Lectures 16-24 Flashcards
What are the chlorophyta?
the green algae
- start of terrestrial green line
Why do we know land plants are monophyletic?
genetic sequencing
- evolved from within a particular group of freshwater green algae = CHAROPHYTA
What charophyta is thought to be the most closely related to land plants?
Zygnema
What is gametic meiosis?
gametes produced directly by meiosis and fuse immediately without mitosis
- most lifecycle = diploid (animals)
What is zygotic meiosis?
when meiosis occure immediately after diploid zygote formation
- lifecycle mainly in haploid form
- fungi
What is sporic meiosis?
mitosis occurs in both haploid and diploid phase and so 2 multicellular phases - all land plants
= alternation of generations
What is the haploid multicellular phase of plants life cycle called?
gametophyte
What is the diploid multicellular phase of plant lifecycle called?
sporophyte
Stages of plant life cycle?
x 6
1) Diploid multicellular stage
2) Meiosis to produce haploid spores
3) Mitosis of spores to haploid multicellular
4) Produce gametes by mitosis
5) Fertilisation of gametes = diploid
6) Mitosis to produce multicellular
What is a clade?
Monophyletic group
- group of organisms containing all decendants of an ancestor
What is a grade?
a group of organisms that share similarities but are not monophyletic
What are the bryophytes?
Liverworts, hornworts and mosses - GRADE
- 470 millions years ago to present
- most closely related to charophyte algae
- haploid dominant
- small diploid sporophyte is parasitic to haploid
What are the liverworts?
bryophytes which are most likely the earliest evolving land plant group
- more or less amphibious thin plants
- patches of cuticle and air pores
- film of water over surface for reproduction as sperm need to swim
What are hornworts?
bryophytes
- resemble liverworts but have a full water-repellent surface cuticle
- has 2 celled stomata in diploid
- very few species
What are mosses?
Bryophytes
- fundamental to worlds carbon balance
- have rudimentary cellular water transport systems —> not xylem
- ~9,000 species
- green spongy = haploid
- extensions are diploid
- need water on surface to reproduce as sperm swim
What are the vascular plants?
clade
- 420 million years ago to present
- all but bryophytes
- vertical water transport by xylem and also tough, rigid body
- diploid sporophytes mainly
- spore-producing bodies held high for long spore distribution
split
1) Non-seed producers = spore producers
2) seed plants
What are the two main groups of living spore-producers?
1) lycophytes
2) ferns and horsetails
- both still rely on water for reproduction
What are lycophytes?
spore producers
- called clubmosses but not mosses
- few species remain
- first plants to have leaves with vascular supply
- thickened stems which gave first trees (though not small)
- phloem
What are horsetails?
- ring of vascular tissue around hollow stem
- strength
- small
What are ferns?
spore producers
- large megaphylls (leaves) which produce spores
- still water-dependent
What are the seed plants?
1) gymnosperms
2) flowering plants = angiosperms
- when gametophyte generation, egg and then next sporophyte generation within sporophyte tissue
- when world dried out = dominant
What are the three types of gymnosperms?
1) Cycads
2) Ginkgo
3) Conifers
What are cycads?
Gymnosperms
- abundant 260-180 mya
- single unbranched stems with mass of tough, highly divided leaves
What is the maidenhair tree? Ginkgo?
only remaining ginkgo species left
- gymnosperm
- changed very little in past 150 mya
What are conifers?
- gymnosperms
- developed novel method of tree growth = expanding ring of mitotically active tissue around tree = grow out as well as up to support
- oldest and largest organisms
- mainly needle-bearers
- 700 species
What are angiosperms?
flowering plants
- only 130 mya
- 250,000-400,000 species
unique
1) showy/often coloured flowers
2) ovule completely enclosed
3) have double fertilisation
What do plants have to cope with on land?
x 6
- Desiccating environments
- Gas exchange requirement changes (aerial environment and due to waterproofing)
- Mechanical support - not water buoyancy
- Transport water, nutrients and photosynthetic products around plant
- Changed nutrient uptake requirements
- Different reproduction conditions
2 benefits of transition to land
- light availability
2. Increased CO2 availability
How do liverworts deal with desiccation and gas exchange?
hug moist ground
- partial cuticle
- suspend metabolism, growth and reproduction when dry out
- rapid recovery when water available
- open air pores for gas exchnage
How do mosses cope with gas exchange and lack of water?
- suspend metabolism if dry out
- cuticle
- doughnut-shaped guard cells
How do most plants deal with desiccation and gas exchange?
- waterproof membrane or cuticle
- cuticle of cutin = hydrophobic polymer
- often uses waxes as well
- barrier to fungal invasion
- in vascular plants stomata have pair of guard cells - all stomata evolved from one (evolved once)
- inside plants open pathways for large surface area for gas exchange
Why do land plants need transport systems?
