PATTERNS OF MOLECULAR EVOLUTION AND EXPRESSION DIVERGENCE OF THE POLYGALACTURONASE GENE FAMILY IN TUNG TREE (VERNICIA FORDII HEMSL.)

Polygalacturonase (PG; EC 3.2.1.15) genes are a major group of pectin-hydrolyzing enzymes responsible for pectin degeneration and play important roles in pollen development and male fertility. Studying the PG gene family could shed light on molecular mechanisms by which PGs regulate pollen development in tung tree ( Vernicia fordii). A total of 35 tung PG genes ( VfPGs) were obtained from tung genome in this study. VfPGs were clustered into 2 main classes which contained 6 clades by phylogenetic analysis. All Class I VfPGs comprised four typical conserved residues while residue III was absent in Class II VfPGs. Evolutionary analyses displayed that Class III PG genes that were present in the most recent common ancestors of land plants could be lost in Euphorbiaceae after their divergence. Whole genome triplication was one of the major forces of the expansion of tung PG family and duplicated VfPGs suffered negative selection during evolution. A total of 107 cis-regulatory elements related to phytohormone responsiveness were identified in the promoters of VfPGs, suggesting phytohormones may play important roles in plant development in tung trees by regulating VfPG activities. Transcriptome-scale gene expression analysis of VfPGs in various tissues displayed that VfPGs were expressed more in flowers than in other tissues. Several VfPGs were predicted to be highly correlated with anther development. This study provides clues for future works aimed at elucidating molecular mechanisms underlying PG-regulated pollen development in tung tree.