Identification of a Natural Hybrid between Castanopsis sclerophylla and Castanopsis tibetana (Fagaceae) Based on Chloroplast and Nuclear DNA Sequences

Castanopsis × kuchugouzhui Huang et Y. T. Chang was recorded in Flora Reipublicae Popularis Sinicae (FRPS) as a hybrid species on Yuelushan mountain, but it is treated as a hybrid between Castanopsis sclerophylla (Lindl.) Schott. and Castanopsis tibetana Hance in Flora of China. After a thorough investigation on Yuelushan mountain, we found a population of C. sclerophylla and one individual of C. × kuchugouzhui, but no living individual of C. tibetana. We collected C. × kuchugouzhui, and we sampled 42 individuals of C. sclerophylla from Yuelushan and Xiushui and 43 individuals of C. tibetana from Liangyeshan and Xiushui. We used chloroplast DNA sequences and 29 nuclear microsatellite markers to investigate if C. × kuchugouzhui is a natural hybrid between C. sclerophylla and C. tibetana. The chloroplast haplotype analysis showed that C. × kuchugouzhui shared haplotype H2 with C. sclerophylla on Yuelushan. The STRUCTURE analysis identified two distinct genetic pools that corresponded well to C. sclerophylla and C. tibetana, revealing the genetic admixture of C. × kuchugouzhui. Furthermore, the NewHybrids analysis suggested that C. × kuchugouzhui is an F2 hybrid between C. sclerophylla and C. tibetana. Our results confirm that C. × kuchugouzhui recorded in FRPS is a rare hybrid between C. sclerophylla and C. tibetana.