Topic 10 Flashcards
What are the key concepts of the evolution of seed plants?
seeds and pollen grains are key adaptations for life on land
gymnosperms bear “naked” seeds typically on cones
angiosperms have seeds encased in “fruit”
What is the evolutionary advantage of seeds?
seeds provide some evolutionary advantages over spores
they may remain dormant for days to years, until conditions are favorable for germination
seeds have a supply of stored food
they may be transported long distances by wind or animals
Why is the change from gametophyte to sporophyte dominant an improved adaptation for land plants?
need water: sporopollenin protects gametes (pollen) from desiccation, they are the ideal thing to have as the dispersal stage for fertilization as they do not depend upon water
protect developing embryo
diploid can be better: diploid cells can respond to varying environmental conditions more efficiently than haploid cells can
What is a downside of embryo retention?
in ferns and horsetails, sporophytes have to live in the same place, as their parent gametophyte
seed plants overcome this limitation: embryo of seed plants are portable and can disperse to new location
What are the characteristics of the evolution of pollen and seeds?
Gametophyte dependent on sporophyte: no longer free-living, gains: nourished and protected by sporophyte, losses: more stress on the sporophyte
Pollen: male gametophyte, gain: protected by pollen grain, water not necessary for fertilization, loss: need some mechanism to carry pollen (air, insects)
Ovule: female gametophyte, gain: protected by sporophyte (non-motile), loss: extra care for sporophyte
Seeds (fertilized ovule): contains next-generation sporophyte, gain: protection, dispersal, food storage, dormancy, loss: great energy cost to plant
In addition to seeds, what features are common to all seed plants?
reduced gametophytes
heterospory (small and large spores)
pollen (small mobile “spores”)
ovules (large retained “spores”)
What are male gametophytes?
within the pollen grain
microscope (1n) produced within microsporangium (2n), develops into male gametophytes (1n)
each pollen grain produces a pollen tube for delivering the sperm to the egg–> fertilization
What are female gametophytes?
integuments + megasporangium + megaspore = ovule
megaspore (1n) produced within megasporangium (2n)
What is pollen?
pollen grains coated with sporopollenin, making them tough
can withstand drying, UV, and physical damage
important since male gametophytes have to disperse to pollinate and then fertilize the ovule
What is pollenation?
pollination represents another drastic change in evolutionary strategy
pollination: transfer of pollen to the part of a seed plant containing the ovules
in non-seed plants, single-celled sperm requires water to swim through (usually short distances)
in seed plants, entire male gametophyte carried inside pollen grain, wind or animals can transport pollen, can travel much greater distances, no need for a film of water
What are seeds?
embryo is sporophyte tissue, along with its food supply (often gametophyte tissue), packaged in a protective coat = seed
range in size from minute orchid seed to gigantic coco-de-mer
main source of variation in seed size is amount of gametophyte-derived food
travel by air, water, in or on animals
seed often has tough coat
tiny embryo protected
What is seed germination?
some seeds need special treatment to germinate (fire or smoke, overwintering, passage through digestive system of vertebrate)
large seeds with a lot of food can germinate and grow in low light
small seeds with little food need light to grow into strong seedlings
What are the two main clades of seed plants?
gymnosperms: naked seeds
angiosperms: fruit covered seeds
What are gymnosperms?
characterized by seed that is relatively “naked”
ovule and seed develops on surface of sporophylls
sporophylls normally in cones (strobila)
What is the phylum ginkgophyta?
1 extant species, Ginkgo biloba
no wild individuals, all are associated with human habituations
common street tree because foliage attractive (yellow in autumn) and able to withstand harsh environment
What is the phylum cycadophyta?
very diverse and dominant in Mesozoic (age of dinosaurs)
now only ~130 species in tropical areas
individuals have large cones and palm-like leaves
What is the phylum gnetophyta?
some are tropical others live in deserts
morphologically diverse
grouping based on molecular evidence
What are conifers?
shapes of leaves vary: scale-like leaves, long needles, flat leaves
dominant trees in boreal and alpine regions in Canada
most are evergreen
well-adapted to cold, dry habitats (thanks to waxy coat and leaf retention)
What is the gymnosperm life cycle?
mature tree is 2n sporophyte
sporangia are located on scale-like leaves (sporophylls) in clusters (cones)
heterospory: megasporangia and microsporangia (on same tree or different trees)
pollination by wind (pollen has two hollow “air bladders”)
seed dispersal also by wind (seed is winged)
What are angiosperms?
just one phylum: anthophyta
make up vast majority of seed plants
includes smallest and tallest seed plants
What is the evolution of angiosperms?
gymnosperms dominant seed plants until middle Mesozoic
huge radiation of angiosperms, and decline of many gymnosperm groups in Cretaceous period
Darwin viewed sudden appearance and rise to dominance of flowering plants in the fossil record as a “mystery”
new fossil discoveries have allowed a plausible hypothesis of how flowering plants might have evolved from gymnosperm-like ancestors
What is the angiosperm phylogeny?
angiosperms/gymnosperms likely diverged
angiosperms related to bennettitales (extinct seed plants with flowerlike structures)
What was the angiosperm radiation?
the diversification of angiosperms is associated with three key adaptations: water-conducting vessels, flowers and fruits
these adaptations allow angiosperms to transport water, pollen, and seeds efficiently
What are the two types of flowers?
monoecious: male and female flowers on same individual plant
dioecious: male and female flowers on different individual plants
What is the angiosperm life cycle?
the female gametophyte, develops within an ovule (contained within an ovary)
a pollen grain on a stigma germinates and the pollen tube grows down to the ovary
double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule
one sperm fertilizes the egg
the other sperm initiates development of food-storing endosperm (sperm combines with two nuclei in the central cell of the female gametophyte)
the triploid (3n) endosperm nourishes the developing embryo
What is the directed-pollination hypothesis?
natural selection favored structures that reward an animal for carrying pollen directly from one flower to another
flowers vary in size, structure, scent, and color in order to attract specific pollinators
What is the co-evolution between plants and pollinators?
mutual evolutionary influence between two species, each of the species involved exerts selective pressure on the other, so they evolve together (co-evolve)
What is the mutualistic relationship between plants and pollinators?
the plant expends less energy on pollen production and instead produces showy flowers, nectar, and/or odors
the pollinator gets a reward such as nectar and/or pollen for pollinating the plant
What is the specific co-evolutionary relationship between flowering plants and their animal pollinators?
- Plants evolve ever more elaborate methods to attract animal pollinators
- Animals evolve ever more specialized body parts and behaviors to obtain the reward (nectar, pollen)
What are the adaptive advantages to angiosperm-style fertilization?
- Food store for seed (endosperm) develops only after fertilization, double fertilization (hence no resources are wasted if egg is unfertilized)
- Female gametophyte reduced in size and hence cheaper to produce
- Production of the small female gametophyte takes only a few days rather than months
What are the key concepts angiosperms and the evolution of seed plants?
angiosperms have seeds encased in “fruit”
fertilization occurs twice = double fertilization (once for embryo and once for endosperm), hence no resources are wasted if egg is unfertilized
pollination is mainly by animals vs gymnosperms which are wind pollinated
co-evolution occurs between plant and pollinators
