Transcript Reproduction – Chapter 42

Plant Reproduction
• Angiosperms represent an evolutionary
innovation with their production of flowers
and fruits
• Flowers are modified stems that house the
gametophyte generation of angiosperms
• Flower organs are believed to be evolved
from leaves; these organs appear as
distinct whorls of parts
• A complete flower has four distinct whorls
of parts
• An incomplete flower lacks one or more of
the whorls
gynoecium
androecium
Enhance
reproductive
success
corolla
calyx
Develops into
fruit
Develop into
seeds
Floral Symmetry
• Primitive flowers exhibit radial symmetry
• Advanced flowers exhibit bilateral symmetry
http://www.flickr.com/photos/mimbrava/22802180/
Pollination
• Plant sexual life cycles are characterized by
an alternation of generations
• In angiosperms, the gametophyte generation
is very small and completely enclosed within
the tissues of the parent sporophyte
• Female gametophyte = embryo sac
• Male gametophytes = pollen grains
• Pollination is the process by which pollen is
placed on the stigma
Pollination
• Self-pollination – pollen from a flower’s
anther pollinates stigma of the same flower
• Cross-pollination – pollen from anther of
one flower pollinates another flower’s
stigma
Pollination
• When pollen reaches the stigma, it
germinates and a pollen tube grows down,
carrying the sperm nuclei to the embryo
sac
• After double fertilization, development of
the endosperm and the embryo begins
• The seed matures within the ripening fruit
Double fertilization
• 2 sperm cells
fertilize 2
cells in the
ovary
• 1 sperm
fertilizes the
egg  zygote
• Other fuses
with 2 polar
nuclei 
endosperm
Pollination
• Some angiosperms are wind-pollinated
– A characteristic of early seed plants
– Gymnosperms and some Angiosperms
(grasses, birches, oaks)
– Pollen only travels short distances (100
meters)
Pollination
• Successful pollination in many angiosperms
depends on a regular attraction of pollinators
• Bees attracted to flowers by color
• Flowers visited by butterflies frequently have a
flat ‘landing platform’
• Flowers visited by moths are often white or pale,
heavily scented
• Flowers visited by birds often are red
• Monkeys are attracted to orange and yellow fruits
Asexual Reproduction
• Asexual reproduction produces a
genetically identical individual (only mitotic
cell divisions occur; no fusion of sperm
and egg)
• Differs from self-pollination (sexual
reproduction, although genetic diversity is
limited)
Asexual Reproduction in Plants
• No alternation of generations
• New plants are cloned from other parts of
the adult plant
• Called “vegetative reproduction”
– Rhizomes – underground stems
– Stolons or runners – long, slender stems that
run along surface of soil
– Fragmentation – adventitious leaves or roots
(reproductive leaves and “suckers”,
respectively)
Adventitious plantlets
Plant Life Spans
• Perennial plants – continue to grow year
after year
– Majority of vascular plants are perennial
• Annual plants – grow, reproduce, and die
within a single year
– Annuals flower, produce fruits and seed (and
then die)
– Many crop plants are annuals
Fertilization in Animals
• Asexual reproduction
– Fission – separation of parent into two or
more offspring of equal size (single-celled
organisms)
– Budding – new individuals split off from
existing ones
– Fragmentation and regeneration – the
breaking of the body into several pieces;
some or all of which develop into new adults
• Budding in Cnidaria
• Fragmentation in
Planaria
Fertilization in Animals
• Parthenogenesis – a specialized form of
asexual reproduction
– Females produce offspring from unfertilized
eggs
– Some species are exclusively parthenogenic
(all females)
– Others switch between parthenogenesis and
sexual reproduction (e.g., Cladocera,
Honeybees)
Fertilization
• Hermaphroditism - A specialized form of
sexual reproduction
– One individual possesses both testes and
ovaries; can produce sperm and eggs
– Some can self-fertilize (e.g., Ctenophores)
– Some hermaphroditic individuals can change
sex
• Protogyny – “female first”
• Protandry – “male first”
Sexual Reproduction
• Most animals reproduce sexually
• Involves fusion of two haploid gametes
(sperm and egg) to form a diploid zygote
• Increases genetic variability
• Advantageous when environmental
conditions are unstable or change often
• Two modes of fertilization in animals
– External
– Internal
Sexual Reproduction
• External fertilization – eggs are shed by
female and fertilized by the male in the
environment
– Moist environments
– No parental care; large numbers of zygotes
with low survival
– Requires synchronization
• Due to environmental cues or pheromones
External fertilization
• Most amphibians
and bony fishes
have external
fertilization
Sexual Reproduction
• Internal fertilization – egg is fertilized
within the female’s body
• Evolved in terrestrial vertebrates
• Requires cooperative behavior leading to
mating
• Requires sophisticated reproductive
systems with copulatory organs
• Fewer zygotes with increased survival;
protection of the embryo and parental care
Internal Fertilization
• Vertebrates that practice internal fertilization
exhibit three distinct strategies for embryonic
and fetal development
– Oviparity – fertilized eggs are deposited outside of
body; some fish, most reptiles, all birds, some
mammals
– Ovoviviparity – fertilized eggs are retained within
female; embryos receive nutrition from yolk; some
fish, some reptiles
– Viviparity – embryos develop within female; receive
nutrition from mother; most cartilaginous fish, all
mammals