TY - JOUR T1 - The role of hybridization during ecological divergence of southwestern white pine (Pinus strobiformis) and limber pine (P. flexilis) JF - Molecular Ecology Y1 - 2018 A1 - Mitra Menon A1 - Justin C. Bagley A1 - Christopher J. Friedline A1 - Amy V. Whipple A1 - Anna W. Schoettle A1 - Alejandro Leal-Saenz A1 - Christian Wehenkel A1 - Francisco Molina-Freaner A1 - Lluvia Flores-Renterıa A1 - M. Socorro Gonzalez-Elizondo A1 - Richard A. Sniezko A1 - Samuel A. Cushman A1 - Kristen M. Waring A1 - Andrew J. Eckert KW - conifers KW - ecological speciation KW - extrinsic barriers KW - hybrid zone KW - introgression KW - population genomics AB -
Interactions between extrinsic factors, such as disruptive selection and intrinsic factors,
such as genetic incompatibilities among loci, often contribute to the maintenance
of species boundaries. The relative roles of these factors in the establishment
of reproductive isolation can be examined using species pairs characterized by gene
flow throughout their divergence history. We investigated the process of speciation
and the maintenance of species boundaries between Pinus strobiformis and Pinus
flexilis. Utilizing ecological niche modelling, demographic modelling and genomic
cline analyses, we illustrated a divergence history with continuous gene flow. Our
results supported an abundance of advanced generation hybrids and a lack of loci
exhibiting steep transition in allele frequency across the hybrid zone. Additionally,
we found evidence for climate-associated variation in the hybrid index and niche
divergence between parental species and the hybrid zone. These results are consistent
with extrinsic factors, such as climate, being an important isolating mechanism.
A build-up of intrinsic incompatibilities and of coadapted gene complexes is also
apparent, although these appear to be in the earliest stages of development. This
supports previous work in coniferous species demonstrating the importance of extrinsic factors in facilitating speciation. Overall, our findings lend support to the
hypothesis that varying strength and direction of selection pressures across the long
lifespans of conifers, in combination with their other life history traits, delays the
evolution of strong intrinsic incompatibilities.