Transcript Plant Diversity I: Colonization by Land Plants

Plant Diversity II: Evolution by Seed
Plants
• cyanobacteria on land – 1.2 billion years ago
• 500 MYA – colonization by plants
• closest relatives of land plants = charophyceans
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molecular comparison of both nuclear and chloroplast genes confirms
morphological and biochemical conclusions that the charophyceans are
ancestors of plants
• plant share characteristics with other more primitive organisms
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multicellular, eukaryotic
photosynthetic autotrophs – brown, red, green algae
cell walls made of cellulose – green algae, dinoflagellates, brown algae
chloroplasts with chlorophyll a and b – green algae, euglenids and a few
dinoflagellates
• A reminder: four unique traits seen in plants and only
charophyceans
– 1. rose-shaped complexes for cellulose synthesis –called
rosettes
• synthesize cellulose microfibrils for the cell walls
– 2. peroxisome enzymes – peroxisomes contain enzymes that
help minimize the loss of organic products as a result of
photorespiration
– 3. flagellated sperm – some species of land plants have
flagellated sperm
– 4. formation of a phragmoplast – involved in the synthesis
of new cell walls during mitosis - via the formation of new
cross walls called cell plates
Defining the Plant Kingdom
• traditional classification schemes equates the plant
kingdom with the presence of embryophytes
• vascular plants form a clade – 93% of all plant species
– categorized into three smaller clades
• 1. lycophytes – club mosses and relatives
• 2. pterophytes – ferns and relatives
• 3. seed vascular plants
– A. gymnosperms - “naked seed” plants
– B. angiosperms – flowering plants
• four key traits define plants – absent in charophyceans
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1. apical meristems
2. alternation of generations & multicellular, dependent embryos
3. walled spores in sporangia
4. multicellular gametogangia
Seed plants
– three key reproductive adaptations
evolved in seed plants:
– 1. increasing dominance of the sporophyte
generation – reduced gametophyte
– 2. advent of the seed – ovules and eggs
– 3. evolution of pollen as an airborne agent
1. Reduced
Gametophytes
• gametophytes of mosses and ferns are
the dominant stage
• gametophytes of seed plants are
mostly microscopic
• miniaturization allows for the
development of their gametophytes
within the sporangium of the
parental sporophyte
• protects the delicate egg-forming
gametophyte from environmental
stress
• moist environment of the sporophyte
shields the gametophytes from drought
and UV radiation
• also allows the growing gametophyte
to derive nourishment directly from
the sporophyte
Bryophytes
Seedless Vascular
Sporophyte
(2n)
Gametophyte
(n)
Sporophyte dependent
on gametophyte
(mosses and other
bryophytes)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Sporophyte
(2n)
Gametophyte
(n)
Large sporophyte and
small, independent
game-tophyte (ferns and
other seedless vascular
plants)
Sporophyte (2n),
the flowering plant
(independent)
Microscopic male
gametophytes (n) in
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Sporophyte (2n),
(independent)
Microscopic female
gametophytes (n) in
inside these parts
of flowers
(dependent)
Reduced gametophyte dependent on sporophyte
(seed plants: gymnosperms and angiosperms)
Seed
Vascular
Mosses and Ferns vs. Seed plants
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mosses:
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development of a sporangium – releases spores for development into gametophyte
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development of a protonema with buds – develop into large gametophytes (moss)
most mosses have separate female and male gametophytes (bisexual gametophyte)
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some gametophytes bear both male and female gametangia
multiple gametangia per gametophyte plant
sperm are released (flagellated) and they fertilize the egg developing within the archegonium – development into a
new sporangium and release of spores
vascular seedless plants:
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development of a sporangium – releases spores for development into gametophyte
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homosporous – one type of spore that ends up developing into female and male gametophytes
homosporous – one type of spore that ends up developing into female and male gametophytes
development of the bisexual gametophyte bearing multiple archegonium and antheridium
fertilization of the egg within the archegonium - development of the sporophyte
development into large sporophytes (plant) – sporophyte bears multiple microphylls (leaves) bearing sporeforming structures on the underside – sporangium in clusters called sori
in gymnosperms and angiosperms – vascular seed plants:
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development of the sporophyte (plant) bearing separate female and male sporangia located in specialized
reproductive structures – e.g. cones or flowers
development of a sporangium – releases spores for development into gametophyte
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heterosporous – development of distinct spores that develop into either male or female
microsporangium – microspore – male gametophyte
megasporangium – megaspore – female gametophyte
microscopic gametophytes bearing either archegonium or antheridium (unisex gametophyte)
fertilization of the egg within the archegonium - development of the sporophyte
Heterospory
• the rule among seed plants
• nearly all nonvascular plants are homosporous – produce
one kind of spore which gives rise to a bisexual
gametophyte
• with the evolution of seed plants – development of
heterospory
