Geographic variation in the structure of oak hybrid zones provides insights into the dynamics of speciation

Abstract

Studying geographic variation in the rate of hybridization between closely related species could provide a useful window on the evolution of reproductive isolation. Reinforcement theory predicts greater prezygotic isolation in areas of prolonged contact between recently diverged species than in areas of recent contact, which implies that old contact zones would be dominated by parental phenotypes with few hybrids (bimodal hybrid zones), whereas recent contact zones would be characterized by hybrid swarms (unimodal hybrid zones). Here, we investigate how the hybrid zones of two closely related Chinese oaks, Quercus mongolica and Q. liaotungensis, are structured geographically using both nuclear and chloroplast markers. We found that populations of Q. liaotungensis located around the Changbai Mountains in Northeast China, an inferred glacial refugium, were introgressed by genes from Q. mongolica, suggesting historical contact between the two species in this region. However, these introgressed populations form sharp bimodal hybrid zones with Q. mongolica. In contrast, populations of Q. liaotungensis located in North China, which show no sign of ancient introgression with Q. mongolica, form unimodal hybrid zones with Q. mongolica. These results are consistent with the hypothesis that selection against hybrids has had sufficient time to reinforce the reproductive barriers between Q. liaotungensis and Q. mongolica in Northeast China but not in North China.

Interspecific gene flow and maintenance of species integrity in oaks

“…Selection against hybrids was suggested as a post-zygotic isolation mechanism that could explain the absence of hybrid swarms as refl ected in the relatively low number of putative hybrids in contact zones between interfertile oak species (Curtu et al 2007a(Curtu et al , 2009). In accordance with this hypothesis ancient contact zones between the two closely related Chinese oaks, Q. mongolica and Q. liaotungensis, showed lower hybrid frequencies than more recent hybrid zones, possibly as the result of selection against hybrids that had reinforced reproductive barriers in the ancient hybrids zones, but not yet in the more recent ones (Zeng et al 2011). Likewise, based on genetic assignment and paternity analyses (see below), higher frequencies of putative interspecifi c hybrids were found in more recent hybrid zones at the northern distribution range of Q. robur and Q. petraea (Jensen et al 2009) than in other studies in Europe (Streiff et al 1999), one of them using the same genetic markers (Curtu et al 2007a).…”

Limited effective gene flow between two interfertile red oak species

“…The relative importance of pre-and post-zygotic isolation may be related to the age of the contact zone between related species (Jensen et al 2009;Zeng et al 2011). For example, ancient contact zones between the two closely related Chinese oaks, Quercus mongolica and Quercus liaotungensis, showed lower hybrid frequencies than more recent hybrid zones, possibly as the result of selection that had reinforced reproductive barriers in the ancient hybrid zones (Zeng et al 2011).…”

Reinforcement in plants

“…In another system, pre-zygotic RI is very strong, but it appears that post-zygotic RI is relatively weak (Kay, 2006). In many plants, selection against hybrids is unknown or assumed, but not measured (McNeilly & Antonovics, 1968;Whalen, 1978;Zeng et al, 2011). More accurate estimates of the pre-existing levels of pre-zygotic and post-zygotic RI are necessary in order to better understand how the homogenizing effect of gene flow limits evolution via reinforcement.…”

“…Some studies just assume that hybrids are maladapted as a result of apparent differential local adaptation of the parents (McNeilly & Antonovics, 1968;Silvertown et al, 2005;Zeng et al, 2011). In the Phlox system, hybrids between P. cuspidata and P. drummondii are vigorous, but suffer low fertility (0-30% compared with nonhybrid individuals), indicating a high cost to hybridization (Ruane & Donohue, 2008).…”