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Reproductive Morphology

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Week 3; Wednesday Announcements: 1st lab quiz TODAY Reproductive Morphology Reproductive morphology - any portion of a plant that is involved with or a direct product of sexual reproduction Example: cones, flowers, fruits, seeds, etc. Basic Plant Life cycle Our view of the importance of gametes in the life cycle is shaped by the animal life cycle in which meiosis (the cell division creating haploid daughter cells with only one set of chromosomes) gives rise directly to sperm and eggs which are one celled and do not live independently. Fertilization (or the fusion of gametes – sperm and egg) occurs inside the animal to recreate the diploid organism (2 sets of chromosomes). Therefore, this life cycle is dominated by the diploid generation. This is NOT necessarily the case among plants! Generalized life cycle -overhead- - alternation of generations – In plants, spores are the result of meiosis. These may grow into a multicellular, independent organism (gametophyte – “gamete-bearer”), which eventually produces sperm and eggs (gametes). These fuse (fertilization) and a zygote is formed which grows into what is known as a sporophyte - “spore-bearer”. (In seed plants, pollination must occur before fertilization! ) This sporophyte produces structures called sporangia in which meiosis occurs and the spores are released. Spores (the product of meiosis) are the first cell of the gametophyte generation. Distinguish Pollination from Fertilization and Spore from Gamete Pollination – the act of transferring pollen from anther or male cone to stigma or female cone; restricted to seed plants. Fertilization – the act of fusion between sperm and egg – must follow pollination in seed plants; fertilization occurs in all sexually reproducing organisms. Spore – the product of meiosis; the first cell in the haploid (1n) or gametophyte generation in plants. In most animals the product of meiosis also functions as a gamete. Gamete – the haploid cells that participate in fertilization, egg and sperm. These are produced by specialized structures in the gametophyte generation of plants called gametangia. In flowering plants the gametophytes are so reduced (pollen grain and embryo sac) that distinct structures are not found. 12 mature sporophyte embryo sporangium diploid (2n) sporophyte zygote generation Fertilization Meiosis haploid (1n) gametes gametophyte spores (egg and sperm) generation Archegonium Antheridium mature gametophyte Variations on the theme .... Each generation (sporophyte/gametophyte) may be free-living or dependent (on the other generation for nutrition) Usually one generation is dominant, but in some algae both are +/- equivalent. green algae - some have sporophyte dominant, some gametophyte dominant, and some are isomorphic Mosses - gametophyte dominant; sporophyte dependent Ferns - sporophyte dominant; gametophyte free-living, but small and short-lived seed plants - sporophyte dominant; gametophyte dependent and reduced Angiosperms - gametophyte VERY reduced - pollen 2-3 cells; embryo sac 8 cells 13 sporangium - a spore bearing case or sac in the case of gymnosperms, the sporangium is where the pollen (microsporangium – pollen sacs) and ovule (megasporangium and associated integument - outer layers that become seedcoat) pollen – the mature microspores or developing male gametophyte – sperm produced within ovule – immature seed the megasporangium and surrounding integuments - the egg produced within archegonium - female gametangia, multicellular sporophyll – sporangium bearing leaf – often modified in structure Cone (strobilis) – a dense cluster of sporophylls on an axis Cones are unisexual and may bear microsporangia – pollen sacs – or megasporangia – ovules – but not both – a cone may be thought of as a reproductive short shoot! Pollen cones – in all conifers pollen cones are simple – a simple cone has a cone axis and one set of appendages, usually called scales – scales have 2 to many pollen sacs. Pollen cones are deciduous soon after pollen is released, and not often used for identification Ovulate cones or Seed cones – in all conifers, seed cones are compound (though not always obvious at maturity!) – seed cones have first order appendages – bracts – and second order appendages – ovuliferous scales – in the bract axils. The central stem axis bears highly modified lateral branches (the ovuliferous scales) in the axils of a reduced leaf. Ovuliferous Scales – bear ovules on adaxial surface (upper surface, closest to the axis) Contrast with abaxial – lower surface (away from axis) Ovules may be inverted (pointing towards axis) or erect Simple vs. Compound cones Simple cone is composed of scales arranged along an axis – reduced branch (Show Pinaceae branch with pollen cones replacing