M4: Nina Wedell

What is reproduction?

passing genes to next generation

What is sex?

recombination

What are the two general modes of reproduction?

1. asexual reproduction

2. sexual reproduction

What is asexual reproduction?

creation of progeny without contribution from another individual

What is sexual reproduction?

requires gametes produced by two different individuals

True or false: sexual and asexual reproduction methods are mutually exclusive?

False! Some organisms can undergo both (fungi or aphids) depending on environmental circumstances!

Why asexuality?

• breed without a partner

• no need for meiosis (5-100x longer than mitosis)

• no STIs, no time wasted on courtship/conflict

• avoids males which cannot make more offspring

• preserves high fitness combinations of alleles, unlike meiosis

What is the ‘two-fold-cost’ of males?

asexual reproduction: need only one individual = one offspring

sexual + isogamy: need two individuals = one offspring

sexual + anisogamy: sperm contributes basically nothing

• need two individuals = one offspring with greater time

Sexual reproduction con?

lethal homozygosity:

• Assortment of chromosomes breaks up favourable dominant/recessive relationships

• Sexual reproduction exposes recessive alleles to selection, lowering average fitness of individuals in sexual populations

• Some offspring have low/zero fitness due to recessive mutations that their parents did not express

Recombination affects on reproduction modes?

• breaks up co-adapted alleles and favourable epistatic effects

• Recombination increases the variability in offspring fitness

  • Often, it reduces the average offspring fitness

How is the prevalence of sexual reproduction a paradox?

asexual reproduction is much more advantageous than sexual however… vast majority of multicellular species reproduce sexually

• therefore sexual reproduction must confer higher fitness than asexual reproduction

What assumptions are made about sexual/aseuxual reproduction that would give them equal fitness?

1. equal number of offspring

   • males provide care = more offspring survive

2. equal survival

   • environment changes, some mothers no longer fittest therefore death of offspring

Why sexual reproduction?

• recombination + crossing over = mechanism to edit mutations

• recombination allows increased variability in subsequent generations

• variability allows capacity to change = adaption to changing environments

How is sex linked to linkage disequilibrium and selection?

• Recombination (sex) brings favourable alleles together faster than mutation

• Recombination (sex) allows chromatids to lose disadvantageous alleles

Does recombination speed up or slow down adaptation?

sped up

• sex brings together mutations that originated in different individuals

How is sex linked to deleterious alleles?

sex can remove deleterious alleles eg. Muller’s Ratchet, deterministic mutation hypothesis

What is Muller’s Ratchet?

asexual populations suffer from Muller’s Ratchet while sexual populations do not

• asexual populations are stuck with whatever mutations they pick up

• sexual populations can recover mutation-free genotypes by recombination

  • multi-mutants can die, removing those mutations from the gene pool

True or false: Muller’s Ratchet explains why sexual reproduction is more common than asexual?

sort of - it is by far not the only reason

• Sexual lineages may get outcompeted by asexuals long before Muller’s ratchet etc. can kick in

What is the deterministic mutation hypothesis?

a way for sex to remove deleterious alleles

• sex concentrate deleterious mutations into certain individuals

• requires a high mutation rate

• mutations = negative effect on fitness

  • synergistic epistasis: mutation has disproportionately large effect

  • agnostic epistasis: mutation has disproportionately small effect

• enables sexual females to eliminate more deleterious mutations from their progeny than they could by asexual reproduction

• slow process

True or false: the deterministic mutation hypothesis explains why sexual reproduction is more common than asexual?

false - while beneficial it is too slow, not significant enough to explain alone

Why is sexual selection favoured?

• sex = genetic recombination

  • crossing over during meiosis

  • mixing of genes from 2 parents

• sex ‘reshuffles’ genes to create new multilocus genotypes in every generation

• ‘red queen’ hypothesis

  • has been proven in experiments like snails

What is the tangled bank hypothesis?

Charles Darwin: A diverse set of siblings may be more fit than a clone, because each sibling uses a slightly different nice = reduce competition

What is the Red Queen Hypothesis?

theory which explains importance of being able to change with your environment hence why sexual reproduction is favoured

• host-parasite coevolutionary ‘arms race’ - both sides must constantly evolve just to maintain the status quo

• evolution by the parasite represents a changing environment for the host - sexual reproduction allows the host to produce offspring resistant to parasite via. recombination

Will the co-evolutionary arms race end?

No - we evolve and evolve but don’t really get anywhere

• parasites/host constantly evolve to infect and resist one another

• alleles that used to confer low fitness can become beneficial again

Link between Red Queen and sex?

• cycles make sex advantageous

• asexual individual might lose all its descendants to a well-adapted parasite

• sex creates and preserves genetic diversity, giving large eukaryotes a fighting chance at out-evolving fast- growing things like bacteria

Advantages to sexual reproduction?

