Species-Specific Gene Expansion of the Gene Superfamily in the Orchidaceae Family and Functional Divergence of Mannan Synthesis-Related Genes in .

Plant genes constitute a supergene family that includes the (CesA) family and nine -like (Csl) families, the members of which are widely involved in the biosynthesis of cellulose and hemicellulose. However, little is known about the superfamily in the family Orchidaceae, one of the largest families of angiosperms. In the present study, we identified and systematically analyzed the family members in three fully sequenced Orchidaceae species, i.e., , , and . A total of 125 superfamily genes were identified in the three orchid species and classified into one CesA family and six Csl families: CslA, CslC, CslD, CslE, CslG, and CslH according to phylogenetic analysis involving nine representative plant species. We found species-specific expansion of certain gene families, such as the CslAs in (19 members). The families exhibited sequence divergence and conservation in terms of gene structure, phylogeny, and deduced protein sequence, indicating multiple origins via different evolutionary processes. The distribution of the genes was investigated, and 14 tandemly duplicated genes were detected, implying that the expansion of genes may have originated via gene duplication. Furthermore, the expression profiles of the genes were investigated using transcriptome sequencing and quantitative Real-time PCR (qRT-PCR) analysis, which revealed functional divergence in different tissues and during different developmental stages of . Three were highly expressed in the flower, whereas and family genes exhibited low expression levels in all tissues and at all developmental stages. The 19 were differentially expressed in the stems at different developmental stages, among which six were expressed at low levels or not at all. Notably, two ( and ) showed significantly high expression in the stems of , indicating a vital role in mannan synthesis. These results indicate the functional redundancy and specialization of with respect to polysaccharide accumulation. In conclusion, our results provide insights into the evolution, structure, and expression patterns of genes and provide a foundation for further gene functional analysis in Orchidaceae and other plant species.