BIOE 109 Summer 2009 Lecture 7- Part I Linkage disequilibrium and the evolution of sex.

BIOE 109Summer 2009

Lecture 7- Part ILinkage disequilibrium and the

evolution of sex

Linkage disequilibrium and the evolution of sex

Q: What distinguishes sexual from asexual reproduction?



A: Meiosis and Syngamy




Male Female

2N 2N




Male Female

2N 2N

Meiosis N N




Male Female

2N 2N

Meiosis N N

Syngamy

2N

A model for the evolution of two sexes


• in many species, sexual reproduction has entailed a shift from isogamy to anisogamy.



Isogamy

+ -



Isogamy

+ -Anisogamy

♂

♀


Mating type (M)

+/-


Mating Gamete type (M) size (G)

+/- Small (S)/Large (L)




Linkage disequilibrium

MG





MG

MG

+S

♂





MG

MG MG

- L+ S

♀♂

MG MG

- L+ S

♀♂

MG MG

- L+ S

♀♂

Recombinants:

MG MG

- L+ S

♀♂

MG

+ L

Recombinants:

fitness due to low sperm number

MG MG

- L+ S

♀♂

MG

+ L

MG

- S

Recombinants:

fitness due to low sperm number

fitness due to inviable eggs

What is linkage disequilibrium?


Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus.


Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus.

Linkage disequilibrium occurs when genotypes at the two loci are not independent of each other.

Q: What causes linkage disequilibrium?


1. Natural selection



• can be produced by epistatic selection




• epistasis occurs when the fitness of a genotype at one locus depends on its genotype at another locus




• epistasis occurs when the fitness of a genotype a one locus depends on its genotype at another locus

2. Random genetic drift

• much weaker than selection in creating disequilibrium.


3. Population admixture



• can be as important as selection in creating disequilibrium.




Q: What eliminates linkage disequilibrium?




Q: What eliminates linkage disequilibrium?

A: Recombination!

The decay of disequilibrium depends on the rate of recombination

How and why did sex evolve?


or… how is sexual reproduction maintained in the face of so many alternative strategies?



Some alternatives:

1. Parthenogenesis (both mitotic and sexual forms)



Some alternatives:


• organisms develop from unfertilized eggs.

Examples: lizards, aphids, many plants

New Mexico whiptail lizard (Cnemidophorus neomexicanus)

C. neomexicanus



Some alternatives:



Examples: aphids, many plants

2. Hermaphroditism (obligate or sequential)



Some alternatives:





• organisms possess both male and female reproductive organs, or change sex at some point in their lives.



Some alternatives:





• organisms possess both male and female reproductive organs, or change sex at some point in their lives.

Examples: many fishes, snails, worms



3. Haplodiploidy



3. Haplodiploidy

• haploid males develop from unfertilized eggs, diploid females from fertilized eggs.



3. Haplodiploidy


Examples: ants, bees, wasps



3. Haplodiploidy


Examples: ants, bees, wasps

4. Pseudogamy

• contact with sperm stimulates development from unfertilized eggs.

Example: some nematodes and freshwater fishes

Amazon molly (Poecilia formosa)

But… of the world’s 2 million named species less than 2,000 are totally asexual

But… of the world’s 2 million named species less than 2,000 are totally asexual

… and they don’t appear to persist very long

Asexual species are typically found at the tips of phylogenetic trees

S = sexual speciesA = asexual species

S A S S S A S A S S

The exception: bdelloid rotifers – no sex for 40 million years!

The “costs” of sex

1. The cost of producing males (or, the two-fold cost of sex).


1. The cost of producing males (the two-fold cost of sex).






2. The cost of finding mates



• exacerbated by low population density.




3. The costs of mating





• mating is a risky business!





• mating is a risky business!• also vulnerable to sexually transmitted diseases.






4. The cost of recombination






4. The cost of recombination

• recombination creates superb combinations of genes then quickly breaks them apart.

Why then does sexual reproduction persist?

1. Adaptive evolution is enhanced



• in asexual species, advantageous mutations must occur in the same lineage:



• in asexual species, advantageous mutations must occur in the same lineage:

advantageous mutation

Abcd Abcd’

Ab’cd’ advantageous mutation



• in asexual species, advantageous mutations must occur in the same lineage.

• in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy).



• in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy):

advantageous mutation

Abcd Abc’d

x Abc’d’

Abcd’ Abcd’ advantageous mutation


2. The Red Queen hypothesis



• originally proposed by Leigh Van Valen in 1973.





• originally proposed by Van Valen in 1973.

• species must continuously “run” (evolve) to track changing environments.





• if species fail to adapt, they may go extinct.





• if species fail to adapt, they may go extinct

• sexual reproduction facilitates this process.

The Red Queen process is an evolutionary arms race


Target species

“Enemies”(parasites, predators, competitors)


Target species


Adaptation


Target species


AdaptationCounter-adaptation

3. Muller’s ratchet

Hermann Muller (1890 – 1967)

A simple ratchet

crank

pawl

pawl

Muller’s ratchetMutation

Muller’s ratchet

• deleterious mutations occur in asexual lineages…

Mutation

Muller’s ratchet


• … causing the least mutated class to dwindle…

Mutation

Muller’s ratchet



• … and be lost by random drift

Mutation

Mutation

?

Muller’s ratchet




• now the ratchet has “clicked” forward once.

Mutation

Mutation

?

Muller’s ratchet




• now the ratchet has “clicked” forward once.

• now the ratchet has “clicked” forward again.

Mutation

Mutation

Mutation

? ?

?

Muller’s ratchet

Asexual populations can only evolve towards ever greater loads of deleterious mutations!

Muller’s ratchet


Does Muller’s ratchet occur in sexual populations?

Muller’s ratchet



NO! Sex breaks the ratchet.

Muller’s ratchet




How? By reconstituting the least mutated classes (by recombination).

Muller’s ratchet




How? By reconstituting the least mutated classes (by recombination).

SEX IS RECOMBINATION!

Q: So why are asexual species at the tips of phylogenetic trees?


S A S S S A S A S S

Q: So why are asexual species at the tips of phylogenetic trees?

A: Because the short-term benefit of asexual reproduction is countered by the long-term advantage of sex.


S A S S S A S A S S

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BIOE 109 Summer 2009 Lecture 7- Part I Linkage disequilibrium and the evolution of sex.

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