The Unique Reproductive Biology of
Angiosperms
Alternation of generations between haploid gametophyte and diploid sporophyte.
This is known as a haplo-diplontic lifestyle. Fertilisation occurs between gametes to
make 2n sporophyte. 2n sporophyte undergo meiosis to make n spores.
This is reminiscent of marine ancestry.
The Lifecycle of a Fern:
Start with small flat gametophyte. This has both an archegonium (makes egg) and an
antheridium (makes sperm). With water, the sperm fertilises the egg to make a zygote.
This zygote is the young sporophyte (2n). The sporophyte is the part of the fern we
typically see. This makes spores through meiosis and eventually makes the
gametophytes. Note that the sporophyte is temporarily dependent on the gametophyte
for nutrition, before becoming independent.
Ferns are homosporous – spores are all the same and give rise to a gametophyte that
gives rise to sperm and eggs.
The Lifecycle of Seed Plants:
These are heterosporous – there are 2 spore types that vary in structure (size). These are
microspores and megaspores.
Microspores –(mitosis)--> pollen (Microgametophyte).
Megaspores -> Ovule (Megagametophyte)
è The megaspore is retained by the sporophyte – it is never dispersed, whilst the
microspores are.
Successive reductions in the gametophyte phase of the haplo-diplontic life cycle during
land plant evolution:
The gametophyte has become reduced over time, whilst the sporophyte has increased.
This is known as extreme progenesis.
The ‘Abominable Mystery’:
= Sudden emergence of fully formed flowers in the fossil record, with no recognised
‘missing links’ in the fossil record.
Gymnosperms are mostly monoecious (male & female cones on the same plant – some
are dioecious and so make one or the other) and have unisexual cones -> male and
female, there are no hermaphrodite structures, most of which are wind pollinated.
Angiosperms are mostly hermaphrodite (flower has both male and female organs in the
same structure) and have co-sexual flowers, vast majority are animal pollinated.
,Hermaphrodite makes more sense for angiosperms being animal pollinate, as they can
both bring and then pick up pollen at the same time.
The Stamens:
Made up of an anther (microsporangium) and a filament to hold it up. Pollen matures
here, eventually it opens and the pollen and is released.
Male Gametophyte – the pollen grain:
In angiosperms there is a split between two types of pollen – bicellular pollen (one
division) made up of one generative cell and one vegetative cell – and tri-cellular pollen
(two mitotic divisions – generative cell divides into two) – this creates one vegetative cell
and two sperm cells.
The exine & intine (sporopollenin) pollen coat is laid down – this has a distinctive shape.
Can be used to find where someone died ?
The Carpel & Pistil:
Carpels can be individual, or fused into pistils (generally more evolutionary time =
fused).
The female gametophyte – the embryo sac:
If you cut the carpel, there are multiple ovules each containing female gametophytes.
Nutrition provided for gametophyte after fertilisation by some kind of plant umbilical
cord.
Female gametophytes made up of 7 cells and has 8 nuclei. Has integuments to protect
the female gametophyte. There is a micropyle – this allows the pollen tube to deliver
sperm to the egg.
Pollination & double fertilisation:
There are 2 sperm cells in the pollen grain. Pollen lands on stigma, if compatible, then T
the pollen grain makes a pollen tube, this grows from the vegetative pollen cell by tip
growth (no cell division involved – just elongation). As the tip grows, the cytoplasm is
compartmentalised to keep the sperm cells at the tip.
This is known as the pollen-pistil interaction.
The pollen tube delivers the two sperm cells to the embryo sac -> unique cellular
interaction between the haploid male gametophyte and the diploid female sporophyte
prior to fertilisation.
Pollen is very dehydrated, so to germinate needs water.
The pollen-pistil interaction summary:
1. Pollen capture
2. Pollen adhesion
3. Pollen hydration
4. Pollen germination
5. Pollen tube penetration of the stigma
,6. Growth of pollen tube towards the ovule
7. Entry of pollen tube into the ovule leading to fertilization
-> gives female part of the plant a chance to select her mate.
Implications of the Pollen-Pistil Interaction:
1. Evolution of pollen discrimination systems: E.g.:
- Incompatibility systems
- Intra-specific self-incompatibility systems (i.e. not self-fertilised)
- Interspecific incompatibility
2. Pollen competition (gametophytic selection):
- Vigorous microgametophytes produce fitter obspring – female is selecting
the best mate.
- Around 70% of genes expressed in the microgametophyte (pollen) are still
expressed in the sporophyte so are subject to selection during pollen-pistil
interaction.
è These are thought to be one of the key reasons for angiosperm success over
gymnosperms as these do not happen in gymnosperms.
How does a pollen tube find its way to the ovule?
Hypothesis: Chemical signals within the style:
Evidence for this:
1. There is a gradient of water potential created by surface lipids of stigma/pollen
coat – gives a polarity to the germination, so pollen grows into stigma rather than
air above it.
2. There are various chemical pollen tube attractants in the stigma and style e.g.
TTS.
3. Synergid peptide signal (LURE) attracts pollen tube to ovule as a prelude to
fertilisation – the very last step, point where pollen is about to reach ovule.
Double fertilisation in Angiosperms:
Pollen tube goes through the micropyle and
enters the synergid cell first. This is since
LURE is made by these cells. The tube
ruptures and releases 2 sperm cells. One
fertilises the egg, and the other sperm cell
(unique to angiosperms) fuses with the two
nuclei of the central cell to create a triploid
endosperm.
Triploid endosperm = 2 nuclei maternal, 1
paternal. 2m:1p is critical for seed size.
Diploid embryo = 1 maternal nucleus, 1
paternal
, The endosperm nourishes the embryo.
Endosperm development:
- Nuclear endosperm is the most common form. Fertilisation is followed by
repeated free-nuclear divisions forming a liquid endosperm. Coconut milk is
an example of this. If a cell wall is formed, then it will form after these
divisions.
- Cellular endosperm: fertilisation is followed by normal cell divisions with
cell wall formation.
- Helobial endosperm (rarest form): cell wall is laid down between the first
two nuclei, after which one develops as a cellular endosperm and the other
as nuclear.
Gymnosperm vs Angiosperm female gametophyte:
Gymnosperms: Food reserves are laid down in the female gametophyte before
fertilisation. The female gametophyte is multicellular.
Angiosperms: They have a smaller female gametophyte of around 7/8 cells. Food
reserves are laid down only after fertilisation and therefore have more ebicient resource
use.
Fruits are derived from various parts of the flower:
Once fertilises, fruits may form. E.g. petals etc. drop ob, ovaries swell and become fruit.
This is also unique to angiosperms – although there are similar convergent evolutions in
gymnosperms e.g. yew.
Fleshy fruits are an adaptation to seed dispersal by animals – some may have laxatives.
Angiosperms feed us.
Man is trying to hit the word count.
Angiosperm Mating Systems & Pollination
Mating (breeding) system: the way in which a group of organisms is structured in
relation to its reproductive behaviour.
The Majority of Flowers are Co-Sexual:
This optimises the benefits of animal pollination (mentioned last lecture).
“The contrivances by which orchids are fertilized are as varied and perfect as any
adaptations in the animal kingdom.”
“An examination of their contrivances will exalt the whole vegetable kingdom in most
persons’ estimation.” – Darwin probably.
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