Additional file 16: of Unexpected patterns of segregation distortion at a selfish supergene in the fire ant Solenopsis invicta

Figure S10. Hypothetical scenario for a historical double crossover involving the putative supergene drive locus as the genetic mechanism responsible for Sb supergene drive reversal in polygyne Solenopsis invicta. (A) Double crossover between the wild-type Sb and SB social chromosomes in the segment between markers i_126 and Gp-9 transferred the complete drive complex (locus) from Sb to SB without altering the congruence of alleles at all three supergene markers observed in progenies with both drive and reversal-of-drive. Two novel recombination products were generated, a SB chromosome containing the drive locus and a Sb chromosome without it, and these presumably must be paired in a queen to yield drive reversal in her progeny (pairing of wild-type SB with wild-type Sb yields supergene (Sb) drive; pairing leading to any of six different drive-locus+ or drive-locus− homozygotes is expected to yield Mendelian proportions, although at least some of these homozygotes are expected to be lethal genotypes in queens; and pairing leading to either of two drive-locus+/drive locus– heterozygotes on otherwise SB or Sb haplotypes in homozygous condition is expected to yield drive that is undetectable). (B) Sequence differentiation between SB (N = 60) and Sb (N = 20) social chromosomes from native (South American) S. invicta, as measured by differences in the pooled SB nucleotide diversity (π) and the median values of absolute sequence divergence (dxy) between individual chromosomes of each type along 5 kb non-overlapping windows. Values significantly greater than zero (substantially above blue line), consistent with recombination of Sb segments into SB haplotypes, are rare and involve small segments. Asterisks indicate three such instances of likely recombination of ≈5-10 kb-size elements. Positions of the three inversions contained within the supergene are shown by the blue bars. (C) Linkage disequilibrium (LD, measured as r2 values) in native S. invicta along the SB social chromosome (N = 60). Outside of the centromere, LD is weak, as expected given the lack of sizeable inversions and predicted resultant free recombination. Data for Panels B and C were obtained from whole-genome sequence assemblies (Y. Zheng et al., unpublished; [104]). Centromere is not shown to scale in any of the figures. (TIF 1.53 mb)