Hybridization is a common phenomenon in plants, facilitating genetic exchange and contributing to evolutionary innovation. In Penstemon, which boasts a diverse array of species with varying pollination syndromes, hybridization has been extensively documented. In this study, we employed nine nuclear microsatellites to investigate the genetic differentiation between Penstemon gentianoides and P. roseus populations, along with two populations of potentially hybrid morphotypes. Our results reveal significant genetic differentiation among populations, particularly between sympatric and allopatric populations of P. gentianoides and P. roseus. Bayesian analyses identified three genetic clusters, the first corresponding to individuals of sympatric and allopatric P. roseus populations, and the second and third to intermixed individuals of sympatric and allopatric P. gentianoides populations and intermediate morphotypes. Sympatric populations of both parental species exhibit signs of admixture. Contemporary and historical migration rates were low and asymmetrical among populations, with higher migration rates from the “fuchsia” morphotype to P. gentianoides populations. The significant and positive correlation between migration rates suggests that migration patterns have remained relatively stable over time. These findings underscore the complex dynamics of genetic exchange and hybridization between Penstemon populations, highlighting the need for further research to understand population differentiation in the presence of gene flow.

We collected a total of 89 plant samples representing two Penstemon species (P. gentianoides and P. roseus) and intermediate morphotypes found at different elevations on the northern side of the volcano La Malinche: P. gentianoides in allopatry (n = 13 samples), P. gentianoides (n = 13), “fuchsia” (n = 20) and “blue” (n = 20) morphotypes in sympatry, and two populations of P. roseus in allopatry (n = 13; Fig. 1A). All leaf samples were dried with silica gel immediately upon collection. DNA was extracted from lyophilized leaf tissues using the cetyl trimethyl ammonium bromide (CTAB) protocol (Doyle and Doyle 1987). To measure genetic variation among samples from different locations, we used nine Penstemon-specific loci of the nuclear microsatellite (nSSRs) set that was developed to be useful across the genus (Kramer and Fant 2007, Dockter et al. 2013, Anderson et al. 2016) and parameters were described in Anderson et al. 2016.