Lack of interruption of the gene network underlying wing polyphenism in an early-branching ant genus

Ants evolved about 140 million years ago and have diversified into more than 15,000 species with tremendous ecological and morphological diversity, yet evolution of the gene regulatory networks (GRNs) underlying this diversification remains poorly understood. Wing polyphenism, the ability of a single genome to produce either winged or wingless castes during development in response to environmental cues, is a nearly universal feature of ants. The underlying wing GRN is evolutionarily labile in worker castes of phylogenetically derived species: it is conserved in winged castes but interrupted at different points in wingless castes of different species. However, it remains unknown whether the wing GRN is interrupted in wingless castes of species from early branching lineages, and if so, whether it is interrupted at similar locations in worker castes of derived species. We therefore used in situ hybridization to assay the expression of nine genes in the wing GRN in wing imaginal discs of larvae from two species from the early branching ('basal') genus Mystrium. These species possess two castes each: Mystrium rogeri has winged queens and wingless workers, and M. oberthueri has wingless queens and wingless workers. In contrast to derived species, we found no evidence of interruption points in the wing GRN kernel of wingless castes. Our finding supports: (1) a "phylogenetic ladder model" of wing GRN evolution, where interruption points move further upstream in the wing GRN as ant lineages become more derived; and (2) that evolutionary lability of the GRN underlying wing polyphenism originated later during ant evolution.