Synapomorphies of Opuntioideae include small deciduous, barbed spines called glochids born on areoles and a bony arilsurrounding a campylotropous ovule. Other prominent morphological characters for this subfamily are presence of cylindrical, caducous leaves that tend to be shed by maturity and the sectioning of the stem into joints or pads known as cladodes. Opuntioideae are unique among cacti for lacking in the stem a thick cortex, an extensive system of cortical bundles, collapsible cortical cells, and medullary bundles. Typically, the epidermis consists of a single layer of irregularly shaped cells, a cuticle at least 1-2 microns thick, and long, uniseriate trichomes in the areoles. Opuntioideae have a hypodermis of at least one layer, very thick walls, and druses, and their cortical cells have enlarged nuclei; the reason for this is unknown. They also possess mucilage cells. Notably, their lack of collapsible cortical cells, ribs, and tubercles mean that they cannot absorb water or transfer it intercellularly as easily as the other cacti, so this may place evolutionary constraints on the aridity of habitats and maximum adult size. One adaptation around this problem is the evolution of flattened cladodes that allow opuntioids to swell up with water, increasing in volume without an increase in surface area risking water loss. Opuntioids also lack fibercaps to their phloem bundles, which in other cacti protect against sucking insects and stiffen developing internodes.
Tribes and genera
Tribes and genera of the subfamily Opuntioideae include: Austrocylindropuntieae
Current research shows conflicting arguments over the phylogenetic accuracy of the tribes and genera listed above. A major challenge in Opuntioideae classification is that the subfamily is known to hybridize, which further complicates how to define at the species level. In addition, the genera within each tribe exhibits variation in morphology, which makes using genetic analysis more important in determining relationships, since defining physical characteristics may be unseen in some genera. In 2009, a study by Griffith and Porter, based on ribosomal intergenic spacer analysis defined four tribes of Opuntioideae:
Core Maihueniopsis - which was shown as monophyletic through genetic analysis
However, this classification has been questioned by additional research that did not yield the same systematic results. Specifically, there was differences in yielded relationships based on what genetic sequences and analysis was being utilized. In 2010, it was proposed by Nyfeller and Eggli, as part of a larger overhaul of Cactaceae systematics, to only recognize Cylindropuntieae and Opuntieae as true tribes of Opuntioideae, since they are the tribes generally agreed upon to be monophyletic. It was proposed to include all other genera into one polyphyleticbasal group until further research concludes more definitive answers to their phylogeny.
