Insights into the population genetics of an extreme habitat specialist, the wood ant commensal Formicoxenus nitidulus

1. Habitat specialists, particularly those that are poor dispersers, are highly susceptible to population isolation as a result of habitat fragmentation. Population isolation can lead to inbreeding, resulting in reduced genetic diversity and an increased risk of local extinction. 2. The shining guest ant, Formicoxenus nitidulus, lives only within the nests of its wood ant hosts. It is thus an extreme habitat specialist, dependent on patchy nesting sites within an already fragmented woodland habitat. We aimed to generate the first data on the population genetics of this species, to characterize its genetic diversity and degree of population isolation. 3. We developed eight novel nuclear microsatellite markers and generated mitochondrial DNA sequence data of the COI region to characterize samples from seven UK F. nitidulus populations collected from nests of three wood ant hosts: Formica aquilonia, Formica lugubris and Formica rufa. These novel nuclear microsatellite markers can be used in future studies of this species across a wider geographic range and may have utility in other closely related species. 4. We found clear differentiation between Scottish and English F. nitidulus populations. The six largest study populations were genetically diverse, showing and showed signs of effective within-site dispersal and low inbreeding. Our data show that wood ant nests often host multiple F. nitidulus colonies. 5. We found genetic diversity has been maintained in this extreme habitat specialist at risk of population isolation. We also demonstrate that a single wood ant host nest can have high conservation importance for the multiple F. nitidulus colonies it supports.