Cytogenetics of the rare and endangered Trigonobalanus doichangensis (Fagaceae) from Northern Thailand

Trigonobalanus doichangensis is rare and endangered in Thailand. The species is found in northern Thailand, in hill-evergreen forests at high elevations. In this study, T. doichangensis was identified for the first time from Khun Mae Kuong Forest in Chiang Mai Province, and this discovery has extended the species distribution eastwards significantly. Samples were collected from trees of this species: twigs with fully grown leaves, fruits and flowers were used for taxonomic identification, whereas shoot and flower buds were used in the chromosome preparation. Results confirmed that T. doichangensis was diploid with chromosome number 2n = 2x = 14 (base number x = 7 confirmed with meiosis), whereas other Fagaceae species from the same forest (Castanopsis, Lithocarpus and Quercus) were diploid with 2n = 2x = 24. The karyotype analysis indicated a common genomic origin. The chromosomal mapping of highly repetitive 18S-25S and 5S ribosomal genes was performed by fluorescence in-situ hybridisation. Trigonobalanus doichangensis showed two pairs of 18S-25S rRNA loci (subtelomeric and paracentromeric) and one pair of 5S paracentromeric rRNA loci. The presence of linked 5S and 18S-25S ribosomal sites indicated an ancestral state of the genome of T. doichangensis.