Current Status of a Model System: The Gene Gp-9 and Its Association with Social Organization in Fire Ants

The Gp-9 gene in fire ants represents an important model system for studying the evolution of social organization in insects as well as a rich source of information relevant to other major evolutionary topics. An important feature of this system is that polymorphism in social organization is completely associated with allelic variation at Gp-9, such that single-queen colonies (monogyne form) include only inhabitants bearing B-like alleles while multiple-queen colonies (polygyne form) additionally include inhabitants bearing b-like alleles. A recent study of this system by Leal and Ishida (2008) made two major claims, the validity and significance of which we examine here. After reviewing existing literature, analyzing the methods and results of Leal and Ishida (2008), and generating new data from one of their study sites, we conclude that their claim that polygyny can occur in Solenopsis invicta in the U.S.A. in the absence of expression of the b-like allele Gp-9b is unfounded. Moreover, we argue that available information on insect OBPs (the family of proteins to which GP-9 belongs), on the evolutionary/population genetics of Gp-9, and on pheromonal/behavioral control of fire ant colony queen number fails to support their view that GP-9 plays no role in the chemosensory-mediated communication that underpins regulation of social organization. Our analyses lead us to conclude that there are no new reasons to question the existing consensus view of the Gp-9 system outlined in Gotzek and Ross (2007)

Selfish genes: a green beard in the red fire ant

A 'green-beard' gene is defined as a gene that causes a phenotypic effect (such as the presence of a green beard or any other conspicuous feature), allows the bearer of this feature to recognize it in other individuals, and causes the bearer to behave differently towards other individuals depending on whether or not they possess the feature(1-3). Such genes have been proposed oil theoretical grounds to be agents mediating both altruism and intragenomic conflicts(1,2), but until now few, if any, of these genes have been identified(4,5). Here we provide evidence of a green-beard gene in the red imported fire ant, Solenopsis invicta. In polygyne (multiple-queen) colonies, all egg-laying queens are Bb heterozygotes at the locus Gp-9 (ref. 6). Previous studies suggested that bb females die prematurely from intrinsic causes(6); we now show that BE queens initiating reproduction are killed by workers, and that it is primarily Bb rather than BE workers that are responsible for these executions. This implies that allele Gp-9(b) is linked to a green-beard allele that preferentially induces workers bearing the allele to kill all queens that do not bear it. Workers appear to distinguish BB from Bb queens on the basis of a transferable odour cue