Natural hybridization between sympatric forest tree species is common in natural populations; it can influence the genetic diversity within species and promote new species formation (Arnold, 2006; Lexer et al., 2005). Loss of diversity from widespread hybridization between a predominantly cultivated species and an indigenous species may also raise concerns among not only conservation geneticists but also breeders (Zhao and Woeste, 2011).
The genus Juglans includes ≈20 species that are distributed over a wide geographical range including southern Europe, east Asia, and the Americas. The species within this genus display differentiation in morphology, particularly in nut characteristics (Krüssmann et al., 1985). Hybridization between species of the genus Juglans is common because of weak reproductive isolation mechanisms between phylogenetically close species (Gunn et al., 2010; Hoban et al., 2009; Ross-Davis et al., 2008; Wang et al., 2015; Zhao and Woeste, 2011). Juglans regia is grown locally in the Balkans, Iran, Turkey, central Asia, the Himalayas, and China. It is the predominant cultivated species that distributes in most area of China (from Liaoning Province to Yunnan Province) because of its economically valuable nut. Although China is considered one of the origin places of J. regia, the natural population is rare in China at present, most of them are landrace populations. Chinese walnut is one of the most widely distributed temperate deciduous native tree species in southern China. It occurs primarily throughout the hilly midelevation area that ranges between the Qinling Mountains–Huai River line and the tropical South (Bai et al., 2014). This region is typical of a subtropical climate with a complex topography. Juglans cathayensis generally grows on mountain slopes or in valleys with 500- to 2800-m elevations. In Hubei Province, China, these two species are frequently sympatric in many regions and hybridization may occur. The nut of J. cathayensis is thick shelled. Its commercial importance is not as significant as that of J. regia; its fruit typically fall in the vicinity of parental plants without collection or dispersal by humans (Ma et al., 2005). For this reason, it may be subject to different evolutionary and ecological processes than J. regia. Thus, some important characteristics of J. cathayensis genes could have an evolutionary and ecological impact on J. regia genetic resources if the hybrids produce successful descendants. Furthermore, new germplasm (e.g., specifically for resistance to diseases and characteristics of fruiting) can also be created. Whether hybrids of the two species persist in natural settings remains unknown.
Morphology may provide some information regarding hybrid identification, but the wide variability of leaf and nut morphology within species limits the utility of morphological characteristics for hybrid verification (Ross-Davis et al., 2008). Furthermore, many hybrid trees may be the result of more complex introgression as backcrosses, F2s, etc. (Hoban et al., 2009). For these reasons, hybrids are often difficult to distinguish by silvic characters, and identification using DNA markers is essential. Simple sequence repeat (SSR) markers exhibit hypervariability and codominance and are, therefore, highly informative. Hundreds of specific SSR primer pairs are currently available for Juglans; they have been demonstrated to be useful in hybrid identification and introgression analyses in the genus Juglans (Hoban et al., 2009; Pollegioni et al., 2009; Ross-Davis et al., 2008; Wang et al., 2015).
To investigate the possibility of genetic invasion, we used 12 microsatellite markers to identify the hybrids between J. regia and J. cathayensis in three regions where these two species co-occur. The objectives of this study were to assess whether interspecific hybrids occur and reproduce in natural populations.
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Juglans populations surveyed, and their geographical parameters.