- water = roots to leaves
2. photosynthetic nutrients = to roots
What carries water in plants and how are they formed?
Xylem
- rings of lignin for support = pre-adaptation as in algae
- cellulose strengthened
- lignin water-resistant and antibiotic
- cells cannot survive being wrapped in lignin - so apoptosis - leaves hollow tubes
- water movement driven by water evaporation from stomata
- lignin provides compression resistance but also lateral flexibility
Transport mechanism for photosynthates?
Phloem
- sucrose
- must be alive so no lignin
What did the first land plants have instead of roots?
Rhizoids
- hair like projections, helped anchor to ground
Why did roots evolve?
- stabilisation
2. Mineral and water uptake
What are mycorrhiza?
- symbiosis between fungi and plants
- in 85% of flowering plants
- 100% gymnosperms
- in fossils from Devonian
What is the structure of VAM - mycorrhiza?
VAM = Vesicular Arbuscular Mycorrhiza
- fungal spores attracted to roots by chemical signals
- hyphae form appressorium on root surface
- force between epidermal cells and break into outer, cortical cells through cellulose
- inside space between membrane and wall = arbuscule - little tree
- big contact zone
What do plants gain from mycorrhiza?
- mobilisation of phosphate
- Plant growth hormones made in fungi
- protection from pathogens
- Maintenance for acid/base balance
What do fungi gain in VAM?
Sugars from plant
What are embryophytes?
All living plants
Where are the eggs and embryos attached to the gametophyte?
Ina archegonium
Examples of physical plant defences?
x 4
- Hairs
- Spines
- Glandular trichomes
- Stinging hairs
How do hairs defend plants?
Hairs = TRICHOMES = unicellular or multicellular outgrowths of the epidermis
- still air over surfaces cutting water loss
- dense hairs are distasteful to mammalian grazers (sharp fragments can penetrate soft palate = irritation or infection)
- Insects cant get to flesh
- Or hooks to catch and trap so cant move and then starve
How can spines protect plants?
- effective against large grazers and browsers
- develop by modification of a lot of different organs
What are glandular trichomes?
Secrete sticky substances to impede animal movement
- some carnivorous genera secrete proteases to digest the trapped insects
How can stinging hair protect plants?
- nettles
- fragile trichomes that break when touched
- embed into skin
- histamine injected cause immediate itching
- many other chemicals give long-lasting burn
How can insects remove trichomes?
bite them off
- some pin them back
What are the two classes of toxins?
nitrogenous and non-nitrogenous
What are the 4 nitrogen based toxins?
- Amino acid analogues
- Cyanogenic glycosides
- Alkaloids
- Proteins
Amino acid analogue toxins?
- eaten then incorporated into animal proteins
- dysfunctional proteins
EXAMPLE
- β-cyanoalanine in Vicia can cause convulsions and death in mammals
- 200mg/kg
Cyanide containing plants?
- release HCN when damaged
- terminal respiration inhibitor = rapid death
- inactive cyanogenic glycoside (usually in vacuole) to enzymes in apoplast - react - HCN
Alkaloids in plants?
- found in roughly 20% flowering plants families
- have nitrogen containing rings
- Hemlock
- Strychnine = common rat poison
Protein toxin in plants
- ricin
- breakdown ribosomes - stop protein synthesis
- slow organ degradation and die
Non-nitrogenous toxins
- St Johns Wort
- hypericin - photosensitivity
- rotenone
Tannins?
- feeding deterrent (something that tastes bad)
- Others include Phorbol esters in bark of paper birch
- only height of grazing arctic hares
Plants that coagulate attackers?
- under epidermis
- cells containing latex
- coagulates by polymerisation
- stick mouth parts together or insect to plant - starve
- also can have poisonous compounds
Hormone defences of plants
Ecdysone
- metamorphosis of insects
- yew
- cause premature moulting and so die as exoskeleton splits
Monarch Butterflies - Ouabain
- milkweed produces ouabain = alkaloid
- Monarch butterfly caterpillars insensitive and sequester it in body
- toxic to birds
- bright colouration
- though some learnt to pick away body parts rich in glycosides
What does the monocot clade contain?
wheat and maize
- 22% angiosperms
Eudicot clade?
75% angiosperms
- split into rosids and asterids
Why are the angiosperms so diverse?
- many differences between different clades
- huge number of species (250,000-400,000)
- diversified quickly within 130 million years
What did the first angiosperm look like?
- woody or shrubby
- didnt have proper xylem vessels
- flowers probably didnt have separate sepals and petals and instead undifferentiated perianth
Why did flowers enable angiosperms to radiate so dramatically
reproductive organ
- once mastered the ability to manipulate somebody, you can radiate into as many different sorts as there are different somebodies to manipulate
Why use animals as pollinators?
