Anna Mosegaard Schmidt:
The invasion biology and sociogenetics of pharaoh ants

Date: 15-08-2010    Supervisor: Jes Søe Pedersen & Jacobus J. Boomsma

Social insect colonies perform a number of tasks affecting the environments they live in. Some unintentionally introduced species have attracted the attention of scientists and general public alike when causing a number of changes to the composition and functioning of ecosystems. Such “invaders” or “tramps”, though often considered negative influences, can also be seen as natural experiments, generating a number of questions in the fields of ecology and evolution. Pharaoh ants (Monomorium pharaonis) are very successful invaders of human habitation in most parts of the world. Individual pharaoh ants are small, their colonies are polygynous (have multiple queens), and consist of multiple interconnected nests that can spread to cover large areas through so‐called budding. Pharaoh ants appear to mate exclusively within their nests, indiscriminately inbreeding without a cost to colony performance. Combining these traits, and adding to them that numerous introductions of the species have resulted in genetically highly differentiated, low diversity colonies, makes pharaoh ants an interesting model system. During my PhD I have thus investigated the potential of pharaoh ants as models for questions of invasion biology and evolution in social insects. I have developed methods for establishing colonies of different genetic composition through controlled crossings of genetically different colonies, and established that measurable genetic as well as morphological variation exists between different laboratory lineages, thus building the foundation for future research on the species. In addition, I have started a selection experiment (still ongoing in collaboration with Dr. T. Linksvayer) using pharaoh ants, which is the first time artificial selection is attempted in an ant species.


Pharaoh ants have been introduced multiple times from an as of yet unknown native range, but little is known about their local mode of spread and propagule pressure. To learn more, I investigated the population genetic structure and phylogeography of pharaoh ants on different geographical scales in Thailand and malaysia, a region where they are very common. Employing species‐specific microsatellite markers, I found a structure of multiple introductions and isolation of colonies even on relatively small geographical scales.

Enemy release is a central concept in invasion biology, frequently proposed to play an important part in the success of introduced species. To investigate the enemy release hypothesis in Pharaoh ants, I screened introduced populations from several localities around the world for the presence of the presumed detrimental intracellular bacterium Wolbachia. Wolbachia was frequently present in introduced colonies, a finding which raises the question how harmful Wolbachia really is for pharaoh ants, as lack of this bacterium does not appear to be a prerequisite for successful introductions.

Having many related individuals living at high densities, as found in social insect colonies, is expected to increase susceptibility to infection and within‐group disease transmission. Such potentially negative effects of sociality are hypothesized to be counteracted by increased within‐colony genetic variation. To test this hypothesis, knowing that pharaoh ant colonies generally have low levels of genetic diversity, I investigated the effect of genetic diversity on disease resistance in pharaoh ants by crossing colonies. I did so by creating different diversity groups, and infecting these with a generalist entomopathogenic fungus. The results showed variation between groups from different colonies, and also showed that higher levels of genetic diversity, be this inter‐or intra‐individual, did not necessarily result in increased pathogen resistance in pharaoh ants. This finding is different from most previous research on the subject, and thus cautions against making general assumptions in the field.

The success of an ant colony depends on the simultaneous presence of reproducing queens and non‐reproducing workers in a ratio that will maximize colony growth and reproduction. Though seemingly very basic, little is known about the effect of colony size on queen‐worker caste ratios in ants. Manipulating colony size through the creation of multiple queenless colonies, we found that smaller colonies produced more new queens relative to workers, and that those queens and workers also tended to be larger. These findings demonstrate high levels of plasticity in energy allocation towards female castes, and suggest that polygynous, budding species may adaptively adjust caste ratios to ensure rapid growth.

The work in this thesis thus sheds some more light on pharaoh ants as introduced species, and takes the first crucial steps towards establishing it as a social insect model organism, while demonstrating some of the questions that may be addressed using this Monomorium‐model.