Constructing sympatry networks to assess potential introgression pathways within the major oak sections in the contiguous US states

Abstract

Societal Impact Statement Increasing evidence indicates gene flow commonly occurs between closely related species in diverse plant genera and can involve numerous species. Here, we present a simple method to quantify and characterize the potential for gene flow among “sympatric” species (they share some part of their geographic distribution), using oaks as a case study. The resulting sympatry networks provide insight into potential interspecific gene flow, from the perspective of each species and across the entire suite of interfertile species. In a rapidly changing world, interspecific gene flow will play an increasingly important role in the conservation of endangered species, both positive and negative. Summary We provide a simple method, using geographic distribution data, to quantify and characterize sympatry networks among a suite of interfertile species to assess potential routes for introgressive hybridization (IH). Sympatry is a necessary condition for IH and is easily determined from widely available geographic distribution data. Oaks, famous for hybridization, present an excellent case study. We use county‐level data for 94 oak species in the contiguous US states to determine patterns of sympatry of species within the four sections, assumed to be interfertile. The pairwise sympatry relationships can generate species‐centric local networks or a syngameon‐wide network. Two different measures of sympatry are used: proportional and relative. Most oak species are exposed to IH from many species, with red oaks having slightly higher local levels of sympatry (5.7 vs. 3.8 spp per county) than white oaks but lower total numbers of sympatric species (13.7 vs. 14.8). The nature of potential IH generally differs for threatened species, which are often embedded in the much larger distributions of widespread species. Potential IH within oak sections is widespread and disproportionately affects rare species. Given that IH has potentially positive or negative impacts on species cohesion, persistence, and adaptation, the geographic connections among species and the ‘local sympatry effect’ can serve as a springboard for hypothesis generation and genetic investigation of introgression patterns.