Genomic and phenotypic architecture of a spruce hybrid zone (Picea sitchensis × P. glauca)

Abstract

Interspecific hybridization may enhance the capacity of populations to adapt to changing environments, and has practical implications for reforestation. We use genome-wide estimates of admixture and phenotypic traits for trees in a common garden to examine the extent and direction of gene flow across a Picea hybrid zone, testing assumptions of the bounded hybrid superiority and tension zone models of hybrid zone maintenance. Seeds were collected from the ecological transition zone spanning from maritime to continental climates across the Picea sitchensis-P. glauca contact zone, and 721 trees were planted in a common garden experiment within the hybrid zone. Individuals were genotyped using a panel of 384 candidate-gene single nucleotide polymorphisms (SNPs) putatively associated with adaptive traits in Picea, and phenotyped at age ten for height and autumn cold hardiness. Low interspecific heterozygosity in hybrids indicated that intrinsic reproductive barriers were too weak to prevent widespread recombination, although introgression appeared asymmetric with P. sitchensis dominating the zone. Whereas marker-based hybrid index was strongly correlated with climate and geography, phenotypic traits exhibited weak or no significant clines. Our results indicated that exogenous selection appeared to play a strong role in the distribution and structure of this hybrid zone, indicative of an environmentally determined bounded hybrid superiority model of hybrid zone maintenance, although endogenous mechanisms could not be ruled out. This study provides insight into the mechanisms underlying adaptation across ecologically transitional hybrid zones that will ultimately provide an additional tool in managing these economically important tree species.

Fine-scale population structure and asymmetrical dispersal in an obligate salt-marsh passerine, the Saltmarsh Sparrow(Ammodramus caudacutus)

“…We documented a high proportion of admixed individuals (52%) among our sampled marshes but an overall deficit of recent-generation (F 1 -F 2 ) hybrids, with only 3% (n ¼ 8) of sampled individuals assigned to the F 1 -F 2 category. A low frequency of F 1 -F 2 individuals indicates an advanced-generation hybrid zone characterized by high rates of recombination and limited reproductive isolation between 2 species (Culumber et al 2010, Hamilton et al 2013. Given the recent divergence between Nelson's and Saltmarsh sparrows (~600,000 yr; Rising and Avise 1993), coupled with the typically slow rate of evolution of postzygotic incompatibilities in birds (Price and Bouvier 2002), a finding of frequent backcrossing events in this system is not unexpected.…”

Relationship of phenotypic variation and genetic admixture in the Saltmarsh–Nelson's sparrow hybrid zone

“…We documented a high proportion of admixed individuals (52%) among our sampled marshes but an overall deficit of recent-generation (F 1 -F 2 ) hybrids, with only 3% (n ¼ 8) of sampled individuals assigned to the F 1 -F 2 category. A low frequency of F 1 -F 2 individuals indicates an advanced-generation hybrid zone characterized by high rates of recombination and limited reproductive isolation between 2 species (Culumber et al 2010, Hamilton et al 2013. Given the recent divergence between Nelson's and Saltmarsh sparrows (~600,000 yr; Rising and Avise 1993), coupled with the typically slow rate of evolution of postzygotic incompatibilities in birds (Price and Bouvier 2002), a finding of frequent backcrossing events in this system is not unexpected.…”

Range shift and introgression of the rear and leading populations in two ecologically distinct Rubusspecies

“…(b) Interspecific heterozygosity vs hybrid index based on SNPs of 17 nuclear genes. Colored dots indicate broadly defined hybrid classes [63]; BCG as backcrosses with R. grayanus ; BCP as backcrosses with R. palmatus ; FN as advanced-generation hybrids.…”

“…Based on the STRUCTURE analysis of the contact zones, we defined pYK* and ab1 populations as parental R. palmatus and R. grayanus populations, respectively. We broadly defined the hybrid classes, as suggested by Hamilton et al [63] (i.e. backcrosses to either parent when hybrid index is more than 85% but not 100%; advanced-generation hybrids (FN) when hybrid index is more than 15% and less than 85%).…”