- more precise and efficient
- dont need huge feathery stigmas
- stop picking pollen from species that arent your own
What do animals get from pollinating?
reward = nectar
- some feed larvae with pollen as rich in amino acids- so some give excess pollen
What is cantherophily?
beetle pollination
- extra pollen
- nectar in a flat surface
- no good colour vision and rely on scent
- white and strong smell
What is myophily?
fly pollinators
- not periodic so can pollinate during winter
- dont feed offspring so need less pollen and only a small amount of nectar in flowers
- pale and yellow flowers
What is melittophily?
Bee pollination
- carry pollen to feed larvae
- perceive depth and long tongues
- deep corolla tube nectar at base
- big to bear weight
- deposit pollen on back
- blue yellow and ultraviolet but not red
- bright colours and nectar guides
- spots of stripes
What is psychophily?
Butterfly pollination
- can see red so bright coloured flowers
- long tongues so nectar in tubes relatively deep
- have to land on flowers
What is phalaenophily?
Moth pollination
- white
- at night
- hovers alongside the flower
- which is bilaterally symmetrical with rims bent backwards
- long nectar tubes
- more nectar as energy expensive
- strong scent
What is ornithophily?
Bird pollination
- Hummingbirds - nectar with long tongue
- or some birds perch
- mainly red - probably so bees dont take nectar as birds dont show preference
What is chiropterophily?
Bat pollination
- at night
- usually white or cream
- strong scent as bats colour blind
- many anthers
- usually hung below foliage
What is deceit pollinations?
give no reward - look like other flowers - saves energy - but frequency limited some mimic female of species - visually or by scent - so mate and deposit on back Some look like rotting meat carrion and dung beetles - looks like or smells dreadful
What is the definition of a flower?
The determinate bisexual reproductive structure of an angiosperm
- just modified leaves
- fixed number of organs
- cant grow indefinitely
Sterile perianth surrounding reproductive organs
Why arent the reproductive organs of gymnosperms flowers?
- not determinate
- not bisexual (male and female pine cone)
- not surrounded by sterile perianth
- Not an angiosperm
How to induce mutants in plants and why
Study what the genes do by seeing how the plant changes with certain mutations
- x rays
- chemicals
What is the model plant?
- why?
Arabidopsis thaliana
- easy to grow
- quick germination and growth
- small
- clear flowers
What is the ABC model of flower development?
4 whorls of organs
- Sepals = A expressed
- Petals = AB
- Stamens = BC
- Carpels = C
- A and C cant coexist (so overlap) as repress each other
- if A lost then C all the way across so goes C, CB, BC, C (Carpels, stamen, stamen, carpel)
- all gone then just leaves
What is the function of the A gene?
AP2 = transcription factor
What is B function in flowers?
2 function mutations
- heterodimers
- MADS box transcription factors
What is C gene function in flowers?
MADS box transcription factors
- ag in arabidopsis
When does the ABC model become present in plant lineage?
no recognisable homologues of ABC genes
- B and C in gymnosperms but not A (no evidence for) - conifer and cycads
What is the function of B and C genes in gymnosperms?
No hermaphroditic flowers
- male and female cones
B and C = conifers male cone
C = female cone
—>so BC is male sex organ like flowers
C from cycads can compliment (vaguely repair) arabidopsis C mutant
What is the ABC model in monocots?
like tulips and lilies
- 2 whorls of tepals rather than petals and sepals
- so B gives tepals (unsure of A presence)
- BC = stamen
- C = carpels
What is a transcription factor?
a protein which controls gene expression
- a gene has a promoter region which RNA polymerase binds to
- transcription factors allow this to happen
- have binding domain (to promoter) and activation domain
- the shape can put it into families like MADS box
Domestication of maize
Maize = tall, unbranched
- lots of ears of corn = from female inflorescence
Teosinte is wild variety maize is from
- change in one gene
What is the gene that differentiates maize from teosinte
TEOSINTE BRANCHED
- encodes a transcription factor from TCP family
- normally repress branching, only switched off in shade so grow taller
MAIZE
= completely inactivates so never branched, even in sunny environment
- artificial selection as easier to harvest and more energy to female inflorescences
Why are angiosperm floral traits under strong selective pressure?
any mutation that makes a flower more attractive to pollinators than other flowers of other plants have strong selection
- converse is true
Delphinium and bees plus hummingbirds
Flowers usually deep blue with white nectar guides
- some white so cannot distinguish the nectar guides
24% fewer visits from pollinators than blue
- 1.2 to 1.4 times as long to fly between white flowers
- 1.1 to 1.5 times as long to extract nectar
So never gets to very high frequency
Petal cell shape controlled by MYB transcription factor
80% flowers have cone shaped petal epidermis cells
- bees strongly favour cone shaped petal cells
- most likely to grip flower
Two ways in which weeds can have herbicide resistance
- resistance through enhanced metabolism
- enhanced metabolism detoxifies herbicides before they act
- may not provide absolute resistance to herbicides - less susceptible - Target site resistance
- absolute resistance
- alter in a way that no longer effective
What problem do you have to worry about with genetically engineered crops?
