A common presumption is that behavior is part of phenotype, and since phenotype arises from genotype (plus/minus Reaction Norm), that there can be a study of “behavioral genetics.” This is certainly an overstatement (or oversimplification) for organisms with extensive and/or complex neural systems, such as humans and mice. Neural systems probably evolved (not initially, but eventually) to disassociate behavior with the kind of pre-determined micro-management of behavior that a simple gene-behavior link requires. However, in organisms with neural systems the size of the period at the end of this sentence, we often do see cases of allelic variations causing behavioral variation in the whole organism. The study discussed here is an example of this.
Solenopsis invicta, a fire ant, can have colonies with a single reproductive queen (these are called mongyne colonies) or a colony wit multiple reproductive queens (called polygyne colonies).
In mongyne colonies, all individuals have a particular allele for one gene. The gene is General Protein-9 (Gp-9), and the allele is the B-like allele.
Polygyne colonies contain individuals with both the B-like allele and the b-like allele (case matters!). This has led to the suggestion that the presence of b-like is necessary and sufficient for the rise of polygyne colonies.
One overarching reason that this sort of thing is important has to do with the eusocial nature of bee, wasp, and ant colonies. In many ants and bees, and some wasps, reproductive altruistic behavior is seen, in which individuals forgo their own direct reproduction in order to assist a single queen and a handful of males produce a large number of offspring. This behavior can be accounted for by the fact that if individuals act more or less alone to raise offspring, they will have just a few offspring, but if they group together in a colony they can produce zillions of close relatives. Producing more offspring directly (more than competing individuals) increases one’s fitness. Contributing to the production of zillions of relatives can also increase one’s fitness. The idea is that a colonial system allows for greater fitness, indirectly, for non reproductive workers than working alone would.
Bees ants and wasps have a genetic system that also results in this interesting fact: From the point of view of a given female, she (the female) is more closely related to a sister than to a daughter. Therefore, contribution to the production of sisters adds up, in terms of fitness, faster than one might expect.
However, indirect fitness (one’s contribution to the production of relatives that re not offspring) if the members of the colony that one is supporting are less related, as would be the case with multiple queens. So why are there ever multiple queens? You can see why this is an interesting question.
There are many species of fire ants, but the monogyne-polygyne polymorphism studied in this paper occurs in only six species in a single clade.
The GP-9 gene codes for a protein that is one of may different proteins that are used as olfactory cues, for communication among ants. It may be the case that this gene relates to the communication that happens between prospective queens and workers, in the process of negotiating which queens will be allowed in a colony (the queens require care and feeding by the workers, so the workers determine the number of queens).
The present study looked at a much larger set of data than previously examined, and confirmed the relationship between b-like alleles and multiple females, but also uncovered important nuances.
…our determination of the social organization of key colonies confirmed the invariant link between the presence of typical b-like alleles and [multiple queens], while our discovery of several novel alleles bearing various combinations of b-like and B-like codons revealed that no single amino acid residue is completely predictive of polygyne behavior.
Where b-like alleles (and multiple queens) occur, one may postulate that there is reduced selection against multiple queens, or increased selection for them. The genetic analysis done here attempts to characterize the selection pattern on this gene. While far from conclusive, the researchers suggest that it is more likely that the presence of b-like alleles causing multiple queen colonies is not so much the result of lack of selection against this, but rather, positive selection for the multiple-queen alleles.
This elevated coding-region variation may result from a lack of negative selection acting to constrain amino acid replacements over much of the protein, different mutation rates or biases in coding and non-coding sequences, negative selection acting with greater strength on non-coding than coding regions, and/or positive selection acting on the protein. Formal selection analyses provide evidence that the latter force played an important role in the basal b-like lineages coincident with the emergence of polygyny.
I am left wondering what the selection force for multiple queens would be, and would like to see the author’s speculation in this area.
Gotzek, D., Shoemaker, D.D., Ross, K.G., West, S. (2007). Molecular Variation at a Candidate Gene Implicated in the Regulation of Fire Ant Social Behavior. PLoS ONE, 2(11), e1088. DOI: 10.1371/journal.pone.0001088