Free tutoring service from students who have taken the course at McGill One-on-one, group or tutorial style sessions Check out our website for more info & to sign up! Email: [email protected] Website: http://peertutors.sus.mcgill.ca/ Facebook: www.facebook.com/suspeertutoring Twitter: @SUSPeerTutoring Biol-111 (2012) – Lecture 5 Rajinder Dhindsa Evolution of Land Plants What types of new challenges and opportunities the plant life would face in living on land as opposed to living in water? How can these new challenges be met and opportunities exploited? 2 Do you have a clicker? (I need to check if my clicker receiver is working 1. Yes. 2. What is a clicker? 3. I don’t like clickers 0% 0% 0% Response Counter Yes. What is a clicker? I don’t like clickers 3 During the evolution of land plants, plants moved to land from water. What challenges did they confront? • Make a list of important necessities of life and then compare how a plant fulfills them living in water and a plant living on land. • Think about the limitations and advantages of living in water and living on land. 4 Important necessities of a plant 1. Optimum hydration: Avoid dehydration of the body. 2. Protection of gametes from dehydration 3. Maintenance of physical structure and posture of the body. 4. Water provides considerable buoyancy. On land they will have to support almost all their weight. 4. to obtain sufficient water and nutrients from surrounding medium. 5. To carry out maximum photosynthesis 5. Maximum body size possible. 5 Plants living in water 1. No deficiency of water. 2. No special protection of gametes needed. They can be released into water. 3. No problem in maintaining physical structure and posture of the body as water provides buoyancy. Organisms in water don’t have to support their weight. 4. Water and mineral enter the plant body by diffusion through simple diffusion through the body surface. No special organs ot structures needed 5. Photosynthesis is limited by attenuation of light intensity under water. 5. In general body size is limited 6 Plants living on land 1. Water availability frequent problem, thus evolution of organs for absorption of water and minerals through roots and root hair. Water conservation through controlled loss through stomata. 2. Special pathways evolved for distribution of water and minerals through long-distance transport (Xylem, phloem) 3. Gametes are protected. Water is made available only in preparation for fertilization. 4. Evolution of seed habit resistant to dehydration. 5. Cell wall thickening in tissues to provide mechanical strength. 6. To benefit from higher light intensity, leaves with large surface evolved for maximum photosynthesis. 5. Large sprophytic body evolved. 7 All adaptations needed for flourishing life on land did not arise simultaneously during the course of evolution. Their sequential acquisition can be seen various land plants. 8 9 Alternation of Generations In sexually reproducing organisms, there is a phenomenon called Alternation of generations. A haploid form called gametophyte with one set of chromosomes alternates with a diploid form called sporophyte with two sets of chromosomes. Gametophyte produces gametes that fuse to form zygote that develops into sporophyte. The latter forms haploid spores through meiosis. Each spore develops into a gametophyte. 10 Advanced green algae are ancestors of plants (Gametes already protected in these ancestors) All green algae have Chara – up to 5 Photosynthesis with chlorophyll a,b cm long cells Cellulose cell walls Haploid dominant Coleochaete Pair of advanced groups jointly have Multicellular thallus growth form Antheridium & oogonium protect gametes Oedogonium with oogonium and 11 antheridium Advanced algae like Chara and Coleochaete as ancestors of land plants Coleochaete-like alga could serve as an ancestor of thallus- based land plants like liverworts (see later) Chara-like alga possesses branching growth patter and could serve as an ancestor for most land plants with erect growth habit. 12 Adaptive changes that evolved for life on land also altered the pattern of alternation of generations Let us follow the changes in relative predominance and independence of sporophytic and gametophytic phases of life- cycle. Remember: gametophyte is haploid (every cell has one set of chromosomes) Sporophyte is diploid (every cell has two sets of chromosomes). Think about possible consequences of this all-important difference. 13 Isogamy: The two gametes are similar in appearance This figure shows the alternation of generations in Chlamydomonas, an unicellular green alga. It has asexual as well as sexual modes of reproduction. The gametophytic phase is predominant. The sporophytic phase is limited to just the zygote. Asexual reproduction is just the multiplication of the haploid gametophytic phase. 