Genetic, morphological, and niche variation in the widely hybridizing Rhus integrifolia-Rhus ovata species complex

Hybridization and introgression are common processes among numerous plant
species that present both challenges and opportunities for studies of
species delimitation, phylogenetics, taxonomy, and adaptation. Rhus
integrifolia and R. ovata are two ecologically important shrubs native to
the southwestern USA and Mexico, and are known to hybridize frequently,
but the morphological, genetic, and ecological implications of
hybridization in these species are poorly studied on a broad geographic
scale. Analyses were conducted using leaf morphology, genetic variation of
plastid and nuclear loci, and species distribution models for both species
and their putative hybrid introgressants across 19 localities in
California and Arizona, USA. These analyses revealed evidence for
morphological and genetic distinction among localities comprising putative
parental species, but a high degree of morpho-genetic intermediacy among
localities with putative hybrids. Comparison of morphological and genetic
population structure among localities revealed evidence for putative local
adaptation or widespread phenotypic plasticity. Multiple regression models
identified a weak but statistically significant negative association
between leaf area and precipitation. Finally, species distribution
modeling inferred northward range shifts over time, with both species
predicted to occupy more coastal regions in the future, possibly
increasing the frequency of hybridization among them. These findings
underscore the importance of integrative assessment of multiple data
sources in the study of hybridizing species and highlight the Rhus
integrifolia-ovata complex as a powerful model for investigating the
adaptive implications of hybridization.