Week 3; Wednesday
Announcements: 1st lab quiz TODAY
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
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
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 in growth in diameter.
17 The division of seed plants into two groups is based on the location of the ovule and the details of pollination Plants with “naked” seeds are referred to as gymnosperms (Greek, “gymno” = naked, “sperm” = seed) – ovules are not contained inside of other structures at the time of pollination – pollen can land directly on the ovule itself.
Plants with ovules that are enclosed in an ovary, so that the pollen cannot land directly on ovules are called angiosperms – flowering plants
NOTE: It is important to realize that the word gymnosperm does not refer to a natural group (i.e., not monophyletic) – there are many different gymnosperm groups, extant and extinct, and we know very little about how their phylogenetic relationships to each other. DRAW TREE with Angiosperms, extant gymnosperms, and other extinct gymnospermous lineages.
Gymnosperms are simply all those seed plant lineages with their ovules exposed to the outside world so that pollen can land directly on them. It would be more appropriate to refer to these plants as gymnospermous (using an adjective), rather than using the word as a noun. However, the word gymnosperm is firmly fixed in the scientific literature and remains in common use, so it is what it is (and extant gymnoperms do appear to be monophyletic.
On the other hand, flowering plants all share a more recent common ancestor with each other than they do with other seed plants so the term angiosperm refers to a monophyletic group.
So, seed plant lineages are morphologically very diverse and each lineage is probably more closely related to some extinct group than to any extant lineage. This creates a situation where it is nearly impossible to reconstruct phylogenetic relationships and just about every possibility under the sun has been proposed – most of these have to do with the placement of Gnetales (Overheads showing – Anthophytes, Gnepines, Gnetifer, Gnetales-sister, etc…)
Bottom line: seed plant relationships are still a fundamental question in plant evolutionary biology! Understanding these relationships will help us understand evolution of morphological and anatomical traits, interpretation of fossils etc.
WEB READINGS for more details about each of the extant gymnospermous seed plant lineages