SS) Compound cone is composed of Bracts subtending Scales. Each scale is interpreted as a reduced branch in the axil of modified leaf (bract). Therefore, the compound cone is a series of reduced branches arranged on an axis (show Pinaceae branch with ovuliferous cone replacing axillary LS bud) The compound nature of the ovulate cones is only really evident at maturity in Pinaceae – in all other conifers (Cupressophytes – includes Cupressaceae, Taxaceae, et al.) – bract and scale are fused during ontogeny. Bract/Scale complex – the product of bract-scale fusion during ontogeny – mimics simple cone 14 Apophysis – the exposed portion of the cone scale in a mature unopened cone Usually lighter in color, varies (smooth, wrinkled, grooved, ridged, etc.) and is important for identification – especially Pinus where apophysis terminates in a small protruberance called the umbo (the visible portion of the scale in the first year of development) Terminal umbo – at tip of scale Dorsal umbo – on back or abaxial portion of scale Boss – raised or rounded projection on umbo 15 Week 3; Friday Announcements: Web readings – to review some material and learn more… How do we go about identifying groups and relationships? We look for similarities, but similarities can reflect 3 different relationships: 1. Shared derived similarity (synapomorphy) a. Example: Feathers on birds - these evolved at the time birds first appeared 2. Shared ancestral similarity (symplesiomorphy) a. Example: keratin scales on reptiles - these are transformed into feathers in birds b. In these examples, feathers are evidence of monophyly in birds, but scales are NOT evidence of monophyly in reptiles. 3. Convergent similarity - similarity due to evolution in parallel in two different organisms – convergent evolution a. Example: wings on birds and bats - another term for this is Parallel evolution These kinds of similarity correspond to the three kinds of groups: 1. shared derived --> monophyly 2. shared ancestral --> paraphyly 3. convergent --> polyphyly In systematics we call similarities CHARACTERS Example: Leaf type à character, The term we use for a character that arose with the evolution of the group and is shared due to common ancestry is: HOMOLOGY A character is a hypothesis of homology – characteristic shared b/c it came from a common ancestor. "Homology - similarity in two or more organisms that can be traced back to the same feature in the common ancestor of those organisms." (Mayr 1969) Example: wings on birds, also wings on bats, but NOT wings on birds AND bats 2 components: When we talk about ‘homology’ as evidence for relationship, we must refer both to a trait and a group of organisms. • Homology is the fundamental idea in evolution! But, determining homology can be quite difficult! • To know for sure that a character is a homology, we need to know the relationships among the species that share the character. TREE-THINKING QUIZ OVERHEAD (character-based questions) 16 Lecture: Major groups of seed plants – seed plant relationships Land Plant Relationships -Overhead- Major groups of organisms in Kingdom Plantae (= green plants) Note that this is one view of land plant relationships. Some molecular evidence contradicts this view, suggesting that ‘bryophytes’ might be related differently than shown here. Also, some evidence suggests that the extant (those alive today) ‘gymnosperms’ may be monophyletic. Plantae “Green Algae” - paraphyletic basal group Land Plants Liverworts \ Hornworts > “Bryophytes” paraphyletic - consists of 3 monophyletic groups Mosses / Tracheophytes (traditionally “vascular” plants, However, mosses have vasculature, but not tracheids) Lycophytes - Lycopodium (club mosses), Selaginella (resurrection plant) Fern allies Sphenophytes - Equisetum (horse tails) Psilotum Ferns - Seed Plants Cycads Ginkgo Conifers - you know these guys Gnetophytes - Gnetum, Ephedra (Mormon Tea), Welwitschia Angiosperms Seed plants (Spermatophytes) can be divided into two extant groups – Angiosperms (> 300,000 spp) and Gymnosperms (ca. 1100 spp.) (OVERHEAD) SYNAPOMORPHIES of seeds and wood We’ve already defined seeds – product of the ovule after fertilization, contains the embryo, nutritive tissue, and seed coat Wood – strictly defined as secondary xylem – explain primary vs. secondary growth Primary growth – from the apical meristems in shoots and roots. Undifferentiated tissue is produced via mitotic divisions leaving behind derivative cells that go on to differentiate into all the different cell types/tissues etc. in the plant, including vascular tissue – xylem and phloem. Secondary growth – between the xylem and phloem remains a group of undifferentiated cells that form a secondary meristem – cambium – produces secondary xylem towards the inside of stem and phloem towards the outside, resulting
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