Genome-wide SNPs data and past distribution models reveal genetic admixture and isolation in the range-restricted <i>Psittacanthus auriculatus</i> mistletoe species

The study of speciation via adaptation to different hosts is a crucial process in the evolution of parasitism. However, historical isolation and ecological factors also contribute to explaining genetic structure across space, particularly for species with restricted host ranges, alongside factors like reproductive systems, genetic drift, and mutations. Using genome-wide data, we determine the genetic structure and genetic diversity in Psittacanthus auriculatus (Loranthaceae) across its restricted geographical range in the xeric valleys surrounded by complex mountain ranges in Oaxaca. We used single nucleotide polymorphisms (SNPs) from 69 genotyped individuals of this hemiparasitic mistletoe from eight populations obtained through Genotyping by Sequencing (GBS). We used these genomic data combined with species distribution and ecological modelling and Approximate Bayesian Computation (ABC) methods, to infer the evolutionary history of P. auriculatus populations. Our analyses yielded two genetic groups, one composed of individuals from the western valleys (northern locations), and the other composed of individuals from the central valleys (southern locations) in Oaxaca, with further genetic substructure in one of the northern locations historically isolated. A scenario of habitat fragmentation during the Late Pleistocene was highly supported by species distribution modelling, in which the predicted distribution of P. auriculatus was contracted and fragmented during the Last Glacial Maximum (LGM) and expanded during the interglacials (Last Inter Glacial and present) for a more connected distribution. Our results underline historical isolation and environmental factors in moulding population genetic differentiation and structure of P. auriculatus mistletoes.