Diversification of angiosperms

What are angiosperms?

The flowering plants

What is the sister group to the rest of the living angiosperms?

Amborella trichopoda

What is a clade?

A group of organisms that share a common ancestor and all of its descendants.

What are the clades within an angiosperm phylogenetic tree?

Monocots and eudicots

How did angiosperms evolve?

They were slow to appear but then diversified very quickly.

What did the first flower (probably) look like?

• Woody or shrubby habit, but with no xylem vessels

• Flowers with no separate sepals and petals (with an undifferentiated perianth)

• Anthers that made a lot of pollen

• Carpels that were not fused together

Why can’t we be certain about this^?

Flowers don’t fossilise well

What are flowers?

The reproductive shoot of a plant where both male and female gametes are produced.

What does a flower need to do to mix the genes in these gametes up effectively?

Transfer its pollen to the carpel of a different flower.

How did flowers enable the angiosperms to radiate so quickly?

They can manipulate different animals to carry pollen and can result in reproduction isolation (which is the first step to speciation).

Why is it more reliable to use animals as pollinators (e.g. compared to wind)?

• More control

• Don’t need to make as much pollen

• Can have a smaller stigma tucked inside the flower

• More choice as to where the pollen goes

What is the benefits of having a smaller stigma tucked inside the flower?

It won’t pick up random pollen from other species, which can potentially clog up the stigma and the stop the pollen it actually wants from germinating.

What does the animal get out of pollinating the plant?

A reward

What can this reward be^?

• Nectar (most common)

• Pollen (as a food source for their larvae because it’s rich in amino acids)

What are pollination syndromes?

When flowers are partitioned into types according to the features they show that suit them for particular pollinators.

Adaptations shown by a plant that suit a particular group of animals, and by those animals that suit a particular flower morphology.

What adaptations can these plants and animals have?

Animals can have both behavioural or morphological adaptations, while plants can only have morphological adaptations.

What are some common insect pollinators?

• Beetles

• Flies

• Bees

• Butterflies

• Moths

What do we call the process of pollination by beetles?

Cantherophily

Why are beetles pollinators?

They acquired a habit of eating pollen from cycads and due to their mouth parts, that are positioned parallel to the axis of the body, they are limited in their ability to manipulate food sources. So, pollinated flowers provide a flat surface of extra pollen and nectar for them.

What do beetle-pollinated flowers usually look like and why?

Greenish/off-white with a strong fruity smell (e.g. roses) because beetles do not have good colour vision and rely heavily on smell.

What do we call the process of pollination by flies?

Myophily

What is true about fly pollinators?

They show the greates variation in methods and habits of pollination of any group of insects.

Why can plants that flower in adverse conditions / odd times of the year rely on flies to pollinate?

Because flies are not strictly periodic

How much nectar do myophilic flowers need and why?

Not much because the flies’ offspring do not require much food.

What do fly-pollinated flowers usually look like and why?

Palely coloured flowers without much of a scent (e.g. daisies) because flies are more visual insects than beetles and have a positive preference for pale and yellow colours.

What do we call the process of pollination by bees?

Melittophily

How do bees acquire the nectar?

They have the ability to perceive depth and long tongues, which is useful because the nectar is kept at the bottom of the deep corolla tube.

What other adaptation do melittophilous flowers have?

They are quite big and have a landing platform for the bees.

What do bee-pollinated flowers usually look like and why?

Brightly coloured (e.g. snapdragons) because bees can see in ultra violet, blue, and yellow, but not well in red.

What do we call the process of pollination by butterflies?

Psychophily

What are the butterfly and psychophilous flower adaptations?

• The flowers have flat surfaces

• The butterflies have long tongues to gather the nectar that is held in deep, enclosed tubes

What do butterfly-pollinated flowers usually look like and why?

They are brightly coloured because butterflies have good colour vision and can see red.

What do we call the process of pollination by moths?

