Journal of Tea Science Research, 2024, Vol.14, No.1, 64-78 http://hortherbpublisher.com/index.php/jtsr 70 Estimating the divergence times of Camellia species provides insights into their evolutionary history. Molecular clock analyses, which use the rate of genetic mutations to estimate the timing of evolutionary events, have been employed to date the divergence of major Camellia clades. Results suggest that the genus Camellia began to diversify during the late Cretaceous to early Paleogene periods, approximately 60-70 million years ago (Cheng et al., 2022). Whole genome duplication events, critical to the evolution of key traits, have been dated to around 30 to 40 and 90 to 100 million years ago (Wei et al., 2018). The divergence between two major lineages, Camellia sinensis var. sinensis and Camellia sinensis var. assamica, is estimated to have occurred approximately 0.38 to 1.54 million years ago (Wei et al., 2018). These events have significantly impacted the genetic makeup and evolutionary history of the genus. 3.3 Hybridization events Hybridization events are well-documented in the Camellia genus, contributing to its genetic diversity. Comparative genomic studies have identified numerous instances of interspecific hybridization, which complicates the phylogenetic resolution of the genus (Li et al., 2019; Lin et al., 2022). The presence of hybridization is further supported by the detection of sequence polymorphisms and structural variations in the chloroplast genomes of different species (Lin et al., 2022). Hybridization has had a profound impact on the genetic diversity and adaptability of Camellia species. It has facilitated the exchange of genetic material between species, leading to the emergence of new traits and increased genetic variability (Li et al., 2019; Lin et al., 2022). This genetic diversity enhances the adaptability of Camellia species to various environmental conditions, contributing to their widespread distribution and ecological success (Li et al., 2019; Wu et al., 2022). Moreover, hybridization events have played a role in the evolution of economically important traits, such as stress resistance and secondary metabolite production, which are crucial for the cultivation and utilization of Camellia plants (Yan et al., 2018; Wu et al., 2022). The evolutionary history of the Camellia genus is characterized by complex phylogenetic relationships, driven by mechanisms such as hybridization, polyploidization, and environmental adaptation. These processes have shaped the genetic diversity and adaptability of the genus, making it a valuable resource for both ecological and economic purposes. 4 Genomic Insights into Adaptation and Evolution 4.1 Adaptive traits The adaptability of Camellia species to diverse environments is underpinned by specific genomic traits that confer resistance to pests, tolerance to environmental stresses, and other adaptive characteristics. High-throughput sequencing and genome-wide association studies (GWAS) have identified numerous genes associated with these adaptive traits. For instance, results showed that the cold tolerance in Camellia japonica var. decumbens has been linked to specific gene expression changes under cold stress. Transcriptome analysis identified differentially expressed genes (DEGs) involved in cold response, including transcription factors and genes related to signal transduction and plasma membrane stabilization (Wu et al., 2019). Additionally, the tea plant Camellia sinensis has been found to possess lineage-specific genes (LSGs) that contribute to stress resistance and secondary metabolite production, such as catechins, which have antioxidant properties (Zhao and Ma, 2019). These findings highlight the genomic basis of key adaptive traits in Camellia species, providing valuable insights for breeding programs aimed at enhancing stress tolerance and other desirable traits. 4.2 Gene flow and genetic variation Gene flow, the transfer of genetic material between populations, is a critical factor influencing genetic diversity and adaptation in Camellia species. Gene flow within the Camellia genus has been studied to understand the evolutionary dynamics and genetic diversity among populations. The chloroplast genome analysis of Camellia japonica revealed significant genetic variation and evolutionary routes influenced by environmental pressures (Li
RkJQdWJsaXNoZXIy MjQ4ODYzNA==