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 (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

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 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)

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.