# Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polylploidy
## 4 Evolution of VOCs: The Role of Polylploidy and Transgenerational Memory  
Floral scent is considered an easily evolved trait, as VOCs are usually acquired and lost in related species [130, 131]. Scent compounds would have mostly evolved due to gene duplication and divergence-convergence [1, 2]. The large gene families, terpene synthases and O-methyl transferases, among others, would have evolved following this mechanism [1, 55]. In addition, these modifications can also occur without the mediation of duplication events on single genes, changing the VOC emission throughout the evolutionary history [132]. Structural mutations modifying the enzyme catalytic sites can also cause the gaining of new functions (or the loss thereof) and hence cause changes in VOC emission. However, more interestingly, scent changes (qualitatively and quantitatively) can arise due to gene expression changes. Gene silencing or activation cannot only affect an individual VOC emission, but it can also have a cascade effect on the complete pathway [46, 133]. New VOC emissions can straightforwardly arise from exogenous gene expression in new tissues. For example, in _Clarkia breueri_, linalool emission (its most abundant floral VOC) most likely evolved from trace emissions in stigma tissue in the ancestor _C. concinna_[134]. Such VOC changes could arise as a consequence of epigenetic regulation of genes, creating advantageous or disadvantageous phenotypes that are subject to natural selection. If new VOCs confer a selective advantage for the plant, the epigenetic change and the VOC production would be retained and inherited [135]. The transgenerational inheritance of floral and leaf VOCs has been recently found in _Brassica rapa_[136], and this could be epigenetically mediated [39]. Therefore, epigenetic change would propose a flexible evolutionary mechanism and help to explain the continuous gain and loss of specific VOCs in related plant lineages. Epigenetic variations would assist plants in surfing on an adaptative landscape in which abiotic and biotic interactions usually involve trade-offs (e.g., herbivory and pollinator attraction) [137, 138]. Furthermore, all angiosperm lineages show vestiges of past rounds of whole genome duplication (WGD), highlighting the role of polyploidy as a major driving force in plant evolution [139, 140, 141]. Polyploidy has multiple effects at different scales, from the molecular to the phenotypic level, owing to the advantageous evolutionary success [38]. Patently, WGD (and the consequent gene duplication) is an important molecular evolutionary mechanism driving the large diversity of plant VOCs. In addition, following WGD, polyploids suffer changes in genomic structure and epigenetic remodeling [142, 143]. Polyploidy usually causes an altered pattern of DNA methylation in genes, promoters and transposable elements [144, 145]. These changes are enhanced in allopolyloids (WGD associated with species hybridization) because of \"genomic shock\". In this case, this methylation repatterning is related to parental dominance (i.e., the higher expression of one parental homeolog) and fractionalization [38]. In addition, changes in smRNAs and chromatin have also been observed after WGD [146, 147, 148]. In this way, autopolyploidy (WGD without hybridization) has been recently considered as an epigenetic macromutation affecting chromatin compaction and altering contacts among chromosomes (i.e., 4D nucleome) as a side effect of the increased nucleus size [149]. This altered epigenetic landscape may promote divergent gene expression by regulating mobile element activity and silencing redundant genes [144, 150]. In addition, these epigenetic alterations could create many opportunities for natural selection to act, enhancing polyploid establishment [151]. Concurrently to the epigenetic changes, WGD also exerts a cornucopia of effects on the plant phenotype, including numerous floral traits [16]. Changes in scent pattern have been shown in some orchid polyploids [152, 153].