Plant Diversity II: Evolution by Seed Plants

1. 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

2. A reminder: several unique traits seen in plants and charophyceans • 1. rose-shaped complexes for cellulose synthesis –called rosettes • synthesize cellulose microfibrilsfor the cell walls • both plants and charophyceans – 20 to 25% of the cell wall is made of cellulose • 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 Charophyceans

3. Defining the Plant Kingdom • ANOTHER REMINDER: four key traits that define plants – absent in charophyceans • 1. apical meristems • 2. alternation of generations & multicellular, dependent embryos • 3. walled spores in sporangia • 4. multicellular gametogangia

4. Defining the Plant Kingdom • traditional classification schemes equates the plant kingdom with the presence of embryophytes • embryo is nourished inside the archegonium • vascular plants 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

5. Seed plants • three key reproductive adaptationsevolved 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

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

7. Heterospory • the rule among seed vascular 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

8. Heterospory • these sporangia can either be located on the same plant = monoeicous • meaning “one house” • or they can be located on “male” and “female” plants = dioecious • i.e. bisexual

9. 2. Ovaries & Seeds • seed plants are unique in that the megasporangiumis retained with the parent sporophyte • the megasporangiumcontains 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

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

11. 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 • multicellular structure - in contrast to the spore • 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 Seed coat (derived from integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) Gymnosperm seed

12. 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

13. 3. Evolution of Pollen • in order to fertilize- the pollen grain begins to 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

14. Gymnosperms • “naked seed” – seeds are not enclosed in ovaries • seeds are exposed on modified leaves (sporophylls) that form cones • in the ferns – development of sporophyll leaves that bear the sporangium (sori) • in gymnosperms – development of modified sporophylls that cluster together to form cones or strobili • 380 MYA – development of heterosporous trees with woody stems – but did not bear seeds = progymnosperms • first seed plant in the fossil record – 360 MYA • now extinct • earliest fossils of gymnosperms – 305 MYA

15. Gymnosperms • drier environment favored gymnosperms over the bryophytes and ferns • 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 Ponderosa pine

16. Gymnosperms • four gymnosperm phyla: Cycadophyta, Ginkgophyta, Gnetophyta and Coniferophyta • Phylum Cycadophyta – cycads • second largest group of gymnosperms • large cones and palm-like leaves • 130 species survive • Phylum Ginkgophyta - ginkos • only one species left – Ginkgo biloba • deciduous leaves - fanlike formation • tolerates air pollution well • trees bear fleshy seeds that smell rancid • Phylum Gnetophyta – three genera alive today • 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 • Phylum Coniferophyta – largest group • “cone-bearing” • 600 species of conifers • many are large trees • most are evergreens – retain their leaves throughout the year Cycasrevoluta Ginkobiloba Welwitschia mirabilis. Ephedra.

17. 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

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

19. 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

21. Key Haploid (n) Diploid (2n) Ovule Ovulate cone Megasporocyte (2n) Integument Longitudinal section of ovulate cone Micropyle Pollen cone Megasporangium Mature sporophyte (2n) Microsporocytes (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS Surviving megaspore (n) Longitudinal section of pollen cone Sporophyll Microsporangium Seedling Germinating pollen grain Archegonium Integument Egg (n) Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n) Life Cycle: The Pine • pollen cones: bear modified leaves or sporophyllseach 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 • pollen development results in the production of 2 sperm cells within the pollen tube

22. Pollen Cones • considered to be simple cones • one single stem axis bearing modified leaves known as 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) air cells Male Pine Cone

23. Seed cones • more complex than pollen cones • compound cone • each consists of a cone axis with scales • the scales bear leaves that are called bracts rather than sporophylls • bracts are associated with reproductive structures = fused megasporophyllthat has two ovules • megasporophyllalso called an ovulate scale megasporophyll sterile bract

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

25. Life Cycle: The Pine Key Haploid (n) Diploid (2n) Ovule Ovulate cone Megasporocyte (2n) Integument Longitudinal section of ovulate cone • the surviving megaspore develops into the female gametophyte • the female gametophyte develops two or three separate archegonia - each will form an egg Micropyle Pollen cone Megasporangium Mature sporophyte (2n) Microsporocytes (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS Longitudinal section of pollen cone Surviving megaspore (n) Sporophyll Microsporangium Seedling Gametophyte Germinating pollen grain Archegonium Integument Egg (n) Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n) Archegonium

26. Life Cycle: The Pine Key Haploid (n) Diploid (2n) Ovule Ovulate cone Megasporocyte (2n) Integument Longitudinal section of ovulate cone • as the eggs mature – the pollen tube is developing its two sperm cells • so the eggs and sperm mature at the same time • the germinating pollen grain develops its pollen tube and two sperm enter into the ovule • through the micropyle • all eggs may be fertilized by sperm BUT only one egg NUCLEUS with fuse with a sperm NUCLEUS to form the zygote • the ovule with the zygote is now called the seed • the developing embryo is retained within the female gametophyte (within the ovule) Micropyle Pollen cone Megasporangium Mature sporophyte (2n) Microsporocytes (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS Longitudinal section of pollen cone Surviving megaspore (n) Sporophyll Microsporangium Seedling Germinating pollen grain Archegonium Integument Egg (n) Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n)

27. A simplified pine cycle

28. 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 (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 grains with pollen tube air bladder tube cell generative cell

29. 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