When I walked into our lecture on social traits at UCL and realized it was about the invasive fire ant in the USA, excitement was not the first thought that came to my mind. To an American, these invasive fire ants mean trouble. Beyond their nasty stings (which are truly awful), they kill native species and damage crops. It is estimated that each year there is an aggregate loss of five million dollars to households, businesses, governments, and institutions. (Wurm 2011) The invasive fire ant thrives in human-modified environments. It was originally introduced in the 1930s through the port of Mobile, Alabama. Today its range extends to 14 southern states and Puerto Rico. Eradication is extremely difficult and often native fire ants are killed either by misidentification or indiscriminate spraying. Despite my hatred of S. invicta, I decided by the end of the talk that they have managed to contribute to scientific advancements in social traits and genetics.
First, let’s talk about the biology of the S. invicta. This species has two forms of colony organization found in the USA. Some populations have monogyne colonies (one queen) and others have polygyne colonies (multiple queens ranging from a few to hundreds depending on the colony’s size). Beyond the number of queens, these colonies differ in the dispersal strategy of virgin queens, founding of a colony, and energy reserves for a queen. Young queens in monogyne colonies store fat and glycogen in preparation for future dispersal when they are mature enough to leave the nest. After the queen flies from the nest and mates, she digs the start of a colony and lays her eggs. In the beginning she does not have her own workers and relies on her energy reserves until the first workers mature. In polygyne colonies the queens do not acquire the significant amount of energy reserves to found new nests alone. Instead these queens tend to travel to existing polygyne colonies. Additionally, in both colonies workers regulate the amount of queens. If a new queen tries to lay eggs in a monogyne colony she is swiftly executed. There are also queen executions by workers in polygyne societies.
Scientists, through genetic sequencing, identified that the variation found in the social organization of the ant colonies are connected with General Protein 9 (Gp-9) in the ant’s genome. This finding was important since for most of the genome, there were no differences between the monogyne and polygyne colonies. There are two types of single gene (allele) variations in Gp-9: the B and b. If you remember high school biology this is similar to your lesson on eye colour. Your eye colour is determined by a combination of alleles inherited by your parents. Now imagine you had inherited an allele combination that determined the entire social order of your town. That is what this allele variation means for the ants. Monogyne colonies have only the BB allele combination and queens with the b allele are executed by workers. In polygyne colonies, only queens with a Bb are allowed to survive but produce workers with BB, Bb, and bb alleles. Queens with bb are not executed because this deleterious combination causes premature death. Scientists believe this may be due to the B allele controlling fat storage in queens. But how do the workers identify the alleles of the queen? Scientists are still not sure of the exact
details but the most widely accepted hypothesis is that the Gp-9 produces something similar to an insect odorant binding protein (OBP). The result is the queen emits a chemical and the workers touch her with their antennas, and use this information to determine whether or not the queen is executed for lacking the proper alleles.
So why is this important? Scientists have determined that a single genetic variation in an ant’s genome can help determine the social order of an entire colony. Scientists have now further tested the theory to other species in Solenopsis and found similar B and b alleles. Scientists believe that monogyne colonies predate polygyne colonies and have used the data from other Solenopsis species to run possible genetic trees to explain the evolution Gp-9. However, there are still many things yet to understand about Gp-9 and its influence on social order.
So what’s the big picture? The results from the work show that genetics can control social behaviour and possibly the social evolution of many species. Genes with this much power, called ‘supergenes’ are extremely important to understanding species’ social behaviours. Scientists need these data to determine if traits are due to a species’ surroundings or is it something inherited through genetics. With the lowering cost of genome sequencing we will see a rise in the identification of these supergenes. While I cannot say we will find genes that determines human’s social behaviours, we may find that in our evolution there may have been supergenes that helped us become well, us.
I still hope for the eradication of the invasive fire ants from the USA. I know I said I valued them for their contribution, and I do, but seriously they still need to leave.
If you are interested in learning more about the science of fire ants check out the articles below!
- Gotzek D, Ross KG (2009) Current Status of a Model System: The Gene Gp-9 and Its Association with Social Organization in Fire Ants. PLoS ONE 4(11): e7713. doi:10.1371/journal.pone.0007713
- Krieger, Michael J.B. (2005) To b or not to b: A Pheromone-binding Protein Regulates Colony Social Organization in Fire Ants. BioEssays 27:91-99, Wiley Periodicals, Inc.
- Wurm Y, et al (2011) The Genome of the Fire Ant Solenopsis invicta. Proceedings of the National Academy of Sciences, 108(14):5679-5684
And a special thanks to our lecturer, Yannick Wurm!
Written by Emily: UCL, BEC
Photos by Bart Drees, Texas A&M Texas Imported Fire Ant Research and Management Project.: http://insects.tamu.edu