sac and embryo Parthenocarpy:  Parthenocarpy Development of fruit w/o fertilization and subsequent development of ovules or embryos Parthenocarpy:  Parthenocarpy Characteristics Remnant ovule sacs (seed coats) w/o any embryos or seeds (e.g. banana) Causes Usually stimulated by pollination or some other stimulus within the style Parthenocarpy:  Parthenocarpy Induction of parthenocarpy Example: Apple fruit set; evidence for a specific role of seeds Extracted immature apple seeds Applied extract to unpollinated flowers Produced seedless apples Conclusion: Hormonal effect induced by gibberellins Xenia and Metaxenia:  Xenia and Metaxenia Xenia: The phenotypic effect of the male genotype on the endosperm of the seed Metaxenia: The phenotypic effect of the male genotype fruit tissues “Window into next generation” Maize: aleurone color and type of endosperm reflects genotype of pollen that fertilized that seed Rosa: time to germination dependent on pollen parent

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