14 Waxy cuticle appears to prevent drying Heterogamy: The two gametes are dissimilar in appearnce and are protected in gametangia. It shows homospory. Alternation of generations in Marchantia – a liverwort. The gametophyte is predominant and its body is called thallus – a flat mat-like structure. A thallus produces male or female umbrella-shaped structures which produce antheridia (containing motile sperms) or archegonia (containing egg). After fertilization, the zygote grows right on the umbrella- shaped structure and produces spores through meiosis. Each spore produces male or female thallus. Non-motile and motile gamete. Sporophyte is matrotrophic. 15 Life-Cycle of a moss – Stomata appear for gaseous exchange Each archegonium produces a single egg. Each antheridium produces millions of sperms (the travelling gamete) Sporophyte is matrotrophic. 16 Anthoceros is a hornwort. The cylindrical sporophyte can be nearly 8” tall and is matrotrophic. Hornworts and vascular plants are believed to share common ancestors. Again, Sporophyte is matrotrophic. Clicker Question Next ! 17 Hornworts - one of the three groups of bryophytes, share common ancestry with vascular plants. 18 Heterospory first appears in the earliest vascular plants – the Pteridophytes – club-mosses or lycopods, ferns and horsetails. 19 Evolution of microphylls (simple leaves with a single vascular projection) and megaphylls (expanded leaves with many vascular veins) to maximize size and light interception Microphylls Megaphylls 20 Selaginella – a lycopod, shows both heterospory as well as heterogamy. Microphylls appear Selaginella is a hetero- sporous pteridophyte (an early vascular plant related to ferns). It produces microspores giving rise to male gametophyte , and megaspores giving rise to female gametophyte. After fertilization, in some species, the zygote undergoes dormancy before germinating (as in seeds). 21 Life cycle of a common fern Ferns include both homosporous and heterosporous species Megaphylls appear Xylem and phloem appear 22 Equisetum – a horsetail, grows abundantly in McGill University Arboretum Up to here, sperms have flagella and are motile 23 Gymnosperm life cycle Naked ovule borne on the scale • Sperms are not motile • But still no flowers • No xylem vessels • Gametophyte matrotrophic • 300 ft tall sporophyte • Same tree bears male strobili and female cones • Naked ovule develops into a naked seed • Polyembryony: several embryos develop within one seed. 24 Angiosperm Life Cycle: Flowers appear, xylem vessels appear, sperms not motile Gametophyte matrotrophic Male gametophyte represented by mature pollen grain consisting of two cells. Female gametophyte represented by 8-celled embryo sac. One of those cells is the egg. Double fertilization Only one embryo per seed Female gametophyte 25 Higher Plant vasculature: Tracheids, Vessels, Seive tubes and Companion cells Phloem seive tubes and Xylem vessels and tracheids companion cells 26 Evolution of Alternation of Generations Gametophyte Gametophyte much Sporophyte predominant and predominant and smaller than nourishes the hidden and nourishes smaller sporophyte but both inconspicuous gametophyte sporophyte live independently 27 Sprophyte/Gametophyte Size Ratio Group Sporophyte Gametophyte Sporophyte/Gametophyte Dependence Algae: Variable Bryophytes Mosses 2 cm 5 cm 0.4 Sporophyte matrotrophic Hornworts upto 20 cm thallus 0.4 20 Sporophyte matrotrophic Pteridophytes: 50-200 cm upto 5 mm 400 Both live independently Gymnosperms >10,000 cm <1 mm 100,000 Microscopic gametophyte and Angiosperms matrotrophic Diploidy means 2 copies of each gene. • Gene-dosage effect • Provides a cushion against harmful mutations. 28 Plants and mobility Plants did not evolve any mechanisms of mobility, except in Tolkien’s fantasies (Lord Of The Rings). Why not? Because they never needed to! They get what they need standing majestically where they are. When they need to either bring something to them or send something away, they recruited slaves – animals! 29 So, how did the plants meet special challenges associated with living on land? 1. Control of water loss through stomata and surface cuticle. 2. Protection of gametes. 3. Expansion of photosynthetic surface (leaves) to take advantage of higher light intensity. 4. Special tissues with thickened cell walls for mechanical strength to support the plant body. 5. Seed habit – Dehydrated seed capable of being stored. 6. Evolution of the diploid sporophyte as the predominant body form to provide cushion against mutations as well as higher gene dosage to increase` body size. 30 .