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Tracing the origin of a social chromosome in the red fire ant

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Genetic basis of red fire ant social organisation

Red fire ants (Solenopsis invicta) demonstrate complex phenotypic innovation, which is associated with how their communities are organised. The phenotypic variation within a species offers unique opportunities to understand the contexts in which traits evolve and how they affect fitness.

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The transition from single-queen colonies of red fire ants to multi-queen colonies and the associated behavioural, physiological and morphological changes is related to significant structural change in the majority of a chromosome. However, little is known in detail about the molecular changes underpinning such processes with regard to ecology. The project SOCIAL CHROMOSOME (Tracing the origin of a social chromosome in the red fire ant) addressed this knowledge gap by sequencing the ant’s genome to determine the mechanisms behind social polymorphism. Scientists uncovered the origin and evolution of the social chromosome by combining structural analyses with genetic and genomic analyses using optical mapping technology. The results were used to directly compare genome structures. They indicated that 3 different socially polymorphic fire ant species shared two large inversions among the non-recombining supergene variant. A supergene is a group of neighbouring genes on a chromosome that are inherited together due to their close genetic linkage and are functionally related in an evolutionary sense. Similarity between species and supergene variants in terms of presence or absence of structural variants suggested a common single ancestor for the social chromosome. Based on molecular dating, the divergence between both forms was estimated to be about a million years ago, which was older than previously believed. Scientists also re-sequenced the genome of 160 haploid males from all sampled phylogenetic groups in order to create a phylogenetic tree for each target locus or gene. Results showed high differentiation and also substantial genetic diversity between non-recombining variants of the supergene across populations and between species. This unexpected level of diversity and differentiation suggested the presence of high levels of gene conversion or hidden recombinations. SOCIAL CHROMOSOME work will help shed light on how the two types of red fire ant colony organisation are formed. It will also have a major impact on research into structural variation, and is of particular interest to those working on non-model organisms.


Red fire ants, social organisation, SOCIAL CHROMOSOME, genome, polymorphism, supergene

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