Lecture 19 - Evolution of sex Flashcards
What is sexual reproduction?
The union of two genomes, usually carried out by gametes
Anisogamy
Distinct male/female functions
What percentage of eukaryotic organisms are sexually reproducing?
99.9%
Sex of dioecious species
Seperate males and females
Sex of hermaphroditic species
Individuals produce both kinds of gametes
Examples of Dioecious species?
Willows and Badgers
Examples of Hermaphroditic species
Roses and earthworms
Parthenogenesis
Type of asexual reproduction
Development of a new individual from an unfertilised egg (apoxmixis)
Essentially a clone of the mother
Examples of parthenogenesis
Komodo dragon, aphids and Daphnia
Gynogenesis
Type of asexual reproduction
Sperm-dependent parthenogenesis
Sperm triggers it but without contributing genetically
Adventitious embyrony
Type of asexual reproduction
Development of a new individual from a somatic cell
Occurs in plants
Vegetative reproduction
Type of asexual reproduction
Development of new individual from a group of somatic cells
In plants
Example of vegetative reproduction
rhizomes of ginger
What is the use of sex?
Not for reproduction as can reproduce asexually
Sex is therefore for recombination
When did recombination first occur?
3 billion years ago as a DNA repair mechanism
When did sex first evolve?
1-2 billion years ago
What are the two parts to the evolution of sex?
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
What is the cost of males?
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
Costs of sex
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
Costs of recombination
Breaks up favourable combination of alleles
Example of recombination breaking up favorable characteristics?
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
What is the distribution of parthenogenesis?
Widely distributed but generally short-lived
What are the two possible long-term advantages to sex?
Sex accelerates adaptive evolution
Sex prevents the accumulation of deleterious mutations
How does sex accelerate evolution?
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
Mullers ratchet
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
Synergistic selection
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
What is the probelm with mullers ratchet?
Not as effective in larger populations
What is the problem with the long term advantages of sex?
They all involve group selection
For sex to persist there must be a short-term advantage
Short -term advantage of sex?
Sex generates a greater number of different genotypes, adapted to a wider range of conditions
What organisms can be studied to find out the short term benefits of sex?
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
Frozen niche hypothesis
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
Sib-competition
Asexual organisms occupy the same niches so compete with themselves
Red queen hypothesis for the evolution of sex
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
Evidence of the red queen hypothesis?
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
