Lecture 19 - Evolution of sex

Question 1

What is sexual reproduction?

Answer 1

The union of two genomes, usually carried out by gametes

Question 2

Anisogamy

Answer 2

Distinct male/female functions

Question 3

What percentage of eukaryotic organisms are sexually reproducing?

Answer 3

99.9%

Question 4

Sex of dioecious species

Answer 4

Seperate males and females

Question 5

Sex of hermaphroditic species

Answer 5

Individuals produce both kinds of gametes

Question 6

Examples of Dioecious species?

Answer 6

Willows and Badgers

Question 7

Examples of Hermaphroditic species

Answer 7

Roses and earthworms

Question 8

Parthenogenesis

Answer 8

Type of asexual reproduction

Development of a new individual from an unfertilised egg (apoxmixis)

Essentially a clone of the mother

Question 9

Examples of parthenogenesis

Answer 9

Komodo dragon, aphids and Daphnia

Question 10

Gynogenesis

Answer 10

Type of asexual reproduction

Sperm-dependent parthenogenesis

Sperm triggers it but without contributing genetically

Question 11

Adventitious embyrony

Answer 11

Type of asexual reproduction

Development of a new individual from a somatic cell

Occurs in plants

Question 12

Vegetative reproduction

Answer 12

Type of asexual reproduction

Development of new individual from a group of somatic cells

In plants

Question 13

Example of vegetative reproduction

Answer 13

rhizomes of ginger

Question 14

What is the use of sex?

Answer 14

Not for reproduction as can reproduce asexually

Sex is therefore for recombination

Question 15

When did recombination first occur?

Answer 15

3 billion years ago as a DNA repair mechanism

Question 16

When did sex first evolve?

Answer 16

1-2 billion years ago

Question 17

What are the two parts to the evolution of sex?

Answer 17

Origen of sexual reproduction (cellular evolution) - the evolution of diploidy (that has the advantage of making deleterious mutations) is likely to have been part of the process

Evolution of and maintenance of sexual reproduction and recombination i.e. how does natural selection favour sex

Question 18

What is the cost of males?

Answer 18

Producing males is costly as does not directly contribute to population growth

Males cannot produce offspring, therefore need to produce two offspring to produce further offspring

Theoretically asexually reproducing organisms should outcompete sexually reproducing organisms as they grow exponentially as all offspring can produce their own offspring

Also in asexual reproduction all of the genes are your own genes whereas in sexual reproduction they are shared 50/50

Question 19

Costs of sex

Answer 19

Cost of finding a mate (locating, competing etc)

Risk of not finding a mate

Rise of disease transmission in mating

Increased predation risk

At the cellular level sexual cycle requires addition time and energy as meiotic cell division takes considerably longer than mitosis

Question 20

Costs of recombination

Answer 20

Breaks up favourable combination of alleles

Question 21

Example of recombination breaking up favorable characteristics?

Answer 21

Primrose (Primula vulgaris) heterostyly

Individuals with Ga genotype have lower fitness than GA or ga

This is due to the positioning of the style and stamen

Question 22

What is the distribution of parthenogenesis?

Answer 22

Widely distributed but generally short-lived

Question 23

What are the two possible long-term advantages to sex?

Answer 23

Sex accelerates adaptive evolution

Sex prevents the accumulation of deleterious mutations

Question 24

How does sex accelerate evolution?

Answer 24

Example

In a population in which AB is optimal but starts with ab

Sexual:

ab can mutate into aB and Ab and sexual reproduction can combine the two to form AB

Asexual

ab can mutate into aB and aB however they cannot combine so need to wait for another mutation to convert either of them into AB

Also in the meantime one of the alleles will outcompete the other

Question 25

Mullers ratchet

Answer 25

Sex prevents the accumulation of deleterious mutations

In a population with deleterious alleles spread across

If there is an event in which individuals with no deleterious mutants become eliminated then there is no way to get less than 1 deleterious mutations

However in sexual you can due to recombination

Question 26

Synergistic selection

Answer 26

Sex prevents the accumulation of deleterious mutants

Recombination could potentially reduce mutational load if deleterious mutation act synergistically

Recombination increases the variance in the distribution of mutations (deleterious and beneficial) so selection is more effective

Question 27

What is the probelm with mullers ratchet?

Answer 27

Not as effective in larger populations

Question 28

What is the problem with the long term advantages of sex?

Answer 28

They all involve group selection

For sex to persist there must be a short-term advantage

Question 29

Short -term advantage of sex?

Answer 29

Sex generates a greater number of different genotypes, adapted to a wider range of conditions

Question 30

What organisms can be studied to find out the short term benefits of sex?

Answer 30

Organisms that have sex occasionally such as strawberry plants

They reproduce asexually when conditions are good to make as many well adapted offspring as possible

When conditions are bad they have sex so they can adapt to the new environment

Question 31

Frozen niche hypothesis

Answer 31

Asexual fish, Poeciliopsis monacha- lucida exist in mexican stream as many different clones

The proportion of fish that are sexual decreases as the number of asexual clones increases

Since asexuals do better when they have more clonal diversity this demonstrates short-term advantage that sexuals usually have due to their diverse genotypes adapted to a wide range of conditions

Question 32

Sib-competition

Answer 32

Asexual organisms occupy the same niches so compete with themselves

Question 33

Red queen hypothesis for the evolution of sex

Answer 33

Coevolution in response to a biotic environment full of parasites

Incredibly strong selective force

Frequency-dependent selection will favour rare host genotyes

Sex produces more variable progeny so is more likely to produce rare genotypes that can escape infection

Question 34

Evidence of the red queen hypothesis?

Answer 34

Snail Potamopyrgus antipodarum

Lives in muds of new zealands lakes, rivers and estuaries

Can be infected with a worm parasite

Both sexual and parthenogenetic snails

HIgh clonal diversity of asexuals

Parthogenetic snails have not been able to outcompete sexual snails

Where there are more parasites in shallow areas there were more sexual snails and more clonal diveristy than in deeper areas with less parasites