• Recombination allows increased variability in subsequent generations

• Variability allows capacity to change = adaptation to changing environments

• Recombination and crossing over = mechanism to edit mutations

  • remove deleterious alleles

• And brings together good mutations

Disadvantageous of sexual reproduction?

• Cost of sharing your genes in offspring - would it not be better to have all of your genes in each offspring?

• Cost of meiosis = breaking up of good gene combinations

  • especially in stable environments

• Cost of producing males

• Cost of courtship and mating - not needed in asexual reproduction

• Increased predation risk

• Disease: STIs

When is asexual favoured over sexual reproduction?

stable environments - both have advantages/disadvantages just depends on environment

Asexuality and sexuality summary:

• Asexuality has 2-fold female fitness advantage, and no search cost or STI’s

• Sex is advantageous because it eliminates bad mutations and brings together good mutations, and generate genetic variationc

Who invests more in reproduction - males or females?

Females!

• males need to reproduce many times to produce maximum offspring - females only once

• females more likely to exhibit parental care

  • offspring carry their genes

  • less to gain by further copulations

MALE BIASED OPERATIONAL SEX RATIO (males mate more)

Benefits/costs of parental care?

• increase offspring survival

• decrease ability to generate further offspring

What is sexual selection?

non-random variance in reproductive success

Forms of sexual selseciton?

1. INTRASEXUAL selection:

   • direct competition for mates between members of the same sex, usually male-male competition

2. INTERSEXUAL selection

   • differences in attractiveness to the opposite sex, usually non-random mate choice by females

Intra-sexual selection:

aka same-sex competition for access to mates

• weaponry

• large body size

• sensory/locomotory apparatus

• dominance hierachies

Inter-sexual selection:

aka mate choice based on individual’s perceived advantage

• direct benefits eg. the best father, mating gifts, high fecundity, good genes

• indirect benefits eg.

How does investment differ by sex?

difference in gamete size/parental investments/resources donated directly to mates affect investment

high investment/donations = low potential reproductive rate = low sexual activity = biased operational sex ration = selection among mates = the better the mate’s quality, the higher the individual’s fitness

low investment/donations = high potential reproductive rate = high sexual activity = biased operational sex ration = competition for mates = the more mates, the higher the individual’s fitness

What is sexual dimorphism?

difference between males and females in a species due to asymmetric nature of sexual selection, directly related to male-male competition and/or female choice

eg. decorated peacock vs. brown peahen

• ornaments can be costly to maintain → direct survival cost

sexual selection pros and cons:

pros:

cons:

• sexual selection has the capacity to evolve maladaptive traits ie. costly ornaments

What is Fisher’s runaway model?

tail length increases due to survival advantage → females notice and start to sexually select for it → longer and longer tails become costly to maintain and are naturally selected against

What is a pre-existing sensory bias?

females associate a benefit in the environment with male phenotype

eg. guppies eat orange fruit (high in carotenoids) → males accumulate and have colourful patterns → female bias towards colourful males

• is this due to pre-existing sensory bias for detecting orange fruit?

SENSORY EXPLOITATION:

males use courtship signals exploiting pre-existing sensory bias in female

What is the chase-away selection theory?

there is a pre-existing bias in females but males do not have the trait yet → mutation produces rudimentary trait in males → females mate more, driving down female fitness → female mating threshold increases → male attraction declines → exaggeration of male display → female fitness declines and so on

Does intra-sexual selection stop after mating?

NO! male-male competition can continue after insemination

• sperm competition when multiple ejaculation occurs

What is the ‘Lottery Principle’?

In many instances sperm competition is analogous to a raffle or lottery. Sperm equal the tickets, fertilisations the prize.

• As in a raffle, the more tickets (sperm), the greater the probability of securing the prize (fertilisation).

• Hence sperm are produced in large numbers

Why do we have two sexes?

possibly sperm competition?

• Fixed amount of resources + zygote fitness size dependent (larger zygote = higher probability of survival)

  • Large gametes = high zygote fitness

  • Small gametes = numerous

• Size dimorphic gametes (anisogamy)

• Conflict over size of fusion partners

How does sperm competition relate to risk and intensity?

• as risk increases, males invest in more sperm

• Within species variation in the NUMBER of competitors = intensity

• Reduce ejaculate expenditure ≥2 competitors due to diminishing returns

Is sperm costly? How?

Yes!

• Energetically costly

• Can reduce male lifespan

• Food shortage reduce sperm number

• Accumulate nutrients

Why tailor ejaculation?

