Adaptive Variation and Introgression of a CONSTANS-Like Gene in North American Red Oaks

Jennifer Lind-Riehl,Oliver Gailing

doi:10.3390/f8010003

Jennifer Lind-Riehl, Oliver Gailing

Open Access

https://doi.org/10.3390/f8010003

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Abstract

Oaks provide a model system to study maintenance of species identity by divergent selection since they maintain morphological differences and ecological adaptations despite interspecific hybridization. The genome of closely related interfertile oak species was shown to be largely homogeneous, with a few genomic areas exhibiting high interspecific differentiation possibly as result of strong divergent selection. Previously, a genic microsatellite was identified as under strong divergent selection, being nearly fixed on alternative alleles in the two interfertile North American red oak species: Quercus rubra L. and Quercus ellipsoidalis E.J. Hill. Further genotyping in two other red oak species—Quercus velutina Lam. and Quercus coccinea Münchh.—revealed a similar bias for the Q. ellipsoidalis-specific allele. To further elucidate the basis of this differentiation, we sequenced the microsatellite in individuals from all four red oak species. Sequence variability was observed in the microsatellite motif which encodes a poly-Q repeat in a COL gene involved in phenology and growth. Furthermore, in neighboring (parapatric) Q. rubra/Q. ellipsoidalis populations, introgression of the Q. ellipsoidalis-specific allele into Q. rubra occurred at a lower rate than introgression of the Q. rubra-specific allele into Q. ellipsoidalis despite symmetric interspecific gene flow, indicating potential adaptive introgression. Introgression of adaptive alleles can be an important mechanism for rapid adaptation to new environmental conditions (e.g., climate change).

Highlights

The transfer of adaptive alleles and traits by hybridization might be an important mechanism of rapid evolution and adaptation to changing environments, e.g., in the face of climate change, and evidence for adaptive trait transfer has been reported in both plant and animal species
Materials and Methods samples were obtained from 16 populations of four red oak species
Quercus velutina and Q. coccinea share the same common allele (138 bp) with Q. ellipsoidalis (Figure 3) which is reflected in the sequence variation between these three species and Q. rubra (Figure 2)

Summary

Introduction

The transfer of adaptive alleles and traits by hybridization might be an important mechanism of rapid evolution and adaptation to changing environments, e.g., in the face of climate change, and evidence for adaptive trait transfer has been reported in both plant and animal species (reviewed in [1,2]). The availability of genomic resources and analytical methods for the identification of loci under strong divergent selection (outlier loci) [3] allows us to trace the introgression of potentially adaptive alleles in interspecific hybrid zones. Oaks generally reveal porous species boundaries, but morphological species’ identity and ecological adaptations (e.g., soil moisture) are generally maintained despite recurrent interspecific gene flow [4,5,6]. Oaks provide a model for the identification of outlier loci under divergent selection with annotated functions and potential roles in stress tolerance and reproductive isolation between species (e.g., [7]). The four interfertile North American red oak species—Quercus rubra L., Quercus velutina Lam., Quercus coccinea Münchh., and Quercus ellipsoidalis E.J. Hill—exhibit porous species boundaries and recent studies found strong evidence for contemporary interspecific gene flow in sympatric and Forests 2017, 8, 3; doi:10.3390/f8010003 www.mdpi.com/journal/forests

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