Genetic architecture and genomic patterns of gene flow between hybridizing species of Picea.

A De La Torre,S N Aitken,P K Ingvarsson

doi:10.1038/hdy.2015.19

A De La Torre, S N Aitken + Show 1 more

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https://doi.org/10.1038/hdy.2015.19

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Abstract

Hybrid zones provide an opportunity to study the effects of selection and gene flow in natural settings. We employed nuclear microsatellites (single sequence repeat (SSR)) and candidate gene single-nucleotide polymorphism markers (SNPs) to characterize the genetic architecture and patterns of interspecific gene flow in the Picea glauca × P. engelmannii hybrid zone across a broad latitudinal (40–60 degrees) and elevational (350–3500 m) range in western North America. Our results revealed a wide and complex hybrid zone with broad ancestry levels and low interspecific heterozygosity, shaped by asymmetric advanced-generation introgression, and low reproductive barriers between parental species. The clinal variation based on geographic variables, lack of concordance in clines among loci and the width of the hybrid zone points towards the maintenance of species integrity through environmental selection. Congruency between geographic and genomic clines suggests that loci with narrow clines are under strong selection, favoring either one parental species (directional selection) or their hybrids (overdominance) as a result of strong associations with climatic variables such as precipitation as snow and mean annual temperature. Cline movement due to past demographic events (evidenced by allelic richness and heterozygosity shifts from the average cline center) may explain the asymmetry in introgression and predominance of P. engelmannii found in this study. These results provide insights into the genetic architecture and fine-scale patterns of admixture, and identify loci that may be involved in reproductive barriers between the species.

Highlights

The genetic architecture of hybrid zones provides the key to understand the sequence of genetic changes that accompany or facilitate speciation (Lexer et al, 2005)
Characterizing genetic architecture allows the study of hybrid zones as evolutionary filters; hybrid zones may allow the spread of advantageous alleles from one species to another; alleles that are adaptive in one species or environment and maladaptive in the other can be maintained (Barton and Gale, 1993; Lexer et al, 2005; Abbott et al, 2013)
Genetic architecture and introgression based on single sequence repeat (SSR) markers Bar plots of posterior estimates of cluster memberships and the frequency distribution of hybrid classes revealed an extensive hybrid zone with a wide variety of hybrid classes, in which hybrid composition exhibited clines along latitude and elevation corresponding to climatic gradients in temperature and precipitation (Figure 1; Supplementary Figure S1)

Summary

Introduction

The genetic architecture of hybrid zones provides the key to understand the sequence of genetic changes that accompany or facilitate speciation (Lexer et al, 2005). It provides information about the interplay between gene flow as homogenizing force facilitating the spread of alleles and maintaining species as cohesive units, and natural selection promoting population divergence (Morjan and Rieseberg, 2004). Hybridization generates new genetic combinations of parental alleles that can be tested by selection These new genetic recombinants may have advantages over parental species because of transgressive segregation or adaptive introgression, or disadvantages because of negative epistatic interactions (Arnold et al, 2012; Abbott et al, 2013). Natural hybrid zones may provide important clues for the identification of important adaptive traits in species with wide distributions and long generations such as trees (Lexer et al, 2007; Lindtke et al, 2012)

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