Ecology & evolution - selfish genes

Why would genes be selfish

They are favoured if they can better transmit themselves, increased proportion in next generations

What are selfish genes

Genes that increase their own transmission - greater than 50% of expression in next generation

Examples of selfish genetic elements

t halotype in mice - a part of the chromosome that has 90% chance of expression. Sperms with this t halotype kill other sperms. The part is inherited as a single unit because an inversion prevents recombination

homing endonuclease genes (HEG) - causes production of an endonuclease that cuts genome at recognition site. HEG exists in the middle of the site so is not cut. DNA is repaired using the HEG strand as a template, so both strands end up with HEG

How does HEG spread

Through heterozygous individuals and outcrossed mating

Inbreeding doesn’t cause HEG increase as all the individuals already have it

Why is the sex ratio of organisms at birth usually equal

Favoured by natural selection

If there are twice as many females, males have twice the fitness of females (males can mate with 2 females while females with 1 male). Therefore it balances out.

Vice versa if there are more males.

What do selfish genes usually have an effect on

Affects the sex ratio drive - usually a selfish X chromosome in the male genotype causes Y-bearing sperm to be killed, ensuring offspring only get the X chromosome.

This causes a female-biased sex ratio, fewer carriers of the Y chromosome.

Paradox of the X chromosome being selfish

Leads to more female offspring with that X chromosome. But as there are fewer males in the population, the fitness of the females decreases.

So the gene pursued its self-expression to the detriment to the rest of the genome.

What are the mechanisms preventing ‘selfish genes’

Parliament of genes - causes biased sex ratios to be rare

Why do biased sex ratios (in the example of selfish X chromosome) want to be avoided

Y chromosome doesn’t want to be eliminated

Males end up producing less sperm because lots of the Y-carrying sperm is killed, so there’s lower success for X-carrying sperm too

Y chromosome being eliminated causes mainly female offspring, the bias of which increases male success (and the female offspring’s success suffers) - so the autosomes also do not want a biased sex ratio

What evolves in response to selfish genetic elements (example of selfish X chromosome)

Suppressors evolve - very diverse, found at Y chromosomes and autosomes

Y chromosome can evolve a suppressor to the Y killer

Do distorted sex-ratios exist in nature

Yes but found with high resistance and lots of suppressors, so the distortion is often not seen

Loop of selfish genes

Initially selected to have a large effect as this increases self-transmission

Their increasing effect eventually causes evolution of suppressors

With suppressors, the selfish genes are selected to cause decreased distortion and their effect decreases

This can repeat → so selfish genes do exist and have impact, but they are usually brought back to favour individual interest

Two outcomes for selfish gene action

Cause small deviations from individual optimum

Cause a large deviation and be suppressed

Outcome of the parliament of genes

Very good at suppressing conflict at an individual level

If genes pull away from individual interest, will be suppressed by the parliament of genes

Conflict will be hidden

How can possible conflict of selfish gene and the parliament of genes / genes and the individual be detected

By crossing species

They will have different selfish genetic elements and the cross-breeding will separate the sex ratio distorters from their suppressors

Therefore the offspring will have sex ratios and viability all over the place - contrast to the expected 50:50 sex ratio and high viability

What does the maximisation approach require

Negligible conflict and genes all pulling in one direction

Why is there negligible conflict (i.e. why does the maximisation approach work so well)

Either the selfish spread of genes cannot distort a trait

Or the selfish spread of genes does distort a trait, but then the parliament of genes will be selected to suppress those genes