# Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants
## 6 Molecular Evolution of Steroidal Saponin Biosynthetic Pathway Genes  
Steroidal saponins are mainly distributed in _Dioscorea_, _Paris_, _Asparagus_, and other plants. At present, only _D. zingiberensis_, _Dioscorea rotundata_, asparagus, _P. dactylifera_, oats, and other plant genomes have been assembled and annotated. However, steroidal saponins are only abundant in _D. zingiberensis_, and the content is relatively small in other plants. Moreover, the genomes of most plants containing steroidal saponins, such as _Allium, Paris, Smilax, Trillium_, and _Polygonatum_, have not yet been reported. In the absence of species genome data, only comparative transcriptome analysis can be used to analyze the biosynthesis and evolution of plant steroidal saponins to a limited extent. Therefore, it is the first/best choice to analyze the biosynthesis and evolution of steroidal saponins by sequencing and assembling the genome of _D. zingiberensis_. Other plants, such as fenugreek and trillium, are also suitable research materials. [PERSON] et al. assembled a high-quality chromosome-level genome of _D. zingiberensis_ by combining Illumina and Oxford Nanopore technologies [20]. Through comparative genomic analysis, it was found that there were a large number of species-specific genes in the genome of _D. zingiberensis_, and these specific genes are significantly enriched in the biological processes of the responses to pathogens, oxidative stress, etc. This finding indicates that _D. zingiberensis_ has evolved a large number of specific genes for coping with stress in order to adapt to the environment, and there are many _D. zingiberensis_-specific genes involved in the biosynthesis of steroidal saponins [20]. These results show that the genome of _D. zingiberensis_ produced a relatively large number of genes involved in steroidal saponin biosynthesis during the evolutionary process, which may also be the premise or basis for steroidal saponin synthesis in _D. zingiberensis_[20]. Furthermore, there are a large number of expanded gene families in the _D. zingiberensis_ genome, and these gene families are significantly enriched in UDP-glycostransferases activity, mono-oxygenase activity, steroid biosynthesis, stress response, and other processes related to steroidal saponin synthesis [20]. Moreover, these expanded gene families also include the gene family members of _OSC_, _CYP450_, and _UGT_, which play important roles in several key steps of steroidal saponin biosynthesis [1; 20]. Thus, compared with other plants, the species-specific genes and significantly expanded gene families in the genome of _D. zingiberensis_ provide abundant genetic and evolutionary resources related to steroidal saponin biosynthesis [20]. The expansion or contraction of gene families is an important factor in species differentiation, phenotypic diversification, and adaptation to natural changes [127; 128]. Similarly, whole-genome duplication (WGD) events are also considered to be the major driving force of species diversity and phenotypic variation [129; 130; 131]. Specifically, the role of WGD and gene tandem duplication is mainly to produce a large number of gene copies in the genome. Further analysis showed that a large number of copies of key genes involved in the biosynthesis of steroidal saponins in _D. zingiberensis_, including _CAS_, _CYP72A_, _CYP90B_, _CYP94_, _S3 GT_, and _F26G_, were produced through WGD and gene tandem duplication events [20]. Combined with the above results, gene tandem duplication coupled with WGD events in the genome may be the driving force to promote the evolution of the steroidal saponin biosynthetic pathway in _D. zingiberensis_[20]. Gene replication can help plants to obtain more gene copies, which may produce new gene functions that are more adaptive than single-copy or low-copy genes [130].