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An ancient and eroded social supergene is widespread across Formica ants

Citation

Brelsford, Alan et al. (2020), An ancient and eroded social supergene is widespread across Formica ants, Dryad, Dataset, https://doi.org/10.6086/D1KD40

Abstract

Supergenes, clusters of tightly linked genes, play a key role in the evolution of complex adaptive variation. While supergenes have been identified in many species, we lack an understanding of their origin, evolution and persistence. Here, we uncover 20-40 MY of evolutionary history of a supergene associated with polymorphic social organization in Formica ants. We show that five Formica species exhibit homologous divergent haplotypes spanning 11 Mbp on chromosome 3. Despite the size of the supergene, only 142 single nucleotide polymorphisms (SNPs) consistently distinguish alternative supergene haplotypes across all five species. These conserved trans-species SNPs are localized in a small number of disjunct clusters that are distributed across the supergene. This unexpected pattern of divergence indicates that the Formica supergene does not follow standard models of sex chromosome evolution, in which distinct evolutionary strata reflect the history of an expanding region of suppressed recombination. We propose an alternative “eroded strata model,” by which clusters of conserved trans-species SNPs represent functionally important areas maintained by selection in the face of rare recombination between ancestral haplotypes. The comparison of whole genome sequences across 10 additional Formica species combined with a chromosome-level F. selysi genome assembly reveal that the most conserved region of the supergene contains the gene Knockout, a transcription factor essential for motor neuron development in Drosophila. The discovery that a very small portion of this large and ancient supergene harbors conserved trans-species SNPs linked to colony social organization suggests that the ancestral haplotypes have been eroded by recombination, with selection preserving differentiation at one or a few genes generating alternative social organization.

Methods

Linkage maps were constructed from ddRAD data for 67 males from a monogyne colony (F. exsecta) and 112 offspring from two F. selysi queens (F. selysi Sp haplotype) using MSTmap.

Usage Notes

EcoRI_ddRAD_oligos: All oligonucleotides were ordered as unmodified DNA, with standard desalting, for use with the custom ddRAD protocol of Brelsford et al. 2016, Heredity, modified to use EcoRI in place of SbfI.

Funding

Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Award: 31003A-173189

Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Award: 31003A-146641

Université de Lausanne

University of California, Riverside