Uncovering Ghost Introgression Through Genomic Analysis of a Distinct East Asian Hickory Species

Although the possibility of introgression from ghost lineages (all unsampled extant and extinct taxa) is now widely recognized, detecting and characterizing ghost introgression remains a challenge. Here, we propose a combined use of the popular D-statistic method, which tests for the presence of introgression, and the full-likelihood method BPP, which determines which of the possible gene-flow scenarios, including ghost introgression, is truly responsible. We illustrate the utility of this approach by investigating the reticulation and bifurcation history of the genus Carya (Juglandaceae), including the beaked hickory Carya sinensis. To achieve this goal, we generated two chromosome-level reference genomes respectively for C. sinensis and C. cathayensis. Furthermore, we re-sequenced the whole genomes of 43 individuals from C. sinensis and one individual from each of the 11 diploid species of Carya. The latter dataset with one individual per species is used to reconstruct the phylogenetic networks and estimate the divergence time of Carya. Our results unambiguously demonstrate the presence of ghost introgression from an extinct lineage into the beaked hickory, dispelling certain misconceptions about the phylogenetic history of C. sinensis. We also discuss the profound implications of ghost introgression into C. sinensis for the historical biogeography of hickory species. [BPP; Carya; D-statistic; gene flow; ghost introgression]