Download PLANT BREEDING SYSTEMS

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.