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PLANT BREEDING SYSTEMS Diversity and Evolution of Reproduction in Angiosperms Plants vs. Animals Plants vs. Animals • Flowering plants are generally hermaphroditic. Plants vs. Animals • Flowering plants are generally hermaphroditic. • Use intermediary agents. Plants vs. Animals • Flowering plants are generally hermaphroditic. • Use intermediary agents. • Can reproduce asexually & sexually. Plants vs. Animals • Flowering plants are generally hermaphroditic. • Use intermediary agents. • Can reproduce asexually & sexually. • Less rigidly controlled development – meristematic tissue everywhere. Asexual Reproduction • Reproduction of genetically identical individuals from a single parent plant. – Via cloning or agamospermy • No meiosis, no fertilization, and no recombination. Advantages of Asexual Reproduction • Parent plants well-adapted to local environment will have offspring with a competitive advantage. Advantages of Asexual Reproduction • Parent plants well-adapted to local environment will have offspring with a competitive advantage. • Colonization with limited dispersal. Modes of Vegetative Reproduction Modes of Vegetative Reproduction • Rhizomes – underground shoots • Tillers – aboveground shoots • Bulblets – “little bulbs” • Bulbils – inflorescence veg buds • Cuttings Agamospermy/Apomixis • “Seeds w/o gametes” – Production of seeds genetically identical to parents asexually, w/o fertilization • ~40 families, 130 genera, 400 species • Obligative or facultative • May have evolved independently multiple times from sexual ancestors. Modes of Agamospermy • Embryo sac develops w/o meiosis w/ unreduced 2n egg cell & develops into zygote. Modes of Agamospermy • Embryo sac develops w/o meiosis w/ unreduced 2n egg cell & develops into zygote. • Embryo sac aborts and a veg cell from surrounding sporophytic tissue (ovary wall) develops into zygote. – Rubus, Taraxacum officinale Sexual Reproduction • Production of offspring through meiosis and fertilization of egg by sperm (postpollination). – Offspring genetically different from parents due to recombination. • Plants can be both asexual and sexual, with a variety of forms. Hermaphroditic Flowers • Self-compatible (SC) – Capable of self-fertilization or cross-fertilization • Self-incompatible (SI) – Only capable of crossfertilization – Inability of hermaphroditic plant to produce zygotes w/ self pollen Autogamy • Self-fertilization • Pollen transfer within or among flowers of same individual • ~25% of plant taxa Advantages of Autogamy Advantages of Autogamy • Insures seed set in absence of pollinators. Advantages of Autogamy • Insures seed set in absence of pollinators. • Overcomes sterility. Advantages of Autogamy • Insures seed set in absence of pollinators. • Overcomes sterility. • Selectively advantageous by transmitting both sets of genes to offspring. – Well-adapted genotypes preserved. Advantages of Autogamy • Insures seed set in absence of pollinators. • Overcomes sterility. • Selectively advantageous by transmitting both sets of genes to offspring. – Well-adapted genotypes preserved. • Only single colonizing individual needed. Disadvantages of Autogamy Disadvantages of Autogamy • Decreases genetic variability. Disadvantages of Autogamy • Decreases genetic variability. • Inability to adapt to changing conditions. Disadvantages of Autogamy • Decreases genetic variability. • Inability to adapt to changing conditions. • Increases inbreeding depression. – Reduces heterozygosity and increases homozygosity of deleterious alleles. – More uniform populations. Cleistogamy (CL) • Flowers never open and only capable of self-fertilization in bud. • Inconspicuous, bud-like apetalous flowers that form directly into seed capsules. • Has evolved independently multiple times – throughout the angiosperms, including some basal lineages. • 488 species, across 212 genera and 49 families. – Violaceae, Fabaceae, Poaceae Cleistogamy (CL) • Mixed mating systems -can produce both CL and CH on an individual. • CL fls are a “back-up” in case pollinators scarce. • CL occur after normal flowering period. – CH fls early spring and CL fls rest of season. • CL fls occur through mutations with loss of SI. Self-incompatibility (SI) • Involves a biochemical rxn in the stigma/style to reject self pollen and prevent pollen tube growth. • Genetically controlled by S-locus – opposite S alleles attract – like S alleles repel Sporophytic SI • Diploid genotype of sporophyte parent determines what matings will be successful. • Interaction between pollen exine and stigma/style tissues. • Pollen will not germinate on stigma of flower that contains either of 2 alleles in sporophyte parent that produced pollen. Gametophytic SI • Haploid genotype of pollen grain (gametophyte) determines what matings will be successful. • Interaction between pollen tube and stigma/style tissues. • Pollen grain will grow in any pistil that does not contain the same allele. • 50% of angiosperms Advantages of Self-Incompatibility