Gymnosperms: Evolution of the Seed Major Themes in Land Plant Evolution Progressive reduction of the gametophyte stage Independent sporophyte Evolution of pollen (replaces swimming sperm) Evolution of the seed (replaces spores as dispersal agent) Homospory  Heterospory (separate male and female spores) also... Improved vascular tissues Increased height and structural strength Anatomical + physiological adaptations to drier environments

Adaptations to Dry Living: Stomata and Cuticle Cuticle = waxy protective layer on the outside of cells that prevents water loss In stomata (singular = stoma), guard cells open to allow CO2 to enter open pores..

Reduction of Gametophyte Gymnosperm Angiosperm Bryophyte Fern Gametophyte is visible Gametophyte is microscopic -

Homosporous vs. Heterosporous Sporangium Megasporangium Microsporangium Megaspores Microspores Spores Gametophyte Mega-gametophyte Micro-gametophyte Archegonium Antheridium Archegonium Antheridium Egg Sperm Egg Sperm Zygote Zygote

Bryophyte life cycle Gametophyte is dominant -

Seedless vascular plant life cycle Sporophyte is dominant - Sperm must still swim to reach egg - Little protection for the embryo -

Appeared ~360 million years ago

Seed plants Gymnosperms - - 4 phyla

Gymnosperms Sporophylls: modified leaves that contain sporangia (spore-producers) FEMALE megasporangium megaspore MALE microsporangium 2N meiosis 1 of 4 cells survives N

male gametophyte grows Gymnosperms Sporophylls: modified leaves that contain sporangia (spore-producers) FEMALE megasporangium megaspore female gametophyte archegonia egg MALE microsporangium microspore male gametophyte grows inside pollen grain sperm 2N N

Development of the gymnosperm seed from an ovule in Pinus sp. Gymnosperm ovules have 1 protective outer layer, or integument, composed of tissue from sporophyte (mom’s tissue) Pollen grain grows a pollen tube of haploid cells to penetrate the micropyle, the opening through the integument; releases sperm Fertilized egg grows into embryo, surrounded by female gametophyte tissue (its food) and the sporophyte’s integument (its protection)

Gymnosperms Strobilus (= cone) is a branch tip specialized for reproduction Leaves are modified into.. Mature female pine cone

Gymnosperms Strobilus (= cone) is a branch tip specialized for reproduction Leaves are modified into sporophylls Megasporangium

Gymnosperm life cycle

Life cycle of Pinus sp. heterospory “ovule” (developing seed) “seed” (after fertilization)

Pine ovule after formation of the megagametophyte, archegonium & egg(s) Stern 1991

Pine ovule at the fertilization stage egg zygote sperm second sperm nucleus Raven et al. 1986

2n n embryo (2n) Gametophyte (n) Pine seed showing embryo (baby sporophyte) within nutritive tissue, surrounded by a seed coat 2n n embryo (2n) Gametophyte (n) Raven et al. 1986

Phylum Ginkgophyta – Ginkgo biloba Only 1 living species, almost identical to fossils 150 million years old

Phylum Cycadophyta – about 160 species

Phylum Gnetophyta (~ 70 species)

Phylum Coniferophyta Conifers – cone bearing entirely wind-pollinated fertilization can take a year to occur

Cross-section of a pine needle showing adaptations to drought Dense packing of cells Sunken stomata Thick cuticle Raven et al. 1986

Transport tissues (secondary xylem & phloem) in a pine tree provide improved long- distance transport and structural support wood Raven et al. 1986

Biggest + oldest Height: coast redwoods (Sequoia sempervirens) >100 m tall Mass: giant redwoods (Sequoiadendron gigantea) > 26 m. circumference Age: bristlecone pine (Pinus aristata) 4900 years old

Economic importance of conifers Pacific Yew (Taxus brevifolia)

Evolution of Plant Biodiversity Gymnosperms dominate Seedless vascular plants # of families Angiosperms First gymnosperms Mackenzie 2003