Co-phylogeny and biogeography of the myrmecophilous beetle Paussus favieri (Carabidae, Paussinae) and its host ant Pheidole pallidula (Hymenoptera, Myrmicinae)

Strict patterns of co-phylogeny have seldom been observed, except among organisms and their symbionts with limited dispersal abilities. In this study, we investigate potential signs of co-phylogeny at the population level between an obligate myrmecophile, the beetle Paussus favieri, and its host ant, Pheidole pallidula. While neither species is physically dependent on the other, as both are fully winged and capable of independent dispersal, Paussus favieri relies entirely on Pheidole pallidula throughout its life cycle. These predatory beetles feed on ants and reproduce in their nests, where they lay eggs and their larvae develop. Therefore, the beetle cannot survive or reproduce without its host, making this an interesting system to explore potential co-phylogenetic patterns. In this paper, we infer population-level phylogenies for both species based on molecular sequence data and apply distance-based and event-based co-phylogenetic methods to search for signs of co-phylogeny, codivergence, or evidence of frequent host population shifts. Molecular phylogenetics reveals significant co-phylogenetic signals, but not phylogenetic congruence, using distance-based methods, as would be expected if the populations of both species shared a similar evolutionary or biogeographic history, without a strict evolutionary dependency. Co-phylogenetic signal without phylogenetic congruence is further explained by event-based methods with a history of codiversification and host population switching, typically occurring among nearby, closely related populations. We discuss the putative mechanisms that might have driven the co-phylogenetic signal between these strictly myrmecophilous beetles and their host ants with particular emphasis on a shared biogeographic scenario within the complex biogeographic history of the Mediterranean Basin.