Bio 144 Plant Diversity Lecture 7 PDF

BIOLOGY 144 7. Gymnosperms and seed evolution - conifers PLANT DIVERSITY  disadvantage of non-seed plant life cycle: water still necessary for male gametes to swim through to find female gametes  New sporophyte also temporarily dependent on small gametophyte p...

BIOLOGY 144 7. Gymnosperms and seed evolution - conifers PLANT DIVERSITY  disadvantage of non-seed plant life cycle: water still necessary for male gametes to swim through to find female gametes  New sporophyte also temporarily dependent on small gametophyte plant in earliest stages (zygote and embryo)  Many sporophytes die  Advantageous for the embryo to utilise the already-established photosynthetic (leaves) and absorptive (roots) capabilities of the Male gametes require water to swim towards previous sporophyte generation archegonia  for this, the female gametophyte (embryo sac) and embryo had to remain within maternal sporophyte  made possible by retention of macrospore (and later female gametophyte) inside macrosporangium – evolution of the ovule  Whole male gametophyte plant is now transferred to the female gametophyte, not just the male gametes  Made possible through vectors such as wind, which move mature end products of microspores (pollen grains) to female gametophyte  Pollen grain germinates on maternal sporophyte and the male gametophyte develops within the microspore wall – now equivalent to pollen grain wall  Spermatophyta – formal name for all plants that produce seeds  All plants wherein the embryo is enclosed in seed tissue and undergoes a period of dormancy  Germination breaks dormancy and allows embryo to develop into adult sporophyte  Spermatophyta:  Gymnospermae – ovule exposed to the atmosphere (cone-bearing plants)  Angiospermae (Anthophyta) – ovules enclosed in sporophyte tissue (carpels and ovaries – flowering plants)  Although sporophyte phases immensely morphologically Naked ovules variable in different gymnosperm phyla, their life cycles, structure of their gametophytes and fertilisation processes are quite similar Enclosed ovules  seed plants evolved  425 million years ago from free-sporing progymnosperms  evolution of the seed had massive ecological implications  extra layer of sporophyte tissue protects the developing embryo  nucellus = macrosporangium of the seed plant  nucellus is surrounded by one of more layers of exptra sporophyte tissue known as integuments, which harden sclerenchymatously to form the seed coat  Micropyle Integument (2N) Nucellus (2N) Female gametophyte (N) Macrosporophyll Archegonium (N) Central stem Egg (N) portion of cone  These adaptations brought improved embryo protection against mechanical damage and desiccation  Evolution of the seed introduced a dormancy phase into the life cycle of plants  Ecological advantages:  Dormancy allows embryo to survive until favourable environmental conditions are present  Leads to optimal germination and seedling development Necessary preconditions for seeds: homospory → heterospory Necessary preconditions for seeds: inclusion of gametophyte inside spore wall Female gametophyte Male gametophyte in in macrospore microspore seed: formation and retention of new diploid embryo inside macrosporangium, surrounded by integument Micropyle Megasporophyll (2N)  Female gametophyte develops inside sporophyte-formed ovule Ovules only partially embedded in sporophyte tissue in gymnosperms (exposed to atmosphere during pollination) Ovule entirely enclosed by sporophyte tissue in angiosperms Seeds are one of the most meaningful developments in the evolution of vascular plants Interpretations of the origin of seeds mostly based on fossil seed structures  Early seeds of the late Paleozoic era (400-250 million years ago) mostly occurred terminally on stem axis bilateral or radially symmetric Archaeosperma one of the oldest plants with fully- developed seedlike structures Lived in the late Devonian (360 million years ago) - more than 40 million years after first vascular plant fossils Archaeosperma fossils are interpreted as a group of loosely-arranged, radially symmetric seeds Every seed: macrosporangium (nucellus) surrounded by protective integumentary layer with fingerlike outgrowths  Other Devonian and (later) Carboniferous fossils show more typical seed structure Macrosporangia progressively more deeply enclosed by integuments This progression shows the evolutionary development of true seeds Seedlike structures have evolved more than once in the vascular plants, but all extant seed plants are monophyletic Archaeosperma  Pollen = male version of “seed" Immature male gametophyte inside microspore wall Evolved through: progressive reduction of male gametophyte + gradual inclusion within spore wall Similar situation evolved independently in Selaginella  Male gametophytes of Selaginella and pollen both develop through meiosis from microsporangium mother cells and are surrounded by a spore wall – what’s the difference?  