Plant Diversity II Evolution by Seed Plants cyanobacteria

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

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

Bryophytes Seedless Vascular Sporophyte (2 n) Gametophyte (n) Sporophyte dependent on gametophyte (mosses and other bryophytes) Microscopic female gametophytes (n) in ovulate cones (dependent) Gametophyte (n) Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants) Sporophyte (2 n), the flowering plant (independent) Microscopic male gametophytes (n) in inside these parts of flowers (dependent) Seed Vascular Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2 n), (independent) Microscopic female gametophytes (n) in inside these parts of flowers (dependent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

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 • the megaspore (n) + megasporangium (2 n) + 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 Integument Female gametophyte (n) Egg nucleus (n) Megasporangium (2 n) Megaspore (n) Unfertilized ovule Micropyle Fertilized ovule Discharged sperm nucleus (n) Pollen grain (n) containing male gametophyte Seed coat (derived from integument) Food supply (female gametophyte tissue) (n) Embryo (2 n) (new sporophyte) Gymnosperm seed

2. Ovaries & Seeds • 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 Seed coat (derived from integument) • 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 Food supply (female gametophyte tissue) (n) Embryo (2 n) (new sporophyte) Gymnosperm seed

• 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 3. Evolution of Pollen

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 • four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta – Phylum Cycadophyta – cycads Cycas revoluta • 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 Ginko biloba Welwitschia mirabilis. – Phylum Coniferophyta – largest group • • “cone-bearing” 600 species of conifers many are large trees most are evergreens – retain their leaves throughout the year Ephedra.

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

Conifer Needles • microphyll line of evolution – distinct line of evolution that came out of the first land plants (i. e. lycophytes) – development of leaves from clusters of sporangia – primitive leaves = enations – small enations are now called microphylls – “micro” refers to the evolution from small enations – evolution of true roots – allows for increased the size of the sporophyte – sporangia became clustered into compact cones or strobili – evolution of heterospory

• 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

Life Cycle: The Pine Key Haploid (n) Diploid (2 n) Ovule Ovulate cone Megasporocyte (2 n) Integument Longitudinal section of ovulate cone Pollen cone Micropyle Megasporangium Mature sporophyte (2 n) Microsporocytes (2 n) Germinating pollen grain MEIOSIS Longitudinal section of pollen cone Microsporangium Pollen grains (n) (containing male gametophytes) MEIOSIS Surviving megaspore (n) Sporophyll Seedling Germinating pollen grain Archegonium Egg (n) Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Embryo (new sporophyte) (2 n) Seed coat (derived from parent sporophyte) (2 n) FERTILIZATION Discharged sperm nucleus (n) Pollen tube Egg nucleus (n) Integument

• pollen cones: bear modified leaves or sporophylls (microsporophylls) each containing two microsporangia – the microsporangium is comprised of diploid cells called microsporocytes (2 n) The Male Life cycle

– microsporocytes divide by meiosis to form pollen grains which are haploid – pollen grains contain the male gametophyte – for the production of sperm The Male Life cycle

• 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 The Male Life cycle

• ovulate cones: made up of scales or megasporophylls • base of the megasporophylls are the ovules The Female Life Cycle Key Haploid (n) Diploid (2 n) Ovule Ovulate cone Pollen cone Megasporocyte (2 n) Integument Longitudinal Micropyle section of ovulate cone Megasporangium Mature sporophyte (2 n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS Surviving megaspore (n) Seedling Germinating pollen grain Archegonium Egg (n) Seeds on surface of ovulate scale Female gametophyte Germinating pollen grain (n) Food reserves Seed coat Discharged (gametophyte (derived from sperm nucleus (n) tissue) (n) parent sporophyte) (2 n) Pollen tube Embryo (new sporophyte) (2 n) FERTILIZATION Egg nucleus (n) Integument

• each ovule contains one megasporangium – the megasporangium contains a cell called a megasporocyte or a megaspore mother cell (2 n) – the megaspore mother cell undergoes meiosis to form 4 haploid cells – only one survives as the megaspore (n) – the remaining degenerate The Female Life Cycle Key Haploid (n) Diploid (2 n) Ovule Megasporocyte (2 n) Integument Longitudinal section of ovulate cone Micropyle Megasporangium Germinating Pollen pollen grains (n) (containing male MEIOSIS gametophytes)

– the surviving megaspore develops into the female gametophyte (archegonia + eggs) – the female gametophyte contains two (or three) archegonia - each will form an egg inside (two or three eggs form by mitosis) – the archegonium with egg + surrounding tissue = female gametophyte The Female Life Cycle Archegonium Egg (n) Female gametophyte Germinating pollen grain (n) Discharged sperm nucleus (n) Pollen tube archegonium egg female gametophyte tissue fills the ovule Egg nucleus (n)

– 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 (2 n) – both eggs in the female gametophyte may be fertilized The Female Life Cycle Surviving megaspore (n) Seedling Germinating pollen grain Archegonium Egg (n) Seeds on surface of ovulate scale Female gametophyte Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Discharged sperm nucleus (n) Pollen Embryo (new sporophyte) (2 n) Seed coat tube (derived from parent sporophyte) (2 n) FERTILIZATION Egg nucleus (n) Integument

– the ovule with the zygote is now called the seed – the developing embryo is retained within the female gametophyte (within the ovule) The Female Life Cycle Surviving megaspore (n) Seedling Germinating pollen grain Archegonium Egg (n) Seeds on surface of ovulate scale Female gametophyte Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Discharged sperm nucleus (n) Pollen Embryo (new sporophyte) (2 n) Seed coat tube (derived from parent sporophyte) (2 n) FERTILIZATION Egg nucleus (n) Integument

A simplified pine cycle

• conifer pollen arrives before the egg is mature • more than a year may pass between pollination & fertilization!!! Pollination

Pollination • 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 • as the eggs mature – the pollen tube is developing its two sperm cells • so the eggs and sperm mature at the same time germinating pollen grains with pollen tube air bladder tube cell generative cell

The Pine 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