Population sampling and species identification

In total, 57 and 36 individuals with typical traits of Q. austrocochinchinensis and Q. kerrii, respectively, were used and 15 morphological intermediates were included in this study. The 108 individuals were sampled in five populations, A–E (Figure ​(Figure11 and Table ​Table1).1). According to a previous investigation (Song et al., 2015), Q. austrocochinchinensis trees can only be found in populations D and E. Of these, E was considered as a pure Q. austrocochinchinensis population (Song et al., 2015) and D is located in the contact zone which contains both Q. kerrii and Q. austrocochinchinensis trees. Six sub-populations, D1–D6, were sampled within D population regions. Two putative Q. kerrii purebred populations were sampled from populations A and B. Population C is a putative hybrid population with morphological intermediates, but trees with typical traits of Q. austrocochinchinensis cannot be found in or close to region C.

Geographic distribution of the study populations of Quercus kerrii and Q. austrocochinchinensis. Population A and B are pure Q. kerrii populations. C is a putative hybrid population without co-occurrence of any Q. austrocochinchinensis trees. D1–D9 are located in the contact zone. E is a population of Q. austrocochinchinensis that has been reported previously in a morphological study as a purebred population (Song et al., 2015). Individuals were colored according to parental species or intermediates (orange, blue, or purple) pre-identified by morphological features.

Sampling information of the study populations.

XSBN, Xi-Shuang-Ban-Na National Nature Reserve; BWL, Ba-Wang-Ling Nature Reserve; N (a/k/i): sampling number of Q. austrocochinchinensis (a), Q. kerrii (k) and morphological intermediates (i). Long, Longitude; Lat, Latitude; Elev, Elevation.

Prior to the experiment, all individuals and their voucher specimens were carefully inspected and identified based on key morphological diagnostic traits, e.g., shape of cupule and trichomes on leaf abaxial surface. The results were used as species information for the studied samples in later analyses. Leaf tissues used for DNA extractions were collected from each individual and dried instantly using silica gel. All vouchers specimens of each tree were stored at the herbarium of the Shanghai Chenshan Botanical Garden (CSH).