Plant Diversity II Evolution by Seed Plants. cyanobacteria on land – 1.2 billion years ago 500 MYA – colonization by plants closest relatives of land

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

• 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– 1. multicellular, eukaryotic– 2. photosynthetic autotrophs – brown, red, green algae– 3. cell walls made of cellulose – green algae, dinoflagellates,

brown algae– 4. chloroplasts with chlorophyll a and b – green algae, euglenids

and a few dinoflagellates

• A reminder: several unique traits seen in plants and charophyceans– 1. rose-shaped complexes for cellulose synthesis –called rosettes

• synthesize cellulose microfibrils for the cell walls– 2. peroxisome enzymes –enzymes to 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

Charophyceans

Defining the Plant Kingdom

• land plants and green algae form a single clade = Viridiplantae

• 1.2 billion years ago – split into 2 clades: Chlorophyta (algae) & Streptophyta (land plants & charophyceans)

• 475 million years ago – the land plants began to evolve from the charophyceans into non-vascular plants

• 420 million years ago – evolution of vascular plants

• traditional botanists call land plants embryophytes– embryo is nourished inside the

archegonium/female gametophyte

Defining the Plant Kingdom

• vascular plants now form a single 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

• ANOTHER REMINDER: four key traits that define land plants – absent in charophyceans– 1. apical meristems– 2. alternation of generations & multicellular,

dependent embryos– 3. walled spores in sporangia

• two kinds of sporangia possible– 4. multicellular gametangia

• male and female

Heterospory

• the rule among seed bearing vascular plants• nearly all non-vascular plants are homosporous – produce

one kind of spore which gives rise to a bisexual/monoecious gametophyte

• with the evolution of seed plants – development of heterospory– megasporangium located on modified leaves called

megasporophylls produce megaspores female gametophytes

– microsporangium located on modified leaves called microsporophylls –produce microspores male gametophytes

– both are found on specialized reproductive structures• e.g cones, flowers

Heterospory

• these sporangia can either be located on the same plant = monoeicous– meaning “one house”– i.e. bisexual

• or they can be located on “male” and “female” plants = dioecious– reproduction requires a male and female plant

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

• 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– also allows the growing gametophyte to derive nourishment directly from

the sporophyte

1. Reduced Gametophytes

Sporophyte(2n)

Gametophyte(n)

Sporophyte dependent on gametophyte (mosses and other bryophytes)

Gametophyte(n)

Sporophyte(2n)

Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants)

Microscopic femalegametophytes (n) inovulate cones(dependent)

Microscopic malegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),the flowering plant(independent)

Microscopic malegametophytes (n)in pollen cones(dependent)

Microscopic femalegametophytes (n) ininside these partsof flowers(dependent)

Sporophyte (2n),(independent)

Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

Bryophytes Seedless Vascular

SeedVascular

2. Ovaries & Seeds• seed plants are unique in that the megasporangium is retained with the

parent sporophyte• the megasporangium contains the developing megaspore

• it is surrounded by layers of sporophyte tissue called integuments– in gymnosperms – the megaspore is surrounded by only one integument– angiosperms usually have two integuments

2. Ovaries & Seeds

Integument

Megasporangium(2n)

Megaspore (n)

Unfertilized ovule Fertilized ovule

Micropyle

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)Pollen grain (n)containing male gametophyte

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

• the megaspore (n) + megasporangium (2n) + 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– these eggs are fertilized by the sperm that will develop from the pollen grain– an embryo results found within a seed

2. Ovaries & Seeds

Seed coat(derived fromintegument)

Embryo (2n)(new sporophyte)

Gymnosperm seed

Food supply(femalegametophytetissue) (n)

• seed = ovule after fertilization – contains the embryo– seed = embryo + food supply + seed coat

(from the integuments) – allows for the developing embryo to resist

harsh conditions• evolutionary advantage of seeds:

– until seeds – the spore was the only protective stage in the life cycle

– unlike spores – seeds carry their own food supply

– unlike spores - a seed can remain dormant for years following its release

3. Evolution of Pollen

• 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– outer wall = protected by a coating of sporopollenin

• 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

3. Evolution of Pollen

• in order to fertilize the egg - the pollen grain must germinate (grow) – it produces a pollen tube – pollen tube allows for the discharge of two

sperm (gametes) into the ovule – sperm 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

Gymnosperms• “naked seed” – seeds are not enclosed in

ovaries• seeds are exposed on modified leaves

(sporophylls) that form cones or strobili– sporophylls bear sporangia for spore

production• 380 MYA – development of heterosporous trees

with woody stems – but did not bear seeds = progymnosperms

• first seed plant in the fossil record – 360 MYA• earliest fossils of gymnosperms – 305 MYA• early Mesozoic era (250 MYA) - domination by

gymnosperms• drier environment favored gymnosperms over

the bryophytes and ferns• most common existing gymnosperms are the

conifers – spruce, pin, fir and redwood

Ponderosa pine

Gymnosperms

Cycas revoluta

Ginko biloba

Ephedra.

Welwitschia mirabilis.

