Potential cause of lethality of an allele implicated in social evolution in fire ants

The gene Gp-9 is believed to have a major effect on colony social organization in fire ants, with the presence of b-like alleles in a colony associated with multiple-queen (polygyne) organization. Queens and workers of polygyne Solenopsis invicta homozygous for the b-like allele designated b suffer reduced viability compared to other genotypes, and bb queens do not survive to become egg-layers. Thus, the b allele effectively acts as a recessive lethal. This allele differs from the remaining b-like alleles (designated b'), as well as all other Gp-9 alleles, by encoding a lysine at position 151 in the protein product, suggesting that this substitution is responsible for its deleterious effects. We tested this hypothesis by comparing frequencies of b'b' and bb homozygotes, first in queens of Solenopsis richteri and S. invicta, then in S. invicta workers from populations polymorphic for the two b-like alleles. We found that almost 20% of S. richteri queens were b'b' homozygotes, compared to the virtual absence of bb homozygotes among S. invicta queens, and that 5-18% of S. invicta workers bore genotype b'b', compared to the apparent lack of bb workers in the same populations. While we cannot entirely rule out involvement of other genes in complete gametic disequilibriurn with Gp-9, our data are consistent with the hypothesis that the Lys(151) residue in GP-9 protein confers the deleterious effects of the b allele in homozygous condition, possibly by impairing the protein's function through interference with ligand binding/release or hindrance of dimer formation.