Heterospory and Seed Plants: Evolutionary Insights into Plant Reproduction

1. Kingdom Plantae:Heterospory and Seed Plants

2. Setting the stage for pollen/seeds • Most of plants discussed so far do not have specialized gametophytes • They are homosporous plants: make one kind of meiospore.

3. Setting the stage for pollen/seeds • Homosporous groups • Bryophyta • Hepaticophyta • Anthocerophyta • Psilophyta • Spenophyta

4. Setting the stage for pollen/seeds • Heterosporous plants: make 2 types of meiospores. • One becomes male gametophyte and makes sperm • One becomes female gametophyte and makes eggs • Thus, make specialized spores and specialized gametophytes. Specialization occurs earlier in life cycle

5. Setting the stage for pollen/seeds • Heterosporous groups: • Some Lycophyta • Some Pterophyta • All seed plant phyla (discussed later today!).

6. Setting the stage for pollen/seeds • Example of heterosporous plant life cycle: Selaginella (Lycophyta) • Seen in lab #3.

7. Setting the stage for pollen/seeds • Heterosporous plant life cycle: Selaginella (Lycophyta) • Photos of structures.

8. Heterospory • Note deadbeat gametophytes • Not free-living. Dependent on sporophyte • Not (very) photosynthetic • Small: microgametophyte reduced to 1 antheridium! Megagametophyte develops mostly within megaspore.

9. Heterospory • Spores specialized • Megaspore makes megagametophyte, which makes archegonia with eggs • Microspore makes microgametophyte, which makes antheridium containing sperm.

10. Heterospory • Valuable ideas for life on the land: • 1) Gametophytes small and protected by spore walls • 2) Deadbeat gametophytes fed by big strong sporophyte (gametophytes can focus on reproduction) • 3) Megaspore: keep it after fertilization and feed embryo. Embryo starts development inside megaspore and protected by its walls. Lets new embryo “eat” its mother (megagametophyte): keeps energy used to make megagametophyte from being wasted • 4) Microspore: disperse it through air, instead of delicate swimming sperm.

11. Plants • Nonvascular Plants • Covered. Reproduce by spores. Gametophytes independent of sporophyte • Seedless Vascular Plants • Covered. Reproduce by spores. Gametophytes independent of sporophyte • Seed Plants • Rest of plant kingdom. Gametophytes dependent on sporophyte.

12. Seed plant features • Have vascular tissue • Make roots, stems, leaves (megaphylls) • Some can do secondary growth in stems/roots: make new phloem and xylem and grow in diameter rather than just length • Life Cycle • Heterosporous, but spores kept by sporophyte • Gametophytes reduced, dependent on sporophyte • Male gametophyte (microgametophyte) dispersed as pollen grain (no free-swimming sperm in water) • Female gametophyte (megagametophyte) kept by sporophyte, becomes part of ovule

13. Seed plant groups • Gymnosperms (“gymno-” means naked, “sperm” means seed): do not make make seeds in sealed container (fruit) • Phylum Cycadophyta (cycads) • Phylum Coniferophyta (conifers) • Phylum Ginkgophyta (ginkgo) • Phylum Gnetophyta (gnetophytes) • Note all gymnosperms do secondary growth (form wood, are shrubs or trees)

14. Seed plant groups • Angiosperms (“angio-” means container, “sperm” means seed): make seeds in sealed container (fruit) • Phylum Anthophyta (flowering plants) • Many of these are woody, but some are herbaceous (don’t do secondary growth).

15. Pine life cycle • Pine life cycle movie (for overview) • Example of gymnosperm life cycle

16. Female cone at pollination stage (top) Pine life cycle • Female cone: Cone scales with ovules on them Cone scale with ovule Mature female cone

17. Pine life cycle • Ovule: integumented megasporangium. Megaspore mother cell does meiosis, forms meiospores, one survives & does mitosis to make female gametophyte. 1 2 3 4

18. Pine life cycle • Female gametophyte has several archegonia, each with an egg. () () 1 2 3 4

19. Pine life cycle • Male cone: made up of microsporophylls • Microsporangia on sporophylls Microsporophyll Sporangium on underside of sporophyll

20. Pine life cycle • Sporangia on microsporophylls have microspore mother cells (2n), which do meiosis to make microspores (1n) • Each microspore does a couple mitotic divisions to form an immature male gametophyte (called pollen grain).

21. Getting male and female gametophytes together • Pollen grain flies through air (wind dispersed) • Arrives at female cone micropyle (gap between integument ends), and winds up next to female gametophyte.

22. Getting male and female gametophytes together • Now, pollen grain germinates: makes pollen tube and spermatogenous cell. Becomes mature male gametophyte.

23. Getting male and female gametophytes together • Pollen tube penetrates into female gametophyte to find archegonium, so sperm and egg can fuse to make zygote.

24. The new baby sporophyte • Zygote grows to embryo as ovule becomes seed • Integument (2n tissue from parent sporophyte) becomes hard protective seed coat • Female gametophyte serves as food for growing embryo. Embryo Remains of seed coat (most dissected away) Female gametophyte

25. Seed has wing: wind dispersed • Winged pine seed helicoptering away (most pine seeds fly this way)

26. Seed germinates and new sporophyte is on its own • Young pine seedling

27. Pine life cycle • Take home lessons: • 1) Pollen grain is immature male gametophyte (microgametophyte) • 2) Male gametophyte matures in ovule, when pollen tube and sperm cell made. Entire male gametophyte is just a few cells. • 3) Ovule contains megasporangium. Meiosis produces megagametophyte, which makes archegonia, still kept in megasporangium. Pine pollen grain

28. Pine life cycle • Take home lessons: • 4) Fertilization takes place and embryo digests female gametophyte. • 5) Seed is baby plant (embryo), in box (seed coat from parent sporophyte tissue), with its lunch (female gametophyte)!.

29. Pine life cycle • Advantages of making seeds and pollen: • 1) Male gametophyte (immature) small, protected by microspore wall, flies through air to female gametophyte • 2) No swimming sperm. Male gametophyte delivers sperm to female gametophyte protected by ovule in female cone • 3) Embryo nourished by megagametophyte and protected by cone of sporophyte • 4) Seed has hard protective coat, supply of food (megagametophyte). Can be dispersed to new location. • 5) Embryo can be dormant: allows new sporophyte to wait for good conditions to germinate.

30. Gymnosperms get their chance to rule the land.... • The Gymnosperm Revolution and the Permian Period • Permian Period (290-248 million years ago) followed Carboniferous: was relatively cool and dry. Most coal swamps dried up. Carboniferous coal swamp

31. Gymnosperms get their chance to rule the land.... • Many SVPs (especially tree species) went extinct. Gymnosperm groups prospered! • Why? Pollen and seeds adapted them to dry conditions. Carboniferous coal swamp

32. Gymnosperms • Phylum Cycadophyta (cycads) • Phylum Coniferophyta (conifers) • Phylum Ginkgophyta (gingko) • Phylum Gnetophyta (gnetophytes)