• four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta– Phylum Cycadophyta – cycads

• second largest group of gymnosperms• 130 species survive

– Phylum Ginkgophyta - ginkos• only one species left – Ginkgo biloba

– Phylum Gnetophyta – three genera alive today• Gnetum – 35 species of tropical trees, shrubs and vines

(Africa and Asia)• Welwitschia – one species, Welswitchia (Africa)• Ephedra – 40 species, desert shrubs

– Phylum Coniferophyta – largest group• “cone-bearing”• 600 species of conifers• many are large trees• most are evergreens – retain their leaves throughout the

year

Phylum Coniferophyta• also called Division Coniferophyta• 575 species• largest genus – Pinus• leaves of conifers are always simple needles or

scales• pine leaves – needles or needle-like

– arranged in clusters or bundles of two to five leaves each bundle

– cluster = fascicle

Pine fascicle

Phylum Coniferophyta• pine needle structure:

– 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 (vascular tissue) 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

• prevents water loss and fungal attacks on the needles• deters insects

Life Cycle: The Pine

• 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

– small pollen cones produce microspores – pollen– larger ovulate cones produce megaspores – egg– ovulate cones also known as seed cones – most

are woody– cones of the juniper can resemble a fruit (berry)

Life Cycle: The Pine• pollen cones: bear modified leaves or sporophylls (microsporophylls) each containing two microsporangia– the microsporangium is comprised

of diploid 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

Maturesporophyte(2n)

Pollencone

Microsporocytes(2n)

Longitudinalsection of pollen cone

Ovulatecone

Longitudinalsection ofovulate cone

Micropyle

Ovule

Key

Haploid (n)Diploid (2n)

Megasporocyte (2n)

Integument

Megasporangium

Germinatingpollen grainPollen

grains (n)(containing malegametophytes)

MEIOSIS

MicrosporangiumSporophyll

MEIOSIS

Germinatingpollen grain

Femalegametophyte

Archegonium

Egg (n)

Germinatingpollen grain (n)

Integument

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATION

Embryo(new sporophyte)(2n)

Food reserves(gametophytetissue) (n)

Seed coat(derived fromparentsporophyte) (2n)

Seedling

Seeds on surfaceof ovulate scale

Survivingmegaspore (n)

Life Cycle: The Pine• grains travel to the ovulate cone where they begin to germinate and form a pollen tube through which the sperm will travel

• pollen tube “digests” its way into female ovule through an opening called a micropyle

• pollen development results in the production of 2 sperm cells within the pollen tube

• two sperm travel into the ovule to fertilizes the eggs


Pollencone

Microsporocytes(2n)

Longitudinalsection of pollen cone

Ovulatecone


Micropyle

Ovule

Key


Megasporocyte (2n)

Integument

Megasporangium



MEIOSIS

MicrosporangiumSporophyll

MEIOSIS


Femalegametophyte

Archegonium

Egg (n)


Integument


Pollentube





Seedling



Life Cycle: The Pine• ovulate cones: made up of scales or megasporophylls

• base of the megasporophyll is an ovule

• each ovule contains one megasporangium– the megasporangium contains

a cell called a megasporocyte or a megaspore mother cell (2n)

– the megasporocyte undergo meiosis to form 4 haploid cells

– only one survives as the megaspore (n)

– the remaining degenerate


Pollencone

Ovulatecone


Micropyle

Ovule

Key


Megasporocyte (2n)

Integument

Megasporangium



MEIOSIS


Femalegametophyte

Archegonium

Egg (n)


Integument


Pollentube





Seedling



Life Cycle: The Pine– the surviving megaspore develops into the female gametophyte

– the female gametophyte develops two or three separate archegonia - each will form an egg inside

– the archegonium with egg + surrounding tissue = female gametophyte

Archegonium

Gametophyte

Femalegametophyte

Archegonium

Egg (n)



Pollentube

Egg nucleus (n)

Life Cycle: The Pine


Pollencone

Ovulatecone

Key


MEIOSIS

MEIOSIS


Femalegametophyte

Archegonium

Egg (n)


Integument


Pollentube





Seedling



– the germinating pollen grain develops its pollen tube and two sperm enter into the ovule through the micropyle

– fusion of egg nucleus and sperm nucleus Zygote (2n)

– both eggs in the female gametophyte may be fertilized

– the ovule with the zygote is now called the seed

– the developing embryo is retained within the female gametophyte (within the ovule)

• as the eggs mature – the pollen tube is developing its two sperm cells

• so the eggs and sperm mature at the same time

A simplified pine cycle

Pollination & The Embryo

• conifer pollen arrives before the egg is mature

• more than a year may pass between pollination & fertilization!!!

• MITOSIS within the pollen grain produces three cells – 2 small cells (which 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 germinating pollen grainswith pollen tube

air bladder

generative cell

tube cell

Pollination & The Embryo• following fertilization – the zygote does not immediately form

– the first cells to form elongate as a suspensor – serves to connect the developing embryo to the food source of the seed

• embryonic development is similar to angiosperms• the zygote splits into a basal cell and a terminal cell

– terminal cell embryo– basal cell suspensor

• the embryo combined with the integuments (derived from the ovule) and its food source – known as the seed

Documents

Plant Diversity II Evolution by Seed Plants. cyanobacteria on land – 1.2 billion years ago 500 MYA – colonization by plants closest relatives of land