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Finding a (pine) needle in a haystack: chloroplast genome sequence divergence in rare and widespread pines

Authors


  • Rich Cronn (US Forest Service, Pacific Northwest Research Station), Aaron Liston (Oregon State University) and PhD student Matt Parks are developing methods to facilitate the application of genomic data to population genetic and phylogenetic questions in non-model organisms. John Syring (Linfield College) studies the systematics, population genetics, and evolution of pines. Justen Whittall (Santa Clara University) studies plant evolution, applying genome-scale approaches to understanding the process of adaptation and speciation. Jason Buenrostro and Cindy Dick are members of the Whittall lab, and are responsible for validating Illumina SNPs.

Richard Cronn, Fax: (541) 750-7329; E-mail: rcronn@fs.fed.us

Abstract

Critical to conservation efforts and other investigations at low taxonomic levels, DNA sequence data offer important insights into the distinctiveness, biogeographic partitioning and evolutionary histories of species. The resolving power of DNA sequences is often limited by insufficient variability at the intraspecific level. This is particularly true of studies involving plant organelles, as the conservative mutation rate of chloroplasts and mitochondria makes it difficult to detect polymorphisms necessary to track genealogical relationships among individuals, populations and closely related taxa, through space and time. Massively parallel sequencing (MPS) makes it possible to acquire entire organelle genome sequences to identify cryptic variation that would be difficult to detect otherwise. We are using MPS to evaluate intraspecific chloroplast-level divergence across biogeographic boundaries in narrowly endemic and widespread species of Pinus. We focus on one of the world’s rarest pines – Torrey pine (Pinus torreyana) – due to its conservation interest and because it provides a marked contrast to more widespread pine species. Detailed analysis of nearly 90% (∼105 000 bp each) of these chloroplast genomes shows that mainland and island populations of Torrey pine differ at five sites in their plastome, with the differences fixed between populations. This is an exceptionally low level of divergence (1 polymorphism/∼21 kb), yet it is comparable to intraspecific divergence present in widespread pine species and species complexes. Population-level organelle genome sequencing offers new vistas into the timing and magnitude of divergence within species, and is certain to provide greater insight into pollen dispersal, migration patterns and evolutionary dynamics in plants.

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