| The evolution of large gene families in the highly organized genomes has been highlighted by evolutionists since the first light dawned on the post-genomic era. rRNA genes, including 18S-5.8S-26S rDNA clusters and 5S DNA, are tamdemly repeated gene families with thousands of copies. The evolution mode of 5S rRNA genes was studies in the present thesis through an extremely endangered rainforest species, Ophiopogon xylorrhizus Wang et Dai, and its closest sister species O. sylvicola Wang et Tang. 294 5S rRNA copies from six individuals collected from three populations of O. xylorrhizus and 45 copies from one individual of O. sylvicola, were randomly chosen from totally 1,085 clones and sequenced. Based on the largest 5S rDNA sequence data that had never been published for single species, sequence differentiation was analysed and gene phylogeny was reconstructed. 18S rDNA at the meiotic chromosomes of O. xylorrhizus was also located by fluorescent in situ hybricization (FISH). The results obtained are briefly shown as follows:1. The sequence lengths of 339 5S rRNA genes spaned variably from 307bp to 548bp, with variations mainly in the non-coding regions. One-base insertions or deletions (indels) were frequently observed flanking coding region. And apart from the short indels, 11 large gaps were also found in the aligned sequences. Diversity of the gene copies was high, and only 13 pairs (3.8%) of the 339 sequences were identical. Sequence differentiation indices (SDI) calculated among the copies from a species or between the species, indicated that O. xylorrhizus' copies were more differentiated than O. sylvicola's.2. Only one gene-tree well resoluted was inferred by PAUP's heutistic method. 0. xylorrhizus and O. sylvicola were grouped into separate clades with 100% bootstrap supports, indicating that all the sequenced copies of each of the species originated from just one ancestral copy, coined here as "founder copy". Moreover, the founder copies of these two species experienced a series of amplificationthrough time, while the rest copies born by the common ancestor of the sister species were lost after speciation. This remarkable copy-sorting have been not reported yet. The gene-tree also showed that all the copies of a species formed a large orthologous gene-family after the founder was continuously amplified.3.In the O. xylorrhizus clade, few sub-clades merely consisted of the copies of one population or one individual; namely, copies from different populations and individuals were mixed in a sub-clade. The gene phylogeny, sequence diversity and sequence differentiation index, all showed that homogenization of the copies was weak, in other words, the copies had evolved independently. This phenomena is special in the tandemly repearted multicopy gene families. It was assumed that gene flow among the populations of 0. xylorrhizus was much more frequently in the past than today. Therefore, it should be some recent changes in environmental factors that blocked the gene flow, and further caused the species dominantly selfing and depressing on seedlings, and finally brought the species into endangerment situation.4.The 18S ribosomal RNA genes (18S rDNA) were mapped on meiotic chromosomes of Ophiopogon xylorrihizus by in situ hybridiztion using digoxigenin-labled 18S rDNA. Two strong pairs and one weak pair of fluorescent signals appeared at three pairs of the chromosomes.
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