Heterochronic shifts in a timing-keeping microRNA are associated with multiple instances of neoteny in plants

The prolonged production of juvenile traits and an associated reduction or loss of adult traits during development (neoteny) can either arise from a change in genes that mediate the timing of the shift between juvenile and adult traits (timing genes) or from changes in genes that are necessary for the development of adult traits (target genes). To date, the relative contribution of each developmental mechanism to the origins of neoteny remain unclear. We examined this question in the plant genus, Acacia Mill., which contains species that undergo the juvenile-to-adult vegetative transition (vegetative phase change) early in shoot development, as well as species that remain permanently juvenile, or have delayed vegetative phase change. Mapping the timing of vegetative phase change onto a molecular phylogeny of Acacia revealed that permanent juvenility has evolved multiple times and is sometimes associated with a delay in vegetative phase change in related species. In three cases, the absence or delay in vegetative phase change was associated with either higher amounts or a delayed decline in the level of miR156, the master regulator of vegetative phase change in plants. These findings support the hypothesis that neoteny in Acacia has evolved not by a loss in the capacity to produce the adult leaf phenotype but by a change in the timing of genes that promote juvenile leaf identity.