Allometric neoteny and the evolution of succulence in cacti

With the objective of analysing the role of heterochrony in the evolution of succulence in the cactus family, a comparative study of xylem development in six species with contrasting morphologies was carried out. Two woody leaf-bearing cacti and four succulent cactus species belong to different subdivisions within the family were analysed. In each species and for different ages, vessel-element length was measured, vessel-element lateral wall-pitting described and the percentage of xylem and parenchyma in the stem quantified. In the succulent species it was found that vessel element length did not change between juvenile and adult wood, that wall-pitting in adult plants was similar to that of seedlings, and that the woody tissue in adult plants was organized in vascular bundles as in the primary tissue of seedlingS. Leaf-bearing cacti, in contrast, changed in both vessel element length and wall-pitting when secondary wood was produced, and the secondary woody tissue of adult plants was organized in a continuous cambial cylinder as in most dicotyledonS. An allometric analysis suggests that a retardation in the developmental rate of woody tissues (allometric neoteny) is the main mechanism in the development of succulence in cacti.