Relatedness and the Evolutionary Pathway to Worker Sterility in Ants

Project Information

AntPolice

Grant agreement ID: 101067861

DOI

10.3030/101067861

Project closed

EC signature date 16 June 2022

Start date 1 September 2023

End date 30 September 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 200 860,40

Coordinated by

OULUN YLIOPISTO
Finland

Periodic Reporting for period 1 - AntPolice (Relatedness and the Evolutionary Pathway to Worker Sterility in Ants)

Reporting period: 2023-09-01 to 2025-09-30

Summary of the context and overall objectives of the project

In some ant colonies, the queens aren't the only ones laying eggs. Workers sometimes lay eggs too, but their sisters "police" this behaviour by destroying their nephews! This "ant policing" could be of profound evolutionary importance. It's thought to have played a key role in the evolution of a sterile worker caste among the ants. Why lay eggs if they will be destroyed? There is another hypothesis for the evolution of worker sterility, however. According to the "monogamy" hypothesis, you don't need policing, queens just need to be monogamous. Monogamy ensures that workers are raising full siblings, the genetic equivalent of their own young. As workers can then pass on their genes indirectly through their mother, sterility can evolve.

AntPolice will test between these alternatives for the evolution of worker sterility by combining two approaches. First, ancestral states will be estimated to determine when in evolutionary time worker sterility evolved and if it was preceded by policing or monogamy (or both). Second, experiments with the ant Formica exsecta will be conducted to test if "ant policing" is good or bad for the colony.

Figuring this out is important. There is no test so far of the policing vs monogamy evolutionary routes to worker sterility. AntPolice will therefore fill a crucial gap in our knowledge about the origins of extreme cooperation. The results are potentially paradigm shifting as high relatedness is the currently the textbook explanation for the evolution of worker sterility.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A large database on worker sterility, policing, and other life history traits for ants was compiled from published sources. This formed the backbone for the ancestral state estimations which were conducted in the R statistical programming language. These analyses showed, surprisingly, that neither worker policing nor monogamy has much effect on sterility. Colony size seems to be the driving parameter.

Field work was conducted at Tvärminne zoological station where 15 colonies of the ant Formica exsecta were collected in the wild and the experiment set up in the lab. The experiment had the following design. Each colony was split into three parts:
1. 500 workers + 1 queen
2. 500 workers + 1 queen
3. 500 workers + 0 queens
This meant that 22500 individual ants were individually sorted! The third part is key. We trick workers into laying eggs by removing their queen. Once workers started laying eggs in the queenless colonies we combine workers from each colony to form a control and our treatment.
Control = 250 workers were swapped from part 2 into part 1
Treatment = 250 workers were swapped from part 3 into part 2
The control is so that we know swapping the workers itself doesn't have an effect. The treatment has egg laying workers, now with a queen. After a few weeks we compared the number of offspring produced by each colony. The results so far indicate that policing affects colony fitness.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Surprisingly, neither worker policing nor monogamy has much effect on sterility. Colony size seems to be the driving parameter. Experiments suggest that policing affects colony fitness.

Formica exsecta worker with eggs. Picture taken during experiments.