PLANT BREEDING SYSTEMS Diversity and Evolution of Reproduction in Angiosperms

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

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 (post- pollination). –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 cross- fertilization –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

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

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

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

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 wind- pollinated 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)

Protandry –Anthers release pollen before stigma receptive –Common in insect- pollinated 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.