Species Delimitation And Speciation Of Pugionium (Brassicaceae)

Pugionium Gaertn (Brassicaceae) is a typical psammophytes endemic to Mongolian plateau of the central Asia. This genus occurs mainly in the arid or semi-arid desert regions with only two species acknowledges based on the recent taxonomic revision, i.e., P. dolabratum and P. cornutum. We used these two species as a model system to examine the accuracy and effectiveness of DNA fragments suggested for plant barcodes. The second part of this thesis was to trace speciation divergences of these two species. The only two species of this genus differ from each other in habitat, morphology and growth form.1. Three kinds of DNA markers were tested, comprising three chloroplast (cp) DNA fragments(rbcL, matK, and trnH-psbA), nuclear ribosomal internal transcribed regions (ITS) and five low-copy nuclear loci. There is no any fixed interspecific variation for three cpDNA fragments and therefore none of them is useful to discriminate these two species. However, a single base mutation in the internal transcribed spacer (ITS) could discriminate between the two species consistently. We found more variations among all individuals of the two species using each of the other five low-copy nuclear loci. However, only alleles from one locus (Det1) of the five low-copy loci related to flowering regulation was able to distinguish the sampled individuals into two species. However, it is difficult to amplify the corresponding fragments of this locus out of Brassicaceae using the designed Det1primers. These results suggested that three cpDNA fragments are useless to discriminate these two closely-related species. In addition, it is still difficult to use any examined low-copy nuclear fragments as the candidate plant barcodes. In addition, these tests further suggested that ITS is a useful fragment as a core or complementary barcode to distinguish the recent diverged species.2. Pleistocene climate change is known to have had an important effect in shaping intraspecific genetic variation in many species; however, its role in driving speciation is less clear. We examined the possibility of a Pleistocene origin of the only two representatives of the genus Pugionium (Brassicaceae), P. cornutum and P. dolabratum, which occupy different desert habitats in northwest China. We surveyed sequence variation for ITS, three chloroplast (cp) DNA fragments, and eight low copy nuclear genes among individuals sampled from eleven populations of each species across their geographic ranges. One ITS-mutation distinguished the two species whereas mutations in cpDNA and the eight low copy nuclear gene sequences were not species-specific. Although interspecific divergence varied greatly among nuclear gene sequences, in each case divergence was estimated to have occurred within the Pleistocene when deserts expanded in northwest China. Our findings point to the importance of Pleistocene climate change, in this case an increase in aridity, as a cause of speciation in Pugionium due to divergence in different desert habitats that formed in association with the expansion of deserts in China.