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1:1 sex ratio is a constraint imposed by what?

Sex chromosomes (equal segregation in the heterogametic sex)


Therefore 1:1 sex ratios can be overcome if sex is not determined by sex chromosomes. True or false?



Even with sex chromosomes they can be distorted. True or false?



Why is a 1:1 sex ratio beneficial? Give 2 reasons.

1. It maximises offspring and grand-offspring (as everyone will have a chance to mate)

2. There is lower variation in fitness means less individuals removed by selection


What is the 1:1 sex ratio often called?

Fisherian sex ratio


What does Fisherian sex ratio refer to?

The primary sex ratio (SR immediately after fertilisation)


What are the 4 major assumptions under Fisherian sex ratio?

1. The cost of parental investment in both sexes is the same

2. There is random mating (or 'population-wide competition for mates')

3. Males and females have similar pay-offs in the environment

4. Parents do not control the offspring sex ratio


Why does selection favour a 1:1 sex ratio?

The sex with the highest reproductive value is selected for; this is the rarer sex. Therefore deviations from equilibrium are corrected for.
E.g. if there are few females then selection will favour females, bringing ratio back to 1:1


What type of selection maintains 1:1 sex ratios?



Selection will favour modifiers that return the ratio to 1:1. True or false?



Fisherian sex ratio assumes that there are equal costs for producing sons and daughters. Is always this true?

No; males are often more costly to produce


Give a referenced example that shows males are more costly to produce.

Clutton-Brock (1981):
Red Deer, males often die in utero. Mothers corrected for this by providing a male-biased feeding rate.


Fisherian sex ratio assumes that mating is random/there is population-wide mate competition. Is always this true?

Mating is non-random/mate competition can be local, i.e. inbreeding.


How does inbreeding affect the sex ratio?

It favours the production of a female-biased sex ratio as fewer/a single male is needed to fertilise sisters. Sperm competition is between relatives, meaning it is reduced.


Give a referenced example of inbreeding affecting the sex ratio.

Werren (1983)
N. vitripennis (parasitoid wasp), female lays eggs in a single host. If she is only one to use that host, flightless brothers fertilise sisters before dispersal.
As no. of foundresses using same host increases, so does male bias in the SR of offspring as more sperm competition.


Fisherian sex ratio assumes that males and females have similar environmental pay-offs. Is this always true?



Give a referenced example of differential pay-offs for males and females.

Krebs, Davies and West (2012):
Dominance in Red Deer, dominant females have access to more resources and therefore it is better to have sons as they are more expensive. Daughters are produced by subordinate females that have access to fewer resources.


Fisherian sex ratio assumes parents do not control the offspring sex ratio. Is this always true?



Give a referenced example to demonstrate how parents can alter offspring sex ratio.

Komdeur et al. (1997):
Seychelles warblers, female offspring stay with the parents as helpers.
In good territories daughters produced as can bring more food in.
In bad territories sons produced as they dispersed and rents don't have to share.


Deviations from 1:1 sex ratio can be induced by ESD. Why?

The offspring control their own sex by responding to environmental cues


Give a referenced example of ESD.

Warner and Shine (2008):
Jacky lizards, females produced in extreme temps. as fare better than males, males produced in intermediate temperatures


When is ESD favoured? Give 2 conditions.

1. There is a differential pay-off between the sexes (i.e. different environmental conditions favour different sexes)

2. There is no parental control over offspring environment


Sex ratio can also be controlled by eusociality. True or false?



Fisherian sex ratios assume autosomal control and Mendelian inheritance. Is this always what happens?



What are the two types of sex ratio distorter (SRD)?

1. Sex-linked drivers
2. Cytoplasmic elements


Cytoplasm is always transmitted by males. True or false?

False, it is only ever transmit by females.


What sex do cytoplasmic elements prefer? Why?

Females as they cannot be transmit by males


What are the three main ways in which cytoplasmic elements can bias the sex ratio to produce females?

1. Parthenogenesis
2. Feminisation
3. Male killing


What is parthenogenesis?

When unfertilised eggs develop as females.


Give a referenced example of cytoplasmic elements inducing parthenogenesis.

Stouthammer et al. (2010):

Trichogramma genus of parasitoid wasp carries Wolbachia bacteria.
Wasps are haplodiploid.
Bacteria cause diploidisation of unfertilised eggs by failure of the 1st or 2nd mitotic division, produces females.