The fire ant social chromosome supergene variant Sb shows low diversity but high divergence from SB.
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Variation in social behavior is common yet little is known about the genetic architectures underpinning its evolution. A rare exception is in the fire ant Solenopsis invicta: Alternative variants of a supergene region determine whether a colony will have exactly one or up to dozens of queens. The two variants of this region are carried by a pair of "social chromosomes", SB and Sb, which resemble a pair of sex chromosomes. Recombination is suppressed between the two chromosomes in the supergene region. While the X-like SB can recombine with itself in SB/SB queens, recombination is effectively absent in the Y-like Sb because Sb/Sb queens die before reproducing. Here, we analyze whole genome sequences of eight haploid SB males and eight haploid Sb males. We find extensive SB-Sb di↵erentiation throughout the >19Mb long supergene region. We find no evidence of "evolutionary strata" with different levels of divergence comparable to those reported in several sex chromosomes. A high proportion of substitutions between the SB and Sb haplotypes are nonsynonymous, suggesting inefficacy of purifying selection in Sb sequences, similar to that for Y-linked sequences in XY systems. Finally, we show that the Sb haplotype of the supergene region has 635-fold less nucleotide diversity than the rest of the genome. We discuss how this reduction could be due to a recent selective sweep affecting Sb specifically or associated with a population bottleneck during the invasion of North America by the sampled population. This article is protected by copyright. All rights reserved.
AuthorsPracana, R; Priyam, A; Levantis, I; Nichols, RA; Wurm, Y
- College Publications