Martin Luther University Halle-Wittenberg

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Dominance hierarchies

Since Darwin faced the problem of the evolution of sterility of the workers in the eusocial Hymenoptera by means of natural selection much effort has been put forward to explain this paradoxon. Our group focused on the reproduction by workers within honeybee (Apis mellifera L.) colonies. We have chosen the Cape honeybee (A.m.capensis Esch.) as a model organism This honeybee subspecies occours in South Africa and is characterized by some unique traits of the worker caste. Workers are able to reproduce parthenogenetically female offspring (thelytoky). The workers are able to develop their ovaries very fast, even in presence of the queen.

Pseudoqueen surrounded by a retinue of subordinate workers. (Photo by S. Härtel)

Pseudoqueen surrounded by a retinue of subordinate workers. (Photo by S. Härtel)

Using quantitative genetic analysis we could show that traits very closly linked with reproductive dominance are highly genetically determined (heritabilities in the broad sense 0.27-0.92) (Moritz & Hillesheim 1985). This phenomenon is rarely found for fitness related traits since natural selection should act to eliminate genetic varianace for such traits as suggested by Fisher. We found evidence for a balancing mechanism, on the one side direct selection for increased worker fitness and opposed to that colony-level-selection against an increasing number of reproductive workers (Hillesheim et al 1989; Moritz 1989). We could show that after queen loss, only a small number of patrilines is monopolizing reproduction (Moritz et al 1996).

The establishment of the reproductive status within the colony depends on the production of traits related to reproduction. One of the most important traits is the production of a queen-like pheromone bouquet. This pheromone can induce retinue behaviour of other workers but can also serve as a suppressing signal for ovary activation in other workers. Together with our cooperators in South Africa (Dept. Zoology and Entomology, University of Pretoria) we could show that workers compete for the production of the strongest queen-like signal (Moritz et al 2000; 2004). Using a newly developed method for extracting pheromones from living bees we could show the development of the competition over time (Crewe et al. 2004) . The competition is followed by a phase of establishment and finally the dominant bee can suppress the subordinate worker bee (Moritz et al 2004).

In polyandrous mating systems reproduction by workers is decreased by a mechanism called worker policing. Due to the skew relatedness in Cape honeybee colonies (caused by thelytokous worker reproduction) worker policing should not be an adaptive trait. Using variable microsatellite markers we found empirical evidence for this hypothesis ( Moritz et al 1999).

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