Selection and Dispersal in a Multispecies Oak Hybrid Zone

Abstract

Abstract. The four western North American red oak species (Quercus wislizeni, Q. parvula, Q. agrifolia, and Q. kelloggii) are known to produce hybrid products in all interspecific combinations. However, it is unknown whether hybrids are transitory resulting from interspecific gene flow or whether they are maintained through extrinsic selection. Here, we examine cryptic hybrid structure in Q. wislizeni through a broad region including contact and isolation from three other western North American red oaks using amplified fragment length polymorphism molecular markers. All four species were simultaneously detected in the genetic background of individuals morphologically assigned to Q. wislizeni, although the contribution of Q. kelloggii was minor. In some cases, introgression was detected well outside the region of sympatry with one of the parental species. Molecular structure at the individual level indicated this was due to long-distance pollen dispersal and not to local extinction of parental species. Species admixture proportions were correlated with climatic variables and greater proportions of Q. agrifolia and Q. parvula were present in the genetic background of Q. wislizeni in sites with cooler and more humid summers, corresponding with habitat preferences of the parental species. Partial Mantel tests indicated that climate was more important than distance from pollen source in this association. Despite high levels of introgression, species integrity was maintained in some populations in close proximity to the other species, providing further support to environmental selection in determining population genetic structure. Thus, the contribution of species mixtures to population genetic structure varies across the landscape according to availability of pollen, but more importantly to varying environmental selection pressures that produce a complex pattern of hybrid and pure gene pools.

Limited effective gene flow between two interfertile red oak species

“…Co-occurring oak species are often differentiated in their fine scale distributions, particularly along water and nutrient availability clines, and hybrids tend to preferentially occur in intermediate habitats (e.g. Curtu et al 2007;de Heredia et al 2009;Dodd and Afzal-Rafii 2004). This distribution pattern of hybrids is consistent with the hypothesis that differential environmental selection contributes to species maintenance.…”

Differences in growth, survival and phenology in Quercus rubra and Q. ellipsoidalis seedlings

“…Thus, ecologically divergent oak species often occur in one stand or region, but in different micro-environments. Despite interspecific gene flow, most adult trees can be assigned to a species based on morphological and/or genetic characters suggesting a role of environmental selection in maintaining species identity (Curtu et al 2007;Dodd and Afzal-Rafii 2004). Likewise pre-zygotic mechanisms (differences in flowering time, crossing incom-patibility, availability of conspecific and heterospecific pollen) can contribute to the reproductive isolation of oak species, and the relative importance of different isolation mechanisms in interfertile oak species is strongly dependent on environmental conditions (Lepais and Gerber 2011;Lepais et al 2009).…”

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).…”

“…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). Environmental variation, disturbance as well as the degree of contact between species can affect the frequency and the spatial distribution of hybrids in natural oak populations (Nason, 1992;Rushton, 1993;Howard et al, 1997;Dumolin-Lapègue et al, 1999;Dodd and Afzal-Rafii, 2004;Tovar-Sanchez and Oyama, 2004;Curtu et al, 2007;Valbuena-Carabañ a et al, 2007). Although hybridization between some oak species, such as the closely related species Quercus robur and Q. petraea, has been analyzed extensively for nuclear, chloroplast and mitochondrial variation, our understanding of the underlying processes is still unclear.…”