Thesis (Ph.D.)--Chulalongkorn University, 2007

Plants in the genus Croton (Euphorbiaceae) have been widely used in folklore medicine and are considered to be a rich source of bioactive compounds. In this study, genetic variation of fifteen Croton species existing in Thailand was evaluated for their phylogenetic relationship. The genes selected for the study included the internal transcribed spacer (ITS) of nuclear DNA and trnL-F gene of chloroplast DNA. The length of ITS sequences of various Croton species was from 620 to 627 bp with 52.5-57.1 % GC content. Genetic variation in the ITS region was mostly from substitutions. For the trnL-F gene, the Croton species had their sequences in the range from 1017 to 1041 bp with 31.0-31.9 % GC content. The genetic variation of this gene in Croton was caused mainly from insertions and deletions. Phylogenetic analysis revealed that the relationship of all fifteen Croton species was monophyletic type. In terms of variation in population, the two well known Thai medicinal plants C. stellatopilosus (Plau-noi) and C. roxburghii (Plau-yai) were extensively investigated for their genetic variation. Both species were studied not only on the ITS region and trnL-F gene, but also on the trnK intron of chloroplast DNA. Based on the obtained ITS sequences of C. stellatopilosus, two groups of individual sequences, designated as Type A and Type B were found as well as various nucleotide additive sequences which were arised from these two groups and designated as “putative hybrids”. These putative hybrids of C. stellatopilosus showed their trnK intron sequences identical to the Type B but different from the Type A. Thus, it was suggested that Type A was paternal parent and Type B was maternal parent of the hybridization of C. stellatopilosus. For C. roxburghii, it showed high genetic variation in their ITS, trnL-F and trnK intron sequences but could be systematically divided into two groups. In conclusion, the technique of direct sequencing was successfully used in this case as a tool for estimating the phylogenetic relationship of Croton and identifying the definite sites of genetic variation in the important Croton species of C. stellatopilosus and C. roxburghii.