The Pharaoh Ant – Monomorium Pharaonis


Pharaoh ant Monomorium pharaonis queen and workers,©Luigi PontieriSocial insects are well-established models for a range of topics such as the evolution of cooperation and conflict and the genetic basis of social behavior. Genomic and transcriptomic tools for an increasing number of social insect species facilitate comparative genomic and transcriptomic studies. However, many basic evolutionary genetic and molecular questions are difficult or impossible to study because most social insects cannot be bred in the lab and readily kept across generations — many social insects require mating flights and generation time is typically years or even decades. The pharaoh ant Monomorium pharaonis is one of only a handful of social insect species that can be easily bred in the lab across generations.

M. pharaonis is considered to be the most widespread and oldest invasive ant: it has been spread worldwide for at least 200 years from its native range in tropical Asia. As a result, genetically distinct and presumably locally adapted lineages are readily collected. M. pharaonis is very well-studied and has the following unique set of characteristics:

  • controlled crosses are easily made in the lab
  • generation time can be very short (6 weeks) relative to most other ants (usually years)
  • inbred lineages can be created and maintained indefinitely: M. pharaonis and some other ants apparently do not have the same csd sex determination system as honey bees
  • hundreds of large colonies are easily maintained in the lab across generations
  • workers lack ovaries so that worker-produced males cannot complicate breeding programs
  • female caste is easily morphologically distinguishable at an early age (2nd instar;see figure below)
  • nestmate discrimination is transient to non-existent, so that colonies can readily be combined and manipulated to control for colony size, genetic background, etc.
  • colonies readily nest between between glass plates (e.g., microscope slides) so that within-nest behavior is easily observed and recorded

Since 2008, we have systematically produced an 8-way intercross with known pedigree. Together with this heterogeneous stock population, we are producing recombinant inbred lines, both of which will be used for finely mapping the genetic basis of complex social traits. We have assembled a transcriptome and draft genome of similar quality as the previously published ant genomes and have produced a linkage map.

We are currently using and further developing this system to study a range of topics associated with the evolution and function of complex social systems.

Reproductive caste is readily determined by the 2nd instar: queens top row, workers bottom row. Female development is shown across the following stages: egg (E); 1st instar (1); 2nd instar (2); small, medium, and large 3rd instar (S3,M3,L3); prepupae (PP); and pupae (P).
M. pharaonis colonies that were initially separate (indicated by color) readily merge,©Luigi Pontieri
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