Interspecific gene flow in sympatric oaks.

Abstract

Variation of chloroplast DNA and nuclear ribosomal DNA (DNA encoding ribosomal RNA) was studied for five species of white oak native to the eastern United States. Although these species differ in many morphological characters and have different (though overlapping) geographical ranges and ecological tolerances, they are interfertile and often grow in mixed stands, and hybrids are occasionally found in nature. AU individuals studied were morphologically typical members of their respective species-i.e., showed no evidence of recent hybrid ancestry. Restriction site markers in the chloroplast DNA reveal several clear cases of localized gene exchange between species, showing that there is appreciable gene flow between sympatric species in this group. One length variant of the nuclear ribosomal DNA, however, is species specific. The sharp morphological and ecological differences between the species, together with the one ribosomal DNA variant, suggest that nuclear genes may be exchanged less freely between species than are chloroplast genotypes.

A nuclear DNA barcode for eastern North American oaks and application to a study of hybridization in an Arboretum setting

“…Our results suggest that the mother tree of these offspring is a hybrid that mates preferentially with white oak, although this result bears additional study. This result is not outlandish, as the individuals sampled are all adjacent to a large forest with large numbers of Q. alba , and Q. stellata is known to hybridize with Q. alba (Whittemore & Schaal, 1991). However, our finding that all acorns cluster predominantly with Q. alba may suggest that the number of Q. stellata SNPs we developed is simply too low to distinguish backcrosses from pure individuals.…”

“…Hybridization in the genus is common and known to be dependent on spatial distribution of parent trees, pollen dispersal and pollination time, and sexual barriers among other factors (Lagache, Klein, Guichoux, & Petit, 2013; Petit, Bodénès, Ducousso, Roussel, & Kremer, 2004). Oak species with similar reproductive strategies and overlapping geographic regions often hybridize in natural stands (Chybicki & Burczyk, 2010; Curtu, Gailing, & Finkeldey, 2007; Dodd & Afzal‐Rafii, 2004; Dumolin‐Lapègue, Démesure, Fineschi, Le Corre, & Petit, 1997; Dumolin‐Lapègue, Kremer, & Petit, 1999; Efrain Tovar‐Sanchez, 2004; Gerber et al., 2014; Hipp & Weber, 2008; Lexer, Kremer, & Petit, 2006; Moran, Willis, & Clark, 2012; Petit, 1993; Petit et al., 2004; Whittemore & Schaal, 1991). Morphological intermediacy, however, is an imperfect predictor of genetic admixture in oaks, making hybrids difficult to identify (Burgarella et al., 2009; Song, Deng, Hipp, & Li, 2015; Wei, Li, Zhang, & Liao, 2015).…”

Detection of hybrids in nature: application to oaks (Quercus suber and Q. ilex)

“…Interspecific hybridization is the most frequently invoked mechanism to account for the existence of plants morphologically and ecologically intermediate between extant oak species (Jensen et al, 1993;Howard et al, 1997;González-Rodríguez et al, 2004) and to interpret the extensive local sharing of organelle and nuclear genes between species (Whittemore and Schaal, 1991;Howard et al, 1997;Petit et al, 1997;Dumolin-Lapègue et al, 1999). However, in some cases, interspecific gene exchanges have been detected with molecular markers in the absence of obvious morphologically intermediate forms (Whittemore and Schaal, 1991;Dodd and Afzal-Rafii, 2004). Moreover, the possibility that shared alleles represent ancestral segregating polymorphisms rather than the outcome of hybridization has been suggested (Muir and Schlö tterer, 2005; but see Lexer et al, 2006).…”

Genetic evidence of reproductive isolation in a remote enclave of Quercus pubescens in the presence of cross-fertile species

“…Related oak species can hybridise relatively easily, and hybrids are usually fertile (Whittemore and Schaal 1991;Rushton 1993;Tovar-Sánchez and Oyama 2004;Curtu et al 2007;Peñaloza-Ramírez et al 2010). Furthermore, related oak species can differ in ecological optima (Whittemore and Schaal 1991;Timbal and Aussenac 1996;Petit et al 2004;Sánchez de Dios et al 2006;Gugerli et al 2007), so that they often occupy different niches and can reveal variable adaptation to local environments, making room for exogenous selection (Barton and Hewitt 1985).…”