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To explore landscape genomics at the range limit of an obligate mutualism, we use genotyping‐by‐sequencing (ddRADseq) to quantify population structure and the effect of host–symbiont interactions between the northernmost fungus‐farming leafcutter ant Atta texana and its two main types of cultivated fungus. Genome‐wide differentiation between ants associated with either of the two fungal types is of the same order of magnitude as differentiation associated with temperature and precipitation across the ant's entire range, suggesting that specific ant–fungus genome–genome combinations may have been favoured by selection. For the ant hosts, we found a broad cline of genetic structure across the range, and a reduction of genetic diversity along the axis of range expansion towards the range margin. This population‐genetic structure was concordant between the ants and one cultivar type (M‐fungi, concordant clines) but discordant for the other cultivar type (T‐fungi). Discordance in population‐genetic structures between ant hosts and a fungal symbiont is surprising because the ant farmers codisperse with their vertically transmitted fungal symbionts. Discordance implies that (a) the fungi disperse also through between‐nest horizontal transfer or other unknown mechanisms, and (b) genetic drift and gene flow can differ in magnitude between each partner and between different ant–fungus combinations. Together, these findings imply that variation in the strength of drift and gene flow experienced by each mutualistic partner affects adaptation to environmental stress at the range margin, and genome–genome interactions between host and symbiont influence adaptive genetic differentiation of the host during range evolution in this obligate mutualism. [ABSTRACT FROM AUTHOR]

Ants are widely recognized as ecologically important members of many low- to mid-latitude ecosystems. Surprisingly, there is very little phylogeographical information on ants at regional scales. We examine here the phylogeography of two partially sympatric species of Trachymyrmex ( Trachymyrmex septentrionalis and Trachymyrmex turrifex) ants in southeastern North America. We test the hypothesis that all Trachymyrmex species found in the USA expanded into North America from refugial populations located in northern Mexico as the post- Pleistocene climate warmed. Phylogeographical theory predicts that these northward-expanding species should exhibit higher genetic diversity in regions closer to Mexico and less diversity in more northern regions. We also examine, in the widely distributed T. septentrionalis, the hypothesis of vicariance that occurred at the formation of the Mississippi Embayment. Phylogeographical patterns indicate that T. septentrionalis has an eastern origin because diversity was highest east of the Mississippi, whereas T. turrifex probably has a Mexican origin because it lacked mitochondrial DNA (mt DNA) variation throughout its range and is currently absent from eastern North America. Both species are characterized by reduced haplotypic variation in the western coastal plain of the Gulf of Mexico ( Texas and Louisiana), which may indicate recent expansion and/or bottlenecks associated with increased aridity and drought in these western regions. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114, 689-698. [ABSTRACT FROM AUTHOR]