Bio153 lecture 10:
plant reproduction
• types of life cycles
evolutionary trends
• reproduction in angiosperms
meiosis = reduction
mitosis = replication
• type I: meiosis of zygote ⇒ 4 haploid
cells; mitosis ⇒ more haploid cells
(multicellular haploid individual) ⇒
gametes; fertilization ⇒ zygote
• most of life cycle is haploid
• fungi; some algae
• type II= gametic meiosis
• mitosis of zygote ⇒ multicellular adult;
meiosis ⇒ 4 haploid cells (gametes);
fertilization ⇒ zygote
• most of life cycle is diploid
• some protists (water molds, brown
algae)
• animals
3 types of life cycles
• first eukaryotic organisms: haploid &
asexual
• 1st step: fusion of 2 haploid individuals ⇒
2n zygote; immediate meiosis restores
haploidy
• introduction of recombination
• type I life cycle = zygotic meiosis
type I: zygotic meiosis
mitosis
gametes
haploid
meiosis
diploid
zygote
type II: gametic meiosis
gametes
haploid
meiosis
diploid
zygote
mitosis
adult
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• type III = sporic meiosis
• multicellular diploid sporophyte ⇒ haploid
spores ⇒ multicellular haploid gametophyte
⇒ haploid gametes; fertilization ⇒ zygote ⇒
sporophyte
• alternation of diploid and haploid
generations
type III: sporic meiosis
mitosis
meiosis
sporophyte
• sporic meiosis ⇒ meiospores
• meiospores: haploid cells that divide
mitotically to form multicellular haploid
individual (gametophyte)
• gametophyte mitotically produces
haploid gametes
• meiospores ≠ gametes: gametes
develop into multicellular body only after
fusion with another gamete
alternation of generations:
evolutionary trends
1. dominance of gametophyte ⇒
dominance of sporophyte
2. sporophyte “parasitic” on gametophyte
⇒ independent gametophyte and
sporophyte ⇒ gametophyte “parasitic”
on sporophyte
gametes
haploid
• many algae; plants
alternation of generations:
gametophyte
diploid
zygote
mitosis
1. isomorphic generations: gametophyte
and sporophyte look the same
- most red, some brown, a few green
algae
2. heteromorphic generations:
gametophyte and sporophyte look
different
- some brown & red algae; green algae
& plants
3. H2O dependent fertilization ⇒ H2O
independent fertilization
(advantage: adaptation to dry env’t)
4. homospory (1 type of spore; bisexual
gametophyte) ⇒ heterospory (2 types of
spores; unisexual gametophyte)
(advantage: avoid self-fertilization)
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5.
isogamy ⇒ anisogamy ⇒ oogamy
• isogamy: gametes
same size & shape
• anisogamy: 1 gamete
(male) is smaller
dominance of gametophtye:
• e.g. bryophytes
• gametophyte is “body”
• sporophyte is a
temporary structure
• oogamy: female
gamete large; nonmotile
reduction of gametophyte
independent gametophtye and
sporophyte:
• early seedless vascular
plants (e.g. Lycopodium)
• gametophyte is
subterranean
• homosporous
(bisexual
gametophyte)
• e.g. ferns
• sporophyte larger
than gametophyte
• some heterosporous,
some homosporous
• motile gametes
gametophyte dependent on sporophyte
“apex” of these evolutionary trends:
the angiosperms
male cones
• seed plants
• gametophyte
highly reduced;
retained in
sporangia
• male gamete =
pollen
• free from water
• gametophyte reduced to a few cells
(3 in male; 7 in female)
• all heterosporous
• enclosed ovules – prevents herbivory
• flowers and pollination
a summary of the life cycles in plants…
female cones
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fern life cycle
bryophyte
life cycle
homosporous
gametophyte
gymnosperm
life cycle
angiosperm
life cycle
• female
cones have
naked
ovule
• no carpel,
style,
stigma
double fertilization
• unique to angiosperms (a type of
double fertilization exists in
gnetophytes)
• 1 sperm fertilizes egg
• 1 sperm fertilizes cell with 2 haploid
nuclei (polar nuclei) ⇒ triploid
cell
• sometimes pentaploid (e.g. lilies)
• triploid cell proliferates ⇒
endosperm (nutritive tissue)
flowers:
• modified leaves; evolved to attract
pollinators
• flower bears sporophylls
• perfect = male and female parts in same
flower
• monoecious = both male and female
parts on same plant
• dioecious = male and female parts on
different plants
4
parts of a flower
petals
(corolla)
sepals
(calyx)
fruit: dispersal mechanism
• ovary develops
into fruit
• fruit may be
dispersed by
wind, on or in
animals
stamens
ovary
promotion of outcrossing
• transfer of pollen from 1
individual to another
• dichogamy: stamens and
carpels mature at different
times
- protandry: males first
- protogyny: females first
• genetic self-incompatibility
• however, self-fertilization
may be favoured in some
conditions
coevolution of plants and pollinators
• directed pollen transfer: big evolutionary step
• resulted in diversification and dominance of
angiosperms
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