Sex allocation conflict in eusocial insects

Question 1

Sex ratio and Sex allocation

Answer 1

Sex ratio: Ratio of reproductive males to reproductive females (not workers)

Sex allocation: The total allcoation of resources to each sex
Example: Female sex allocation is much higher in rover ants due to female size (does not means there is female bias)

Question 2

Assumptions from fisher’s theory that is broken by eusocial insects

Answer 2

1) Mother controls sex allocation/ ratio (can be controlled by workers)

2) Equal relatedness between males and females

Question 3

Haplodiploidy

Answer 3

Mother’s produce males when the egg is unfertalised (0.5 related) and females when the eggs is fertilised (0.5 relatedness.

This means that females are diploid and males are haploid.

Haplodiploid relatedness of sisters:
Under MONOGAMY: relatedness asymmetry
- 0.25 to brothers
- 0.75 to sisters

Under POLYGAMY:
- 0.25 to brothers
- 0.25 to sisters (if all indivs have different fathers)

Question 4

Conflict of interests between Queens and workers in monogamous populations

Answer 4

Workers: they are 3x as related to sisters than to brothers so they favour a 3:1 sex ratio
- Males will have 3x the reproductive success as there are 3x fewer of them. Females have 3x lower reproduce success but workers are 3x more related to them.

Queens: they are equally related to sons and daughters so favour a 1:1 ratio.

Who wins?
- In most ant populations study showed that workers tend to win and there is a 3:1 sex ratio. (Triver’s and Hare) -> but this study looked at mass ratio which does not reflect investment as respiration cost per g is lower for larger individuals -> over estimation of cost for large individuals
- There are unbiased sex allocation examples (termites/ solitary bees) and the queen can win.
- Example: Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs -> lay more females in female specialised colonies and able to exclusively lay haploid eggs to minimise worker manipulation in male speciliased colonies -> workers still able to manipulate a bit and equiblria was between

However, in this study there was huge unexplained split sex ratios between colonies.

Theories:

Inclusive fitness theory:
- Predicts that sex investment ratios in eusocial Hymenoptera are a function of the relatedness asymmetry

Alternative theory:
- The Wilson–Nowak provides an alternative theory for 3:1 investment.
- Natural selection is selecting for 1:1 numerical sex ratio.
- If there was a 1:1 investment, then there would be more males as as they are smaller and require less investment-> they would not mate and be waste.
- This selects for a female biased investment -> nothing to do with worker control
- Selection acts to maximise the number of females and males mated -> no queen worker conflict -> both want to maximise the number of offspring mated.

Question 5

Split sex ratios

Answer 5

Colonies can be both monogamous and polygamous.

• Singly Mated Queen
  o sister-sister: r=0.75
  o sister-brother: r=0.25
  o Workers more related to sisters, favor a female biased sex ratio
• Multiply Mated Queen
  o sister-sister: r=0.25 (as low as)
  o sister-brother: r=0.25
  o Workers equally related to sisters and brothers – don’t favor a female biased sex ratio

The sex ratio within a colonies depends on the sex ratio of the overall population as this can lead to frequency dependent selection for a certain sex. -> too many of one sex reduces their reproductive value as fewer of the other sex to mate with.
- Polygamous colonies are rare results in a female bias so polygamous colonies become male biased.
- Monogamous colonies are rare results in a male bias so monogamous colonies become even more female bias (1)

The more rare the monogamous/ polygamous colonies, the more extremely the workers will promote either a female or male bias due to more extreme frequency dependent selection.

Conclusion:
If some queens mate multiple males, and workers can control sex allocation, then theory predicts that Workers favour production of : (1) females in colonies where the queen mated singly; (2) males in colonies where the queen mated multiple males.

Question 6

Example of split sex ratio

Answer 6

Study on Woodant (Sunstrom, 1994)
- They observed singuarly and multiply mated colonies and recorded the sex ratios.
- ->Single: mainly females
- ->Multiple: mainly males
- Evidence of split sex ratio under worker control

66% variance in investment in queens by workers is explained by mating freq of queen with males (level of polygamy) – this is huge! Usually >10% is good in ecology!

Meta analyses: Stuart West and team
- Variation in relatedness asymmetry (due to mate number or queen replacement) and variation in queen number (which also affects relatedness asymmetry in some conditions) explained 20.9% and 5% of the variance in sex allocation among colonies

Question 7

How do workers control sex ratios?

Answer 7

Count how many times the queen has mated
- They do this by analysing the variation in hydrocarbon signal (smell) from worker cutical which is inherited from the father.
- Higher variation= multiply mates
- No variation= singuarly mated
- successful unless males mated smell similar

Control the sex ratio in response
- Selectively neglect or destroy males
- > example: Narrow headed ants destroy males between larvae and pupil stage
- raise females differently into queens or workers (change fraction raised queens:workers)
- > Example: L. acervorum workers rear eggs randomly with respect to sex and must achieve their favoured sex ratios by selectively biasing the final caste (queen or worker) of developing females.

Question 8

Other factors effecting split sex ratios

Answer 8

 mating freq of queen (and other analogous reasons)
 no. queens per colony (multiple = 1:1, single= 3:1)
 when queen dies

Question 9

Overview

Answer 9

Fisher predicts a 1:1 sex ratio which is not seen in eusocial insects as 2 assumptions are violated:
- Mother’s control sex ratio
- Equal relatedness
- > Monogamous: s-s = 0.75, s-b=0.25, m-m/f=0.5
- > poligamous: s-s = 0.25, s-b=0.25, m-m/f=0.5

Instead see 3:1 and split sex ratios

Why?

If all queens are singly mated, then:
* Queens favor 1:1 sex allocation
* Workers favor 3:1 sex allocation

Who wins?
* The data show 3:1 sex allocation, suggesting that workers are in control
* There is some evidence that queens maintain some control

However, split sex ratios are common.
* If some queens mate multiple males, and workers can control sex allocation, then theory predicts that Workers favor production of:
 Females in colonies where the queen mated singly
 Males in colonies where the queen mated multiple males
* The data from a range of species support this incredibly well (Woodant- 66% explained)

How do workers control sex ratios:
* Workers use hydrocarbons to tell how many times their queen has mated
* Workers can manipulate the sex ratio by either selectively killing, or by changing the fraction of females that they rear as workers (or queens).