• sperm are a limited commodity

• want to maximise fertilisation returns

  • males can detect already mated females, female quality and rival male presence

0-1 sperm competitors → increase sperm numbers

>2 sperm competitors → decrease sperm ejaculate

• high intensity of sperm competition

can sperm competition promote bigger sperm?

YES! (how wild)

How do females select for sperm?

• Selective sperm ejection

• Selective sperm use

• Selective egg hatching

• Selective re-mating

Why should selection be stronger on males than on females?

ie. Why is the variance in fitness of males LARGER than the variance in fitness of females?

• because of the variation in mate numbers between males = Greater selection on males generates differences between the sexes

How is female reproductive fitness determined?

Numbers of Eggs and Numbers of Clutches of Eggs that a female lays

How is male reproductive fitness determined?

Numbers of Mates a male has

variance in fitness = ???

Variance in fitness = strength of selection

True of False: ave male fitness > ave female fitness?

FALSE!!

• Average male fitness must be equal to average female fitness

True of False: variation in male fitness = variation in female fitness?

FALSE!!

• variation in male fitness >> variation in female fitness

• Variance in fitness differ between the sexes generating sex differences

What are the costs of sex maintenance/paradox?

• Sex makes some loci homozygous, which can be fatal

• Recombination breaks up co-adapted alleles

• Cost of courtship and mating

• Two-fold cost of males

What are the benefits of sex?

• Increase genetic variability (better for changing environments)

  • Red queen hypothesis

• Can edit out deleterious alleles/mutations

  • Deterministic mutation hypothesis

  • Homozygous lethality

• Speed up adaption/evolution

  • Recombination increases efficacy of natural selection

• Lower extinction rates

What is the link between mutation and evolution?

If mutations are common, sexual populations evolve faster

If mutations are rare, sexual and asexuals evolve at the same rate

What is group selection?

the theory that natural selection acts on a whole group rather than individuals (individual selection)

• animal behaviours like sacrificing for another kin

What are the theories for origins of sex?

1. Stress

2. RNA viruses

3. Parasitic DNA

4. Cannibalism

Explain Stress as an origin of sex

• Primitive form of sex: an organism with damaged DNA replicating an undamaged strand from a similar organism to repair itself

• Suggest sex is an adaptation for dealing with stress - particularly if causing DNA damage

• Perhaps environmental stresses leading to DNA damage were common to survival of early microorganisms - suggest ongoing selection during the prokaryote to eukaryote transition

• Bacterial transformation or archaeal DNA transfer gave rise to sexual reproduction in eukaryotes

Explain RNA viruses as an origin of sex

• Origin of sex in the RNA world based on sexual interactions that occur in extant single-stranded segmented RNA viruses (e.g. influenza virus), and in extant double-stranded segmented RNA viruses (e.g. reovirus)

• Exposure to conditions that cause RNA damage could have led to absence of replication and death of these early RNA life forms

• Sex allow re-assortment between two individuals with damaged RNA, resulting in undamaged combinations of RNA segments coming together = survival

Explain Parasitic DNA as an origin of sex

sex originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) promoting their transmission and propagation

Explain Cannibalism as an origin of sex

• Sex evolved as a form of predation

• One primitive organism ate another one, but instead of completely digesting it, some of the eaten organism's DNA was incorporated into the DNA of the eater

What is the origin of sexual reporduction?

• can be traced to early prokaryotes, around two billion years ago (Gya), when bacteria began exchanging genes

• Although these processes are distinct from true sexual reproduction (recombination), they share some basic similarities and result in gene exchange

• Question pondered on by Aristotle

What are matign systems about?

maximising the survivorship of offspring

• if young require care, and a singly parent cannot provide for the brood/clutch then monogamy or cooperative breeding

What is the ecology of sexual selection?

1. Males compete with one another for access to females

2. Like competition for scarce resources, male reproduction is limited by the spatial and temporal availability of receptive females

3. The intensity of sexual selection depends on the rarity of receptive females in relation to the abundance of competing males

4. Sexual selection favours male attributes that permit their bearers to find and monopolise their mates

5. Ecological constraints on male monopolisation attempts leads to a species-specific pattern of male-female association, called a ʻmating systemʼ

What is a mating system?

environment affects male-female association

What are the determinants of spatial and temporal distribution of receptive females?

• Abiotic factors: weather, latitude, seasonality

• Habitat and food availability

• Oviposition (where to lay eggs) sites

• Species life history

• Genetic quality

• Other males and females

What affects the opportunity of sexual selection>

• spatial and temporal distribution of receptive females

• Who controls reproduction? Males/females?

• Spatial and temporal distribution of receptive females

• Can males monopolise females and/or resources?

• Operational sex ratio

What key decisions are mainly controlled by females?