– megasporangium located on leaves called megasporophylls
produce megaspores – female gametophytes
– microsporangium located on leaves called microsporophylls –
produce microspores – male gametophytes
– both are found on specialized reproductive structures
• e.g cones, flowers
• these sporangium can either be located on the
same plant = monoeicous
• or they can be located on “male” and “female”
plants = dioecious
2. Ovaries & Seeds
• seed plants are unique in that the megasporangium is retained within the
parent sporophyte
• the megasporangium containing the developing megaspore is surrounded
by layers of sporophyte tissue called integuments
– in gymnosperms – the megaspore is surrounded by only one integument
– angiosperms usually have two integuments
• the megaspore + megasporangium + integuments = ovule
• inside each ovule is a future female gametophyte that develops from the
megaspore
– gametophyte can produce one or more egg cells within the ovule
Integument
Female
gametophyte (n)
Spore wall
Megasporangium
(2n)
Megaspore (n)
Unfertilized ovule
Egg nucleus (n)
Male gametophyte
(within germinating
pollen grain) (n)
Micropyle
Fertilized ovule
Discharged
sperm nucleus (n)
Pollen grain (n)
Seed coat
(derived from
integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
2. Ovaries & Seeds
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seed = ovule after fertilization - development into the embryo
– seed = embryo + food supply + seed coat (from the integuments)
– allow for the developing embryo to resist harsh conditions
– multicellular structure - in contrast to the spore
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evolutionary advantage of seeds:
– until seeds – the spore was the only protective stage in the life cycle
– spores are more “hardy” than the parental plant
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e.g. moss spores survive better in harsh conditions better than the moss
– but most spores can only survive under certain environmental conditions
– unlike spores – seeds carry their own food supply
– unlike spores - a seed can remain dormant for years following its release
Integument
Female
gametophyte (n)
Spore wall
Megasporangium
(2n)
Megaspore (n)
Unfertilized ovule
Egg nucleus (n)
Male gametophyte
(within germinating
pollen grain) (n)
Micropyle
Fertilized ovule
Discharged
sperm nucleus (n)
Pollen grain (n)
Seed coat
(derived from
integument)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
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the microsporangium produces microspores
microspores develop into pollen grains
a pollen grain contains the male gametophyte enclosed within a
pollen wall
outer wall is made by the sporophyte, inner wall is made by the
gametophyte within
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outer wall = protected by a coating of sporopollenin
composition of sporopollenin is still not known!
designed to protect the pollen grain on its “travels”
transfer of pollen to the ovule = pollination
pollen grains are carried away from the parent plant by wind,
insects
or they can travel to the female reproductive structures within
the same sporophyte
in order to fertilize, the pollen grain begins to germinate (grow)
and produces a pollen tube – allows for the discharge of two
sperm (gametes) into the ovule – unites with the egg
developing within female gametophyte (within the ovule)
in non-vascular plants (bryophytes) and seedless vascular
plants (ferns) – the sperm is flagellated and swims to the
female gametophyte in order to fertilize the egg which is also
free living
in vascular seed plants – the female gametophyte produces an
egg which never leaves the sporophyte ovule
3. Evolution
of Pollen
Gymnosperms
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“naked seed” – seeds are not enclosed in ovaries
seeds are exposed on modified leaves (sporophylls) that form
cones
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380 MYA – development of heterosporous trees with woody stems
– but did not bear seeds = progymnosperms
first seed plant in the fossil record – 360 MYA
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now extinct
earliest fossils of gymnosperms – 305 MYA
drier environment favored gymnosperms over the bryophytes and
ferns
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in the ferns – development of sporophyll leaves that bear the
sporangium (microphylls and sori)
in gymnosperms – development of modified sporophylls that cluster
together to form cones or strobili
start to see a die off of many of the bryophyte and fern species
gymnosperms with their thick cuticles and reduced leaves as
needles – adapted well to the dry climates
251 MYA – boundary between the Paleozoic (“old life”) and
Mesozoic (“middle life”) eras
early Mesozoic era - domination by gymnosperms
toward the end of the Mesozoic (65 MYA) – increased dominance
of angiosperms in some ecosystems
most common existing gymnosperms are the conifers – spruce, pin,
fir and redwood
Gymnosperms
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four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and
Coniferophyta
– Phylum Cycadophyta – cycads
• second largest group of gymnosperms
• large cones and palmlike leaves
• 130 species survive
– Phylum Ginkgophyta - ginkos
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Cycas revoluta
only one species left – Ginkgo biloba
deciduous leaves - fanlike formation
tolerates air pollution well
trees bear fleshy seeds that smell rancid
Ginko biloba
Welwitschia
mirabilis.
– Phylum Gnetophyta – three genera alive today
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tropical and desert species
Gnetum – 35 species of tropical trees, shrubs and vines (Africa and Asia)
Welwitschia – one species, Welswitchia (Africa)
Ephedra – 40 species, desert shrubs
Ephedra.