Advantages of Self-Incompatibility • Prevents selfing and expression of deleterious genes that are heterozygous in parents. Advantages of Self-Incompatibility • Prevents selfing and expression of deleterious genes that are heterozygous in parents. • Reduces inbreeding depression. Advantages of Self-Incompatibility • Prevents selfing and expression of deleterious genes that are heterozygous in parents. • Reduces inbreeding depression. • Increases genetic exchange/diversity. Advantages of Self-Incompatibility • Prevents selfing and expression of deleterious genes that are heterozygous in parents. • Reduces inbreeding depression. • Increases genetic exchange/diversity. • Ability to adapt to changing conditions. Disadvantages of Self-Incompatibility Disadvantages of Self-Incompatibility • Relies on effective cross-pollination, seed dispersal and establishment. Selfers vs. Outcrossers • • • • SC Small flowers (few) Unscented flowers Nectaries & nectar guides absent • Maturation of reproductive parts – Anthers near stigma – Style included • All fruits mature • Low pollen/ovule ratio • SI or SC • Large showy flowers (many) • Scented flowers • Nectaries & nectar guides present • Differential maturation of reproductive parts – Anthers far from stigma – Stigma well-exserted • Only some fruits mature • High pollen/ovule ratio Strategies to Prevent Self-fertilization Physical Separation of Reproductive Parts (Herkogamy) • Within flowers • Among flowers Heterostyly • Flowers in different individuals of the same species having 2 or 3 different style lengths – With stamen lengths varying inversely • Distyly • Tristyly Distyly • 2 floral morphs. • “Thrum” flower – long filaments w/ short styles • “Pin” flower – short filaments w/ long styles • Only pollinations between different floral morphs are successful. • E.g.: Primula Tristyly • 3 floral morphs • Style long, stamens short and medium • Style medium, stamens short and long • Style short, stamens medium and long Physical Separation of Reproductive Parts • Unisexual flowers – Staminate and carpellate flowers • Monoecy • Dioecy Monoecy • Common in windpollinated plants. • Common in temperate regions. • Self-pollination possible but less likely. Dioecy • 4% of angiosperms – Scattered throughout • Common in tropical regions and oceanic islands • Gen small fl size • 100% outcrossing, but inefficient • Often controlled by sex chromosomes – Silene Polygamous Flowers • Both bisexual and unisexual fls on the same plant. – Androdioecy = bisexual and staminate individuals in a population. – Andromonoecy = bisexual and staminate flowers on same individual. • Euphorbia, Solanum – Gynodioecy = bisexual and carpellate individuals in a population. • Sidalcea hendersonii, Silene – Gynomonoecy = bisexual and carpellate flowers on same individual. • Silene, Solidago – Polygamodioecy = some plants with bisexual and staminate flowers & some plants with bisexual and carpellate flowers in a population. – Polygamomonoecy = bisexual, staminate, and carpellate flowers on same individual. Evolution of Dioecy • From hermaphroditism – Vestigial sex organs – Few families entirely dioecious • From monoecy • From SC – W/in groups that have lost original GSI system • From distyly – Unequal pollen flow & gender function – Change in pollinator frequency – Non-functional anthers at low level in female flowers – Non-functional pistil in male flowers Temporal Separation of Reproductive Parts (Dichogamy) Temporal Separation of Reproductive Parts (Dichogamy) • Protandry – Anthers release pollen before stigma receptive – Common in insectpollinated plants • Geranium maculatum – 1st day flower – 2nd day flower Temporal Separation of Reproductive Parts (Dichogamy) • Protogyny – Stigma receptive before pollen release – Less common than protandry • Magnolia grandiflora – 1st day flower – 2nd day flower Geitonogamy • Self pollination between different flowers on same plant. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. • Ancestral angiosperms were SC, hermaphroditic. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. • Ancestral angiosperms were SC, hermaphroditic. • SI has evolved many times. – SC has evolved from SI plants as well. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. • Ancestral angiosperms were SC, hermaphroditic. • SI has evolved many times. – SC has evolved from SI plants as well. • Physical and temporal separation have evolved many times. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. • Ancestral angiosperms were SC, hermaphroditic. • SI has evolved many times. – SC has evolved from SI plants as well. • Physical and temporal separation have evolved many times. • Dioecy has evolved many times. Evolution of Breeding Systems • Evolutionary trends go both ways and in a variety of ways. • Ancestral angiosperms were SC, hermaphroditic. • SI has evolved many times. – SC has evolved from SI plants as well. • Physical and temporal separation have evolved many times. • Dioecy has evolved many times. • Breeding systems not fixed, but labile.