The role of hybridization during ecological divergence of southwestern white pine (Pinus strobiformis) and limber pine (P. flexilis)

Abstract

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.

Combining mitochondrial and nuclear genome analyses to dissect the effects of colonization, environment, and geography on population structure in Pinus tabuliformis

“…Restriction‐site‐associated DNA sequencing (e.g., genotyping‐by‐sequencing, GBS) is an effective approach for generating genetic data over thousands of loci and is increasingly being used to obtain population genomic perspective in non‐model forest trees (Parchman, Jahner, Uckele, Galland, & Eckert, ). However, to date, only a few investigations have explored GBS for dissecting the organization of genetic variation across landscapes in conifers (Johnson, Gaddis, Cairns, Konganti, & Krutovsky, ; Menon et al., ), probably due to difficulties in optimizing the technique in complex conifer genome analysis.…”

“…Restriction-site-associated DNA sequencing (e.g., genotyping-by-sequencing, GBS) is an effective approach for generating genetic data over thousands of loci and is increasingly being used to obtain population genomic perspective in non-model forest trees (Parchman, Jahner, Uckele, Galland, & Eckert, 2018). However, to date, only a few investigations have explored GBS for dissecting the organization of genetic variation across landscapes in conifers (Johnson, Gaddis, Cairns, Konganti, & Krutovsky, 2017;Menon et al, 2018), probably due to difficulties in optimizing the technique in complex conifer genome analysis. This study aims: (a) to gain a better view of genomewide patterns of diversity and population structure in P. tabuliformis; (b) to reconstruct the recent demographic history of this species; and (c) to evaluate the impact of environment, geography, and colonization history on intraspecific differentiation.…”

An Economical Approach to Distinguish Genetically Needles of Limber from Whitebark Pine

“…Our results, derived mainly from sampling the region of the Greater Yellowstone Ecosystem, should be general to the entire range of limber and whitebark pine in western North America. This prediction stems from the well-established phylogenetic relationships that resolve limber pine as an ancient North American resident and the distantly related whitebark pine as a Pleistocene immigrant into North America (e.g., [12,14,15]).…”

“…These phylogenetic studies resolve limber pine within a North American clade that includes such species as southwestern white pine (Pinus strobiformis Engelm.). Limber pine and closely related species have been in residence in western North America for millions of years [15]. In contrast, the above phylogenetic studies resolve whitebark pine within a primarily Eurasian clade and it is closely related to such species as Swiss stone pine (Pinus cembra L.) and Siberian pine (Pinus sibirica Du Tour).…”

Surface Fire to Crown Fire: Fire History in the Taos Valley Watersheds, New Mexico, USA

“…Nutt.). Southwestern white pine and P. flexilis James (limber pine) likely hybridized within the study area [18], but we considered the sampled fire-scarred trees and remnant wood to be southwestern white pine due to the growth form, site conditions, and elevation.…”