An Investigation On Ecological Adaptation,Speciation And Phylogenetic Studies In Betulaceae Based On The Transcriptome Data

Due to the wide application of the second generation sequencers, it has become relatively cheap to obtain transcriptome data, which provide a basis for clarifying ecological adaptation and constructing phylogenetic trees based on’big dataset’. As a group with wide distribution and diverse adaptation in addition to well resolved phylogeny, the family Betuaceae poses as a good candidate for being used as an example to study ecological adaptation and construct phylogeny based on transcripome data. In this dissertation, we firstly revealed ecological adapation of Corylus mandshurica and C. avellana with sequenced transcriptome data. We then explored molecular mechanisms behind the southern vs northern distribution in the genus of Ostryopsis and potential genes involved in the species divergence and speciation. With newly sequenced other transcriptomes of the Betulaceae, we further discussed adapative genes in Ostryopsis and different ways to construct phylogenetic trees for the Betulaceae. The major results were summarized as follows.1. Corylus mandshurica was endemic to China with a high tolerance to fungal infections and coldness compared to its congeneric species, C. avellana. In this study, we sequenced transcriptome data of C. mandshurica and compared the gene expression and divergence between these two species. We detected a set of genes related to taxol synthesis in C. mandshurica. We found that most differentiated genes were highly correlated to different adaptations to fungal infection and coldness between these two species. These findings provided tools to improve our understanding of local adaptation, genetic breeding and taxol production in a Chineae hazelnut.2. Ostryopsis was a small genus with only three species. Due to the clear geographic isolation of O. davidiana from O. nobilis and O. intermedia and sympatry of O. nobilis and O.intermedia, it offered us an excellent model system to study ecological adapation and speciation. We firstly identified differentially differentiated genes across the three species, finding genes involved in ferric uptake might contribute to geological adaptation of these species, and genes related to plastid and reproduction were repeatedly enriched in differentially differentiated orthologs closer between O. davidiana and O. nobilis and closer between O. intermedia and O. nobilis. We then investigated whether adapative evolution occurred in these genes and found a large fraction of the adapative genes were related to plastid and reproduction, suggesting a crucial role of these genes in species divergence and speciation of the Ostryopsis genus. Besides, as a pioneering species and a dominant shrub on loess plateau, Ostryopsis davidiana displayed strong tolerance to drought. In order to reveal genetic bases of drought tolerance in this species, we studied thoroughly its transcriptome according to known mechanisms of plant drought response, and identified genes involved in different aspects of drought tolerance, including those related to regulation of stomatal closure by abscisic acid, the production of osmotic compounds, xanthophyll cycle, wax formation and synthesis of aquaporins. These genes likely played an important role underlying drought tolerance of this shrub species. These findings helped us understand drought tolerance of this species and facilitated further comparative studies with other species showing high drought tolerance.3. Our knowledge of the molecular phylogeny of Betulaceae mainly comes from analysis of chloroplast DNA and ITS sequences. However, incongruent phylogenies have been reported. Based on transcriptomes from11species in Betulaceae and one transcriptome of Quercus rubor, we identified1460orthologs longer than300bp across the12species. We tried three different ways to construct phylogenetic trees and demonstrated that the length of orthologs used in phylogenetic study was most critical and a length of10000bp of concatenated sequence was enough to get high support in most branches of the phylogeny. Studies on four low copy genes lent additional support to the demand on longer sequences in phylogeny construction. We also evaluated positive selection in Betulaceae, and found coherent results for the evolution of the Ostryopsis genus.