Molecular genetic relationship of Cassia in Thailand

The genus Cassia (family Leguminosae, subfamily Caesalpinioideae, tribe Cassieae, subtribe Cassiinae) is a tropical plant group of either trees, shrubs, or herbs. These plants are so useful from their herbal medical property and their quality woods. According to Flora of Thailand, there are 22 species of Cassia found in the country. However, this taxonomic classification does not agree with those of other research groups. For instance, Flora Malesiana re-investigated an intergeneric relationship of the subtribe Cassiinae (Cassia/Senna/Chamaecrista) and then moved 12 Cassia species to the genus Senna and four species to the genus Chamaecrista. To solve this taxonomic problem, molecular techniques such as PCR and DNA sequencing were introduced to study evolutionarily genetic relationships among Cassia species in Thailand. Chloroplast trnL intron sequence and nrDNA ITS regions included 5.8S subunit were used as the gene targets for this phylogenetic study. A data matrix of trnL intron sequences of 16 Cassia that of ITS regions of 13 Cassia were prepared and thereafter used for a phylogenetic reconstruction. Phylogenetic trees from trnL intron and ITS sequences including 5.8S subunit, also the combined data of both regions revealed two major clades. One clade contained Cassia species moved to the genus Senna following Flora Malesiana and having a subclade of Cassia species remaining to be Cassia sitting within this major clade. The other major clade contained Cassia moved to the genus Chamaecrista. Moreover, trnL intron sequence data of nine more members of the subtribe Cassiinae retrieved from GenBank database, were included to the data matrix and phylogenetically reanalysed. The tree results followed the formula pattern and supported the recommendation of Flora Malesiana. Moreover, Cytological investigations of these Cassia species were also performed and revealed that chromosomes approximately of this genus were too small to be exactly counted. Most of them were found to be 2n = 22 to 28. All of this M.Sc. thesis has pointed an importance of using molecular techniques to clarify such taxonomic problem.