Evolution of sex The nature and dynamics of beneficial variation

Why sex? In with the good? Out with the bad?

Why not sex? The twofold cost of sex

In with the good Creates novel/adaptive genotypes more rapidly (Fisher-Muller) Red Queen hypothesis Variation among siblings = “tangled bank”

Fisher-Muller: recombination (A) enables assembly of adaptive alleles more rapidly than asexual populations (B)

In with the good: Fisher-Muller Sex generates novel combinations of alleles, some of which may have greater fitness. The fate of a beneficial mutation with sex and recombination depends less on the genetic background on which it arises (less interference among loci). Also known as the The Vicar of Bray Hypothesis after: an English cleric noted for an ability to change his religion whenever a new monarch ascended the throne [to emphasize] that there may be great advantages of easily and gracefully adapting to changed circumstances (Bell 1986) Probability of fixation of beneficial mutations is higher in sexuals with frequent recombination than asexuals with no recombination. The effect substantially favors sexual individuals ONLY if beneficial mutations occur frequently throughout the genome.

RecombinationNo recombination the “Vicar of Bray” hypothesis

The most curious part of the thing was, that the trees and the other things round them never changed their places at all: however fast they went, they never seemed to pass anything. "I wonder if all the things move along with us?" thought poor puzzled Alice. And the Queen seemed to guess her thoughts, for she cried, "Faster! Don't try to talk!“ Not that Alice had any idea of doing THAT. She felt as if she would never be able to talk again, she was getting so much out of breath, and still the Queen cried "Faster! Faster!" and dragged her along. "Are we nearly there?" Alice managed to pant out at last. "Nearly there!" the Queen repeated. "Why, we passed it ten minutes ago! Faster!" And they ran on for a time in silence, with the wind whistling in Alice's ears, and almost blowing her hair off her head... until suddenly, just as Alice was getting quite exhausted, they stopped, and she found herself sitting on the ground, breathless and giddy. The Queen propped her up against a tree, and said kindly, "You may rest a little now." Alice looked round her in great surprise. "Why, I do believe we've been under this tree the whole time! Everything's just as it was!“ "Of course it is," said the Queen, "what would you have it?" "Well, in OUR country," said Alice, still panting a little, "you'd generally get to somewhere else -- if you ran very fast for a long time, as we've been doing." "A slow sort of country!' said the Queen. "Now, HERE, you see, it takes all the running YOU can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!" Lewis Carroll Alice Through the Looking Glass 1865 Alice Through the Looking Glass

The tangled bank hypothesis "It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us." A diverse set of siblings may be more fit than a clone, because each sibling uses a slightly different niche.

Out with the bad Allows reconstruction of mutation-free individuals Removes deleterious mutations See Wiki: Kondrashov’s hatchet, deterministic mutation hypothesis

Muller’s ratchet The inevitable loss of the “least-loaded” (in terms of bad mutations) class Especially common in small, non- recombining populations in which the probability of back-mutation (to restore the good allele) is low Thought to explain the decay of the Y chromosome

Out with the bad: Muller’s ratchet

Epistasis and Kondrashov’s hatchet Asexual favored Sexual favored

RecombinationNo recombination

Prokaryotic recombination  eukaryotic sex But some of the key variables can be manipulated in prokaryotes

A test of Kondrashov’s hatchet

In with the good Fisher-Muller: increase rate of adaptation?

(rec+) – (rec-) Greater adaptation in rec+ lines A test of Fisher-Muller

spoT mutations fix more quickly in rec+ lines

spoT mutations become deleterious in rec- lines owing to interference among beneficial mutations in other competing clones

A test of Fisher-Muller

Fitness is affected by the presence of other competing phage. Open symbols: low MOI Closed symbols: high MOI MOI = (low) MOI = 5 (high) Sexuals worse than asexuals at high MOI!