Strategies to Identify Adaptive Genes in Hybridizing Trees like Oaks and Poplars

Abstract

Ecologically divergent, hybridizing species such as oaks and poplars provide models to identify genomic regions under selection and adaptive alleles that are transferred between species in hybrid zones. Oaks show patterns of genomic divergence characteristic for early stages of speciation with gene flow, in which large genomic regions are homogenized by interspecific gene flow interspersed by smaller regions (outlier regions) with high interspecific differentiation as result of divergent selection. These outlier regions can be identified using genome scans in hybrid zones and anchored to the Quercus robur genome sequence which will become available in the near future. Combined outlier and association genetic approaches can assess the role of individual genes in outlier genomic regions in adaptive trait variation. In contrast, hybridizing poplar species show a pattern of genomic divergence with large genomic regions of high interspecific differentiation punctuated by smaller regions of low differentiation as the result of interspecific gene flow. Genome scans in multiple hybrid zones of interfertile poplar species and in populations outside the area of sympatry will allow for the identification of genes that are exchanged between species by interspecific gene flow using the Populus trichocarpa genome sequence as a reference. Again association genetic approaches can be used for the characterization of variation in these introgressed genes with adaptive trait variation. In the present paper, the application of genomic approaches to identify genes for adaptive species divergence and reproductive isolation, and introgressed genes between species is discussed.

Genetic structure of Populus hybrid zone along the Irtysh River provides insight into plastid-nuclear incompatibility

“…P. alba and P. tremula are model species in which to study the later stages of tree speciation, and the maintenance of species identity with gene flow 36 . Our current analysis of the P. alba and P. tremula hybrid zone along the Irtysh River further supports the concept that high hybridization rates and appreciable hybrid fitness do not necessarily conflict with the maintenance of species integrity 31 .…”

Sharing of chloroplast haplotypes among red oak species suggests interspecific gene flow between neighboring populations

“…The frequency of hybridization between interfertile species is dependent on pre-and post-zygotic isolation mechanisms, both of which can be affected by the environment (Seehausen et al 2014). Observational and experimental evidence suggests that selection is a major post-zygotic mechanism in the maintenance of species integrity in hybridizing oaks (Dodd and Afzal-Rafii 2004;Curtu et al 2007Curtu et al , 2009de Heredia et al 2009;Gailing and Curtu 2014;Gailing 2014). Thus, related oak species frequently hybridize in sympatry (Rushton 1993), yet they remain phenotypically and genetically distinct and maintain specific local adaptations, for example to drought (Abrams 1990(Abrams , 1992Lévy et al 1992;Brendel et al 2008).…”