• Egg investment: what material and how much

• Mate choice: which males will donate sperm

• Fertilisation: which sperm to use for each egg

• Offspring investment: how much maintenance and care to each embryo and offspring

How do males influence female decisions?

• Resources transferred to females: may influence egg investment, mate choice, or fertilisation

• Elaborate courtship: may influence mate choice and fertilisation

• Sexual coercion: may overcome female preferences

• Infanticide: may overcome female decisions about offspring investment

Explain this graph

How are mating systems classified?

• Based on number of mating partners and the number of reproductive events

• Influences the variance in mating success

• Impacts on the opportunity for sexual selection (Imates ) = variance in fitness of each sex/mean fitness for each sex

What is the equation for spatial distribution?

What is another way to say spatial distibution?

sexual selection

What affects the intensity of sexual selection?

Female distribution affects the intensity of sexual selection

• Spatial distribution affected by habitat (e.g. host plants) and population density

• Can males monopolize resources?

What is the effect of sex ration?

• Influence the number of receptive females on a patch

• Hence directly influence the spatial distribution of females

• Stronger sexual selection when males are abundant

How are m* and I_mates related?

relationship is proportional

• the more clumped females are the fewer males can mate

should spatial and temporal distribution of females on male mating variance be examined separately or simultaneously?

SIMULTANEOUSLY

How do we reproduce sexually?

1. semelparity (once)

2. iteroparity (many)

Interoparity

• Reproduce >once in successive years or breeding seasons

• Variation in number of clutches and number of offspring per clutch

Monomogamy

• male and female pair mate only with one another during a single breeding season

• when both sexes are needed for a successful reproduction

• could also evolve as a paternity assurance for males = mate

  guarding

• Males often help in raising young

Polyandry, polygyny and promiscuity

Polyandry = females mate with >1 male during a breeding season

Polygyny = males mate with > 1 female during a breeding season

Promiscuity = both males and females mate with many partners during a breeding season

Effect on monandy and polyandry and iterparity on I_mates

MONANDRY: female mates once + produces only one clutch of offspring

• she awards her entire reproductive output to a single male

POLYANDRY: female makes more than once + partitions her clutch into sub-clutches

• each mating male sires only a fraction of the offspring

• variance in reproductive success in males goes down

ITERPARITY: females produces more than one clutch +

• variance in offspring numbers can be partitioned into within- and among- female components

• Multiple reproductive episodes by females erode I_mates

• As clutch number increases, I_mates becomes a smaller fraction of the total variance in offspring number

When is I_mates eroded most and least?

most: polyandrous, iteroparous

least: monandrous, semelparous

How is protandy a male strategy?

• Related to temporal distribution of females

• Males emerge before females

• Maximise no. of mating with virgin females

• Increased fertilisation returns

What are leks?

• Courtship arenas where males display

• Often involve intense male-male competition leading to a dominance hierarchy

• Dominance reflects male fitness and assessed by females

What is cooperative breeding and parental care

• >2 individuals care for the brood

• Variation in reproductive skew

• Habitat saturation?

• Increase survival of offspring?

• Low mortality and small clutch size

• Constant environments

What is a sex role reversal?

• males provide nutrients

• pollen availability varies

• female nutritional need

• male donations > female investment

• result in sex role reversal

How do hemaphrodites affect mating systems?

• The condition of having both male and female reproductive structures

• Simultaneous hermaphrodites

• Sequential hermaphrodites; males first = protandry, female first = protogyny

  • choose which sex is more beneficial

• low population density - finding a mate is hard

What are alternative mating strategies?

• Not all males get to mate

• Do the best of a bad job?

• Means more males get to mate

• Success is directly dependent on the strategy played by competitor

• Game theory analyses

alternative mating strategy: male polymorphism

• alpha, beta, gamma male morphs

• males control resources

  • alpha fight other alpha males for female access

  • beta males mimic females

  • gamma males are tiny and hide among females

alternative mating strategy: unequal fitness

• large males: guard dead insects

• medium males: produce salivary gifts to attract females

• small males: force copulations on females

describe three male strategies

Isopods:

• equal fitness

• frequency dependent equilibrium

Lizards:

• not equal fitness; cyclical dynamics

• blue beats orange, yellow beats blue, orange beats yellow

Scorpion flies:

• not equal fitness; ‘best of a bad job’

• large male: guard insect, medium male: salivary gift, small male: forced copulation

What are mating systems determined by?

• environment and species’ life history

What affects the spatial (m*) and temporal (t*) distribution of receptive females?

the environment

What dictates opportunity for sexual selection?

spatial (m*) and temporal (t*) distribution of receptive female

Alternative mating tactics =

more males get to mate. The variance in male mating success, and therefore the opportunity for sexual selection decreases