Phalaenophily

What are the adaptations of phalaenophilous flowers?

• Nectar in long tubes

• Higher quantity of nectar is produced (because moths have higher energy requirements - hovering around the flower rather than landing on it)

What do moth-pollinated flowers usually look like and why?

They are white, which stands out well in the night and have a strong scent (e.g. gardenia).

What do we call the process of pollination by birds?

Ornithophily

What is the most common bird that pollinates?

The hummingbird (not all birds do pollinate)

What are the adaptations of ornithophilous flowers?

• They are brush or tube shaped and they secrete nectar is short spurs

• They are tough because the beak is stronger than a tongue

• They secrete a lot of nectar because birds need a lot of energy

What do bird-pollinated flowers usually look like and why?

Red (e.g. hibiscus) because birds can see red well and it means that no bees are around.

What do we call the process of pollination by bats?

Chiropterophily

What are the adaptations of bats and chiropterophilous flowers?

• Bats have long tongues and narrow snouts

• The flowers produce the most nectar

• The flowers are large, with narrow tubes or bowls from which to lap nectar and they hang below the foliage (making it easier for the bats to access them at night)

What do bat-pollinated flowers usually look like and why?

White and cream with a very strong scent (e.g. cacti) because bats are colour-blind, so strong colours are not relevant.

What is deceit pollination?

Persuading pollinators to pollinate the flower without actually giving it a reward.

What is one way of doing this^?

By producing flowers that contain no reward but that look similar to ones that do.

What are the advantages and disadvantages to this method^?

Advantages: it saves energy

Disadvantage: it is only succesful when the mimicking flowers are at low frequencies in the community

What’s an example of a flower that produces mimicking flowers?

Orchids (of the genus Ophrys) produce flowers that mimic the female of a species of insect which releases instinctive behaviour in the animal, meaning that it cannot learn to avoid these flowers.

What is sapromyophily?

These flowers attract the sort of insects that feed or lay its eggs on rotting protein, and they do this by looking like something dead and smelling awful.

What is Goethe’s theory?

The foliar theory of flowers, which argues that all parts of a flower are simply modified leaves.

How can we define a flower?

The determinate bisexual reproductive structure of an angiosperm.

What does determinate mean?

The flower produces a fixed number of organs and then stops - it can’t keep growing indefinitely.

What is the classic genetic approach to a biological problem?

Look for mutants (an individual which has a problem with the function that is interested in).

How can we generate mutants?

By treating seed with X rays or certain chemicals

What are ‘model’ species?

Species that are easy to grow in a greenhouse, easy to cross-pollinate, and which have flowers that are easy to see and that grow quickly.

What are the two main species that have been used to study flower development?

The garden snapdragon and the thale cress

What is the main model of flower development?

The ABC model

How many whorls of organs does a flower have?

Four (sepals, petals, stamens, carpels)

How many genes are needed to produce these?

Three (A, B, C)

Describe the ABC model.

• In the presence of only A, sepals will develop

• In the presence of A and B, petals will develop

• In the presence of B and C, stamens will develop

• In the presence of only C, carpels will develop

What is the key point of this model^?

That none of the organs develop directly as a result of their position, but only as a result of the expression of the genes.

The limits of zones A and C are set by…

…mutual interaction - they repress each other (e.g. expression of A represses expression of C).

What does removing all genes result in?

Leaves

How did this model arise?

From mutagenesis programmes with Antirrhinum and Arabidopsis.

The A function mutant in Antirrhinum is called…

…ovulata

The A function mutant in Adabidopsis is called…

…apetala2 (ap2)

What does ap2 do?

Encodes a protein that regulates the transcription of other genes (a transcription factor).

What are the B function mutants in Antirrhinum?

deficiens (def) and globosa (glo)

What are the B function mutants in Arabidopsis?

apetala3 (ap3) and pistillata (pi)

What do all these four genes do^?