– produce the compound called ephedrine
– Phylum Coniferophyta – largest group
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“cone-bearing”
600 species of conifers
many are large trees
most are evergreens – retain their leaves throughout the year
Phylum Coniferophyta
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also called Division Coniferophyta
575 species
largest genus – Pinus
leaves of conifers are always simple needles or scales
pine leaves – needle-like
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arranged in clusters or bundles of two to five leaves each
bundle
cluster = fascicle
fascicle forms a cylindrical rod if the needles are held together
needle is comprised of a outer epidermis coated with a thick
cuticle
below that is one to two layers of cells = hypodermis
stomata are recessed in sunken cavities
veins and associated tissues run down the center of the needle
and are surrounded by an endodermis
also contain resin canals – occur in other parts of the pine
these canals are lined with special cells that secrete a resin –
aromatic and antiseptic
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combination of turpentine and a waxy rosin
rosin prevents water loss and fungal attacks
deters insects
although most pine roots have mycorrhizal fungi
fossilized resin = amber
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pine tree is the sporophyte
sporangia are located on scale-like leaves
(sporophylls) packed into cones – strobili (single
= strobilus)
two types of cones produce two types of spores
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small pollen cones produce microspores – pollen
larger ovulate cones produce megaspores – egg
ovulate cones also known as seed cones – most are woody
but those of the juniper can resemble a fruit (berry)
Life Cycle: The
Pine
• pollen cones: bear modified
leaves or sporophylls each
containing 2 microsporangia
– the microsporangium is comprised of cells
called microsporocytes (2n)
– microsporocytes are also known as
microspore mother cells
– microsporocytes divide by meiosis to form
pollen grains which are haploid
– pollen grains contain the male
gametophyte – for the production of sperm
– grains travel to the ovulate cone where it
begins to germinate and forms a pollen tube
through which the sperm will travel
– pollen tube “digests” its way into and
through the female reproductive structure
through an opening called a micropyle
– development results in the production of 2
sperm cells within the pollen tube
Life Cycle: The Pine
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Megasporocyte (2n)
Integument
Longitudinal
section of Micropyle
ovulate cone
Megasporangium
Mature
sporophyte
(2n)
Microsporocytes
(2n)
Germinating
Pollen pollen grain
grains (n)
MEIOSIS
(containing male
gametophytes)
MEIOSIS
Longitudinal
section of
Sporophyll
pollen cone Microsporangium
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Integument
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves Seed coat
(gametophyte (derived from Discharged
sperm nucleus (n)
tissue) (n)
parent
sporophyte) (2n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
FERTILIZATION
Egg nucleus (n)
Pollen Cones
• considered to be simple cones
– one single stem axis bearing
microsporophylls
• cones typically occur in clusters near
the ends of branches
• pollen is liberated to the wind and
blown away
• pollen has one cell and two large air
bladders that increase its buoyancy in
air
• wind dispersal is inefficient – so few
pollen grains actually land on the
ovulate cone
• but conifer forests are very dense
microsporophyll
microsporangium
microspore
(pollen)
Male Pine Cone
• ovulate cones: bear modified leaves each contains two ovules
• each ovule contains one
megasporangium
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the megasporangiumcontains a cell called a
megasporocyte (2n)
megasporocytes are also known as megaspore
mother cells (2n)
the megasporocyte undergo meiosis to form 4
haploid cells
only one survives as the megaspore (n)
the remaining degenerate
the surviving megaspore develops into the female
gametophyte
the female gametophyte develops two or three
separate archegonia - each will form an egg
as the eggs mature – the pollen tube is developing
its two sperm cells
the eggs and sperm mature at the same time
all eggs may be fertilized by the sperm – only one
egg nuclei with fuse with a sperm nuclei to form
the zygote
the ovule is now the seed & its developing
embryo is retained within the female gametophyte
Life Cycle: The Pine
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Megasporocyte (2n)
Integument
Longitudinal
section of Micropyle
ovulate cone
Megasporangium
Mature
sporophyte
(2n)
Microsporocytes
(2n)
Germinating
Pollen pollen grain
grains (n)
MEIOSIS
(containing male
gametophytes)
MEIOSIS
Longitudinal
section of
Sporophyll
pollen cone Microsporangium
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Integument
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves Seed coat
(gametophyte (derived from Discharged
sperm nucleus (n)
tissue) (n)
parent
sporophyte) (2n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
FERTILIZATION
Egg nucleus (n)
Seed cones
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more complex than pollen cones
compound cones
each consists of a cone axis with axillary buds
the buds bear leaves that are called bracts
rather than sporophylls
• associated with each bract are fused
megasporophylls that form a scale
(ovuliferous)
megasporophyll
bract
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Pollination
&
The
conifer pollen arrives before the egg is mature
more than a year may pass between pollination & fertilization
Embryo
mitosis within the pollen produces three cells – 2 small
(degenerate) and one large cell
this large cell divides to form: a generative cell and a tube
cell
the tube cell elongates to form the pollen tube
the generative cell forms 2 sperm
following fertilization – the zygote does not immediately form
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the first cells to form elongate as a suspensor
it pushes the other cells deep into the megasporocyte – these will form a
proembryo – will become the embryo
embryonic development is similar to angiosperms
the embryo combined with the integuments (derived from the
ovule) and its food source – known as the seed
so the seeds are also borne on the cone
germination of a new seed – new sporophyte
from the time pollen and ovulate cones appear - takes three to
four years for the male and female gametophytes to be
produced!!!
seeds of nearly all species of pines are edible
high protein content in the seed
germinating pollen grains
with pollen tube
air bladder
tube
cell
generative cell
Life Cycle: The Pine