Point of germination – pollen grains differ from Selaginella male gametophyte in terms of position where gametophyte emerges from spore wall  Selaginella male gametophyte germinates through tetrad scars left by three other daughter microspores descended through meiosis from mother cell  In contrast, first true pollen grains emerge from point opposite meiotic scars  In seed plants these scars are absent – have been lost through evolutionary time  Gymnosperms 4 extant gymnosperm phyla: Coniferophyta, Cycadophyta, Gnetophyta and Ginkgophyta 2 extinct phyla: Progymnospermatophyta and Pteridospermatophyta Relationships between gymnosperm phyla remain contentious Today it is clear that the gymnosperms are monophyletic But the relationships between the phyla remain vague Progymnospermatophyta Pteridospermatophyta  Gimnosperme Gymnosperms share several characters : seeds absence of flowers ovules (unfertilised seeds) rest on modified leaves ovule only partially enclosed in sporophyte tissue Word “gymnosperm” derived from Greek: “gymnos” = naked and “sperma” = seed Details of the reproductive cycle differ between the phyla Ginkgo and cycads have motile, flagellate male gametes Sperm of other two phyla are immotile (no flagella) Female cones differ in size – small (few grams) to massive (45 kg+) structures 1. Phylum Coniferophyta (conifers) Most well-known group of gymnosperms – includes pines, cedars, cypresses and sequoias Includes among the oldest, tallest and most massive living things habitat – cold, temperate or arid habitats worldwide Sources of wood, paper, resin, turpentine and taxol (cancer treatment agent) and many other commercially valuable products  Pinus: pine tree >100 pine species on earth today Entirely confined to Northern Hemisphere Needle-shaped leaves in groups of two to five borne on short branches Leaves adapted to minimise water loss: Covered in thick cuticle Stomata deeply sunken Presence of resin ducts Resin ensures that the plant does not freeze also protects against insect / fungal damage Commercially important: terpentine and rosin Pinus life cycle All seed plants are heterosporous – spores develop into two different types of gametophyte Sperm cells develop within pollen grains Pollen formed in microsporangia in 1-4 cm long male cones (microstrobili) Male cones borne in groups of 30-70 at tips of lower branches Papery bracts (microsporophylls) arranged spirally around central axis 2 microsporangia develop abaxially (bottom side) of each microsporophyll Pinus life cycle Masses of microspore mother cells inside sporangium undergo meiosis to produce four microspores Microspores develop inside microspore wall into 4-celled pollen grains Every pollen grain develops two air chambers to aid wind dispersal Female cones (macrostrobili) form on the same tree Larger than male cones, cone bracts (macrosporophylls) become woody 2 ovules develop on adaxial (top) surface of each macrosporophyll  Pinus life cycle Each ovule contains a macrosporangium known as micropilum the nucellus archegonium Nucellus surrounded by thick sporophyte layer, the integument Tiny opening in integument = micropyle Integument will harden to form seed coat Single macrospore mother cell in every macrosporangium undergoes meiosis to form 4 macrospores 3 disintegrate, only 4th will develop into female gametophyte gametophyte development takes more than a year integument nucellus At maturity, 2-6 archegonia develop on the female micropylar end of the female gametophyte gametophyt e  Pinus life cycle Every archegonium contains a single massive egg cell First spring of cone development: macrosporophylls of female cone open and allow pollen grains to enter Micropyle secretes sticky pollination droplet that traps pollen Droplet evaporates, in the process drawing pollen grain through micropyle towards the nucellus Cone closes, female gametophyte and archegonia only reach maturity a year later Pollen tube germinates from pollen grain atop nucellus Digests nucellus tissue as it elongates in order to reach archegonium  Pinus life cycle 1 of 4 cells in pollen grain, the generative cell, divides via mitosis One of 2 resultant cells divides to form two sperm cells Germinated pollen grain with two male gametes = mature male gametophyte After pollen tube reaches archegonium, approximately 15 months after pollination, sperm cells released One sperm fuses with egg to form zygote Remaining gamete and rest of male gametophyte disintegrate Zygote develops via mitosis into embryo, within developing seed Seeds released and after dormancy period germinate to form pine tree sporophyte generation  Pollen and seeds Homospory to heterospory Gametophytes retained in spore wall Embryo (2N) Seed Microsporophyll (2N) Seed coat (2N) Microspore mother cells Integument (2N) (2N) meiosis Ovule Micropyle 2 to 6 archegonia (N) microspore (N) Female gametophyte (N) Nucellus (Macrosporangium) (2N) mitosis Macrosporophyll (2N) Macrospore mother cell (2N) Male gametophyte Macrosporangium wall (2N) Pollen = mature male gametophyte within microspore; only 4-5 cells, no antheridium