Encode MADS box transcription factors

What do all these four genes function as?

Heterodimers (e.g. two def genes will not homodimerise because only the heterodimer is functional).

Why are the mutants phenotypically the same?

Because absence of either gene means that the heterodimer can’t form.

What is a transcription factor?

A protein, encoded by a genes, whose job is simply to turn on or off the expression of other genes.

How do they work^?

The promoter is a region of DNA near the gene where RNA polymerase attaches to begin transcription (the process of making messenger RNA). Transcription factors have two key parts: a DNA-binding domain, which attaches to the promoter region of the A gene, and an activation domain, which helps recruit RNA polymerase to start transcribing the gene into mRNA. This process is crucial for turning the gene on or off, depending on the needs of the cell.

What is true because the DNA binding domains of transcription factors only come in certain shapes?

More than one transcription factor will have the same sort of DNA binding domain and we can out these transcription factors into families according to the structures of their DNA binding domains.

What is the MADS box family?

A large family of transcription factors

What is the C function mutant for Antirrhinum?

plena (ple)

What is the C function mutant for Arabidopsis?

agamous (ag)

What do both of these genes do^?

Encode MADS box transcription factors

What has no recognisable homologues of the ABC genes (although they do have MADS box genes)?

Ferns, mosses, liverworts

What does have homologues of B and C function genes?

Conifers and cycads (which are both gymnosperms).

What do gymnosperms do?

They don’t produce hermaphroditic flowers, but instead, two sorts of cones (male and female).

What is true about B function genes in gymnosperms?

Genes that encode B function MADS box transcription factors are present in the male cones only, suggesting that they have a role in male reproductive structure development (like they have a role in producing anthers in angiosperms).

What is true about C function genes in gymnosperms?

They are expressed in both cones (male and female), suggesting that they confer the reproductive indentity on an organ (whereas B genes determine which sort of reproductive structure you are).

What happens if gymnosperms express B and C?

They become a male reproductive structure

What happens if gymnosperms express only C?

They become a female reproductive structure

What don’t the gymnosperms have?

A function genes

What is the main difference between monocot and eudicot flowers?

Monocot flowers have two whorls of brightly coloured organs (tepals) around their reproductive structures, and no sepals.

In monocots, what do all the gene expressions result in?

• A and B give petals

• Tepals result from B function in whorl one

What is true about the BC part of the ABC model?

It is ancestral to all the seed plants (angiosperms and gymnosperms).

What is gene duplication?

The process by which a segment of DNA, including a gene or a group of genes, is copied, resulting in two or more identical or similar copies of that gene within the genome.

How can gene duplication occur?

• Unequal crossing over

• Transposition

• Polyploidy

What is unequal crossing over?

This occurs when chromosomes pair up at meiosis, and is the result of a chiasma forming imperfectly.

What will this lead to^?

A duplication of a stretch of DNA on one chromosome and the deletion of the stretch of DNA from the opposite chromosome (this is usually deleterious).

When is unequal crossing over most common?

In repetitive regions - so when it has happened once, it is more likely to happen again.

What are the outcomes of gene duplication?

• Pseudogenes

• Subfunctionalisation

• Neofunctionalisation

What is a pseudogene?

Once a gene has been duplicated, it may not fill any function, and then it is gradually degraded by mutations to a non-functional gene.

What is subfunctionalisation?

When the two duplicates split the roles of the original gene between them.

What is neofunctionalisation?

When the new copy of the gene is recruited to new roles, because the original copy is still there fulfilling the original function.

What is transposition?

Where a chunk of DNA is moved from one chromosome to another, usually because it is attached to a transposon.

What is a transposon?

It places a chunk of DNA into a new gene randomly (the disrupted gene).

What is the outcome of transposition?

The chromosome which has lost the stretch of DNA is detrimental, while the chromosome that has gained the stretch of DNA may be more succseful (the DNA can be recuited to new functions).