Abstract
BackgroundThe species status of two closely related Chinese oaks, Quercus liaotungensis and Q. mongolica, has been called into question. The objective of this study was to investigate the species status and to estimate the degree of introgression between the two taxa using different approaches.Methodology/Principal FindingsUsing SSR (simple sequence repeat) and AFLP (amplified fragment length polymorphism) markers, we found that interspecific genetic differentiation is significant and higher than the differentiation among populations within taxa. Bayesian clusters, principal coordinate analysis and population genetic distance trees all classified the oaks into two main groups consistent with the morphological differentiation of the two taxa rather than with geographic locations using both types of markers. Nevertheless, a few individuals in Northeast China and many individuals in North China have hybrid ancestry according to Bayesian assignment. One SSR locus and five AFLPs are significant outliers against neutral expectations in the interspecific F ST simulation analysis, suggesting a role for divergent selection in differentiating species.Main Conclusions/SignificanceAll results based on SSRs and AFLPs reached the same conclusion: Q. liaotungensis and Q. mongolica maintain distinct gene pools in most areas of sympatry. They should therefore be considered as discrete taxonomic units. Yet, the degree of introgression varies between the two species in different contact zones, which might be caused by different population history or by local environmental factors.
Highlights
Natural hybridization occurs frequently in plants and animals [1]
The number of alleles, observed heterozygosity (HO), expected heterozygosity (HE) across all loci were similar between Q. liaotungensis and Q. mongolica
The present study revealed a clear differentiation of the Chinese oak gene pool into two entities corresponding to Q. liaotungensis and Q. mongolica
Summary
Natural hybridization occurs frequently in plants and animals [1]. Analysis of natural hybridization and hybrid zones provides insight into the processes of introgression, speciation and reproductive isolation [1,2,3,4]. While contemporary hybridization and introgression have long been thought to threaten species persistence, more recent work suggests that these processes are not necessarily a major impediment to effective species delimitation [5]. They can lead to species barriers of varying strength across different contact zones, a feature of great potential interest to understand the evolution of reproductive isolation as well as its breakdown [6,7]. The oaks (Quercus) should be good models to evaluate the effects of hybridization and introgression on species delimitation. The objective of this study was to investigate the species status and to estimate the degree of introgression between the two taxa using different approaches
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