Foreign genes into natural environment
- seed
- pollen
Does it have a selective advantage and how likely is this to happen
- maize cant grow without fertilisers so rarely find outside of supported fields
- also rarely crosses as few close relatives in UK
Why is it harder to define plants into species?
fewer barriers to gene flow
- no behaviour and little differences between reproductive organs are largely absent
What is the process of introgression?
- hybrids backcrossing into a parent population
- effectively very fast mutations as rapid exchange in alleles
Why do plants tolerate hybridisation better than animals?
Plant embryogenesis is simpler than animal
- few problem in development
Why aren’t all hybrids sterile?
- often problems during meiosis - irregular chromosome pairing
- most gametes are inviable
- BUT most long-lived and may vegetatively reproduce
- all sterile hybrids are fertile at some low level
- restitution events
What is a restitution event?
error in meiosis where all chromosomes into one daughter gamete
- roughly 1% occurrence
- get two diploid at end of meiosis
How are autopolyploids formed?
Diploid gamete from restitution
- fertilised by normal and triploid zygote formed
- doesn’t disturb development
- pollen and egg viability low as unbalanced chromosome combination usually lead to gametophyte death
- BUT RESTITUTION AGAIN
- then triploid gamete fertilised
- autotetraploid
How are allopolyploids formed?
Hybridisation
- restitution event
- self-fertilise
- one generation allotetraploid AABB
- much more likely
What is the genetic consequence of autopolyploids?
4 copies of each gene
- 2 alleles then 5 different genotypes
- tetrasomic inheritance
a lot more genetic variance
What is the genetic consequences of allopolyploid?
depend on evolutionary relationships of the two parental types
- between extremes
- complete recognition between closely related
- very different then no recognition (amphidiploidy)
- so 2 always inherited together
- permanent true breeding heterozygote
If not at extremes then mix of tetrasomic and diploid inheritance
Establishment of polyploids?
- hard in parent habitat
- due to infertility
- but edges of populations
- common in europe at retreat of glacier
Example of modern polyploidy speciation
20 times to give T.miscellus
- 12 to give T.mirus
America in last 70 years
- three european species Tragopogon
How does gene duplication occur in plants?
x 3
- unequal crossing over
- Transposition
- Polyploidy
How does unequal crossing over cause gene duplication?
Chiasma forms imperfectly
- duplication of part of DNA on one and the deletion on the other
- deletion usually deleterious so individual with this usually dies
- Duplication usually survives
- once happened then it is more likely to occur again as more likely in repetitive regions
How does transposition duplicate DNA?
DNA from one chromosome to another because it is attached to a transposon
- loss may end up in individual that then dies
- the one with the duplication more likely to survive
How does polyploidy lead to gene duplication?
doubling of DNA available
- enormous opportunity
Why is gene duplication useful for new function?
Harder to change necessary genes
- if duplicated then only one needed and other can be subject to change and new function
What is a pseudogene?
A non-functional sequence degraded by mutations
What is subfunctionalisation?
The DNA duplicates and the roles of each split between original single
- so both do parts of same function as whole one
What is neofunctionalisation?
New copy of gene recruited to new roles
- can be related to original function of completely different
Which genes have the most effect if replicated?
Transcription factors
- more effect on plant overall
How are transcription factors classified into families?
There are conserved regions which involve the binding to the promoter region
- conserved within families
- but actual role is almost completely unpredictable
What is the MYB transcription factor family?
Evolved through partial duplications
- existed before plants and animals diverged
- but main duplication in plants
- characterised by helix-turn-helix region at N terminus (52 aa long)
- many repeats of this, 1, 2 or 3
What is the function of 3 repeat MYBs
Very conserved function
- all involved in cell cycle
In animals associated with promotion of cellular proliferation (speed and timing of cell cycle)
Function of 1 repeat MYBs
Subgroup which is very conserved
- recognising and binding telomeres of chromosomes
- in plants, animals and fungo both structurally and functionally
Function of 2 repeat MYBs
Original R2R3 MYB duplicated several times
- more than 120 R2R3 in arabidopsis
- many different functions
How has the duplication of genes for enzymes of secondary metabolism been essential in angiosperm radiation?
Not essential for survival
- but allowed for chemical defences
- scent and colour
- defence, attraction and signalling
Why is gene duplication not necessary for new enzyme evolution?
Mutation at a single locus can be sustainable if it doesnt encode for survival
- as this is essentially